Android中measure过程、WRAP_CONTENT详解以及xml布局文件解析流程浅析(下)

最新推荐文章于 2021-05-26 02:49:58 发布

MaximusKiang

最新推荐文章于 2021-05-26 02:49:58 发布

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分类专栏： Android_UI 文章标签： android android开发 layout 源代码

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本文深入探讨Android中View的测量过程，解析MeasureSpec类的作用及其三种模式，并通过实例代码展示了自定义ViewGroup如何避免重写measure方法。

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本文原创，转载请注明出处：http://blog.youkuaiyun.com/qinjuning

上篇文章<<Android中measure过程、WRAP_CONTENT详解以及xml布局文件解析流程浅析(上)>>中，我们

了解了View树的转换过程以及如何设置View的LayoutParams的。本文继续沿着既定轨迹继续未完成的job。

主要知识点如下：
1、MeasureSpc类说明
2、measure过程详解(揭秘其细节);
3、root View被添加至窗口时，UI框架是如何设置其LayoutParams值得。

在讲解measure过程前，我们非常有必要理解MeasureSpc类的使用，否则理解起来也只能算是囫囵吞枣。

1、MeasureSpc类说明

1.1 SDK 说明如下

A MeasureSpec encapsulates the layout requirements passed from parent to child. Each MeasureSpec

represents a requirement for either the width or the height. A MeasureSpec is comprised of a size and

a mode.

即：
MeasureSpc类封装了父View传递给子View的布局(layout)要求。每个MeasureSpc实例代表宽度或者高度

(只能是其一)要求。它有三种模式：

①、UNSPECIFIED(未指定)，父元素部队自元素施加任何束缚，子元素可以得到任意想要的大小；

②、EXACTLY(完全)，父元素决定自元素的确切大小，子元素将被限定在给定的边界里而忽略它本身大小；

③、AT_MOST(至多)，子元素至多达到指定大小的值。

常用的三个函数：

　 static int getMode(int measureSpec) : 根据提供的测量值(格式)提取模式(上述三个模式之一)

static int getSize(int measureSpec) : 根据提供的测量值(格式)提取大小值(这个大小也就是我们通常所说的大小)

static int makeMeasureSpec(int size,int mode) : 根据提供的大小值和模式创建一个测量值(格式)

以上摘取自: <<MeasureSpec介绍及使用详解>>

1.2 MeasureSpc类源码分析其为View.java类的内部类，路径：\frameworks\base\core\java\android\view\View.java

[java] view plain copy print ?

public class View implements ... {
...
public static class MeasureSpec {
private static final int MODE_SHIFT = 30; //移位位数为30
//int类型占32位，向右移位30位，该属性表示掩码值，用来与size和mode进行"&"运算，获取对应值。
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
//向右移位30位，其值为00 + (30位0) , 即 0x0000(16进制表示)
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
//向右移位30位，其值为01 + (30位0) , 即0x1000(16进制表示)
public static final int EXACTLY = 1 << MODE_SHIFT;
//向右移位30位，其值为02 + (30位0) , 即0x2000(16进制表示)
public static final int AT_MOST = 2 << MODE_SHIFT;
//创建一个整形值，其高两位代表mode类型，其余30位代表长或宽的实际值。可以是WRAP_CONTENT、MATCH_PARENT或具体大小exactly size
public static int makeMeasureSpec(int size, int mode) {
return size + mode;
}
//获取模式，与运算
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
//获取长或宽的实际值，与运算
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}
...
}

public class View implements ... {
	 ...
	 public static class MeasureSpec {
        private static final int MODE_SHIFT = 30; //移位位数为30
        //int类型占32位，向右移位30位，该属性表示掩码值，用来与size和mode进行"&"运算，获取对应值。
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

        //向右移位30位，其值为00 + (30位0)  , 即 0x0000(16进制表示)
        public static final int UNSPECIFIED = 0 << MODE_SHIFT;
        //向右移位30位，其值为01 + (30位0)  , 即0x1000(16进制表示)
        public static final int EXACTLY     = 1 << MODE_SHIFT;
        //向右移位30位，其值为02 + (30位0)  , 即0x2000(16进制表示)
        public static final int AT_MOST     = 2 << MODE_SHIFT;

        //创建一个整形值，其高两位代表mode类型，其余30位代表长或宽的实际值。可以是WRAP_CONTENT、MATCH_PARENT或具体大小exactly size
        public static int makeMeasureSpec(int size, int mode) {
            return size + mode;
        }
        //获取模式  ，与运算
        public static int getMode(int measureSpec) {
            return (measureSpec & MODE_MASK);
        }
        //获取长或宽的实际值 ，与运算
        public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }

    }
	...
}

MeasureSpec类的处理思路是：

①、右移运算，使int 类型的高两位表示模式的实际值，其余30位表示其余30位代表长或宽的实际值----可以是

WRAP_CONTENT、MATCH_PARENT或具体大小exactly size。

②、通过掩码MODE_MASK进行与运算 “&”，取得模式(mode)以及长或宽(value)的实际值。

2、measure过程详解

2.1 measure过程深入分析

之前的一篇博文<< Android中View绘制流程以及invalidate()等相关方法分析>>，我们从”二B程序员”的角度简单解了measure过程的调用过程。过了这么多，我们也该升级了，- - 。现在请开始从”普通程序员”角度去理解这个

过程。我们重点查看measure过程中地相关方法。

我们说过，当UI框架开始绘制时，皆是从ViewRoot.java类开始绘制的。

ViewRoot类简要说明: 任何显示在设备中的窗口，例如：Activity、Dialog等，都包含一个ViewRoot实例，该

类主要用来与远端 WindowManagerService交互以及控制(开始/销毁)绘制。

Step 1、开始UI绘制，具体绘制方法则是：

[java] view plain copy print ?

路径：\frameworks\base\core\java\android\view\ViewRoot.java
public final class ViewRoot extends Handler implements ViewParent,View.AttachInfo.Callbacks {
...
//mView对象指添加至窗口的root View ，对Activity窗口而言，则是DecorView对象。
View mView;
//开始View绘制流程
private void performTraversals(){
...
//这两个值我们在后面讨论时，在回过头来看看是怎么赋值的。现在只需要记住其值MeasureSpec.makeMeasureSpec()构建的。
int childWidthMeasureSpec; //其值由MeasureSpec类构建 , makeMeasureSpec
int childHeightMeasureSpec;//其值由MeasureSpec类构建 , makeMeasureSpec
// Ask host how big it wants to be
host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}
...
}

路径：\frameworks\base\core\java\android\view\ViewRoot.java
public final class ViewRoot extends Handler implements ViewParent,View.AttachInfo.Callbacks {
    ...
    //mView对象指添加至窗口的root View ，对Activity窗口而言，则是DecorView对象。
    View mView;    
    
    //开始View绘制流程
    private void performTraversals(){
    	...
    	//这两个值我们在后面讨论时，在回过头来看看是怎么赋值的。现在只需要记住其值MeasureSpec.makeMeasureSpec()构建的。
    	int childWidthMeasureSpec; //其值由MeasureSpec类构建 , makeMeasureSpec
        int childHeightMeasureSpec;//其值由MeasureSpec类构建 , makeMeasureSpec
        

        // Ask host how big it wants to be
        host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    	...
    }
    ...
}

这儿，我并没有说出childWidthMeasureSpec和childHeightMeasureSpec类的来由(为了避免额外地开销，等到

第三部分时我们在来攻克它，现在只需要记住其值MeasureSpec.makeMeasureSpec()构建的。

Step 2 、调用measure()方法去做一些前期准备

measure()方法原型定义在View.java类中，final修饰符修饰，其不能被重载：

[java] view plain copy print ?

public class View implements ... {
...
/**
* This is called to find out how big a view should be. The parent
* supplies constraint information in the width and height parameters.
*
* @param widthMeasureSpec Horizontal space requirements as imposed by the
* parent
* @param heightMeasureSpec Vertical space requirements as imposed by the
* parent
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
//判断是否为强制布局，即带有“FORCE_LAYOUT”标记以及 widthMeasureSpec或heightMeasureSpec发生了改变
if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
//清除MEASURED_DIMENSION_SET标记，该标记会在onMeasure()方法后被设置
mPrivateFlags &= ~MEASURED_DIMENSION_SET;
// measure ourselves, this should set the measured dimension flag back
// 1、测量该View本身的大小； 2 、设置MEASURED_DIMENSION_SET标记，否则接写来会报异常。
onMeasure(widthMeasureSpec, heightMeasureSpec);
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
throw new IllegalStateException("onMeasure() did not set the"
+ " measured dimension by calling" + " setMeasuredDimension()");
}
mPrivateFlags |= LAYOUT_REQUIRED; //下一步是layout了，添加LAYOUT_REQUIRED标记
}
mOldWidthMeasureSpec = widthMeasureSpec; //保存值
mOldHeightMeasureSpec = heightMeasureSpec; //保存值
}
...
}

public class View implements ... {
   	...
    /**
     * This is called to find out how big a view should be. The parent
     * supplies constraint information in the width and height parameters.
     *
     * @param widthMeasureSpec Horizontal space requirements as imposed by the
     *        parent
     * @param heightMeasureSpec Vertical space requirements as imposed by the
     *        parent
     * @see #onMeasure(int, int)
     */
    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
   		//判断是否为强制布局，即带有“FORCE_LAYOUT”标记 以及 widthMeasureSpec或heightMeasureSpec发生了改变
        if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
        	//清除MEASURED_DIMENSION_SET标记   ，该标记会在onMeasure()方法后被设置
            mPrivateFlags &= ~MEASURED_DIMENSION_SET; 

            // measure ourselves, this should set the measured dimension flag back
            // 1、 测量该View本身的大小 ； 2 、 设置MEASURED_DIMENSION_SET标记，否则接写来会报异常。
            onMeasure(widthMeasureSpec, heightMeasureSpec);

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("onMeasure() did not set the"
                        + " measured dimension by calling" + " setMeasuredDimension()");
            }

            mPrivateFlags |= LAYOUT_REQUIRED;  //下一步是layout了，添加LAYOUT_REQUIRED标记
        }

        mOldWidthMeasureSpec = widthMeasureSpec;   //保存值
        mOldHeightMeasureSpec = heightMeasureSpec; //保存值
    }
   	...
}

参数widthMeasureSpec和heightMeasureSpec 由父View构建，表示父View给子View的测量要求。其值地构建

会在下面步骤中详解。

measure()方法显示判断是否需要重新调用设置改View大小，即调用onMeasure()方法，然后操作两个标识符：

①、重置MEASURED_DIMENSION_SET ： onMeasure()方法中，需要添加该标识符，否则，会报异常；

②、添加LAYOUT_REQUIRED ：表示需要进行layout操作。

最后，保存当前的widthMeasureSpec和heightMeasureSpec值。

Step 3 、调用onMeasure()方法去真正设置View的长宽值，其默认实现为：

[java] view plain copy print ?

/**
* Measure the view and its content to determine the measured width and the
* measured height. This method is invoked by {@link #measure(int, int)} and
* should be overriden by subclasses to provide accurate and efficient
* measurement of their contents.
*
* @param widthMeasureSpec horizontal space requirements as imposed by the parent.
* The requirements are encoded with
* @param heightMeasureSpec vertical space requirements as imposed by the parent.
* The requirements are encoded with
*/
//设置该View本身地大小
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no contraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
//@param size参数一般表示设置了android:minHeight属性或者该View背景图片的大小值
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
//根据不同的mode值，取得宽和高的实际值。
switch (specMode) {
case MeasureSpec.UNSPECIFIED: //表示该View的大小父视图未定，设置为默认值
result = size;
break;
case MeasureSpec.AT_MOST: //表示该View的大小由父视图指定了
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
//获得设置了android:minHeight属性或者该View背景图片的大小值，最为该View的参考值
protected int getSuggestedMinimumWidth() {
int suggestedMinWidth = mMinWidth; // android:minHeight
if (mBGDrawable != null) { // 背景图片对应地Width。
final int bgMinWidth = mBGDrawable.getMinimumWidth();
if (suggestedMinWidth < bgMinWidth) {
suggestedMinWidth = bgMinWidth;
}
}
return suggestedMinWidth;
}
//设置View在measure过程中宽和高
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= MEASURED_DIMENSION_SET; //设置了MEASURED_DIMENSION_SET标记
}

 /**
    * Measure the view and its content to determine the measured width and the
    * measured height. This method is invoked by {@link #measure(int, int)} and
    * should be overriden by subclasses to provide accurate and efficient
    * measurement of their contents.
    * 
    * @param widthMeasureSpec horizontal space requirements as imposed by the parent.
    *                         The requirements are encoded with
    * @param heightMeasureSpec vertical space requirements as imposed by the parent.
    *                         The requirements are encoded with
    */
   //设置该View本身地大小
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
       setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
               getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
   }
   
   /**
    * Utility to return a default size. Uses the supplied size if the
    * MeasureSpec imposed no contraints. Will get larger if allowed
    * by the MeasureSpec.
    *
    * @param size Default size for this view
    * @param measureSpec Constraints imposed by the parent
    * @return The size this view should be.
    */
   //@param size参数一般表示设置了android:minHeight属性或者该View背景图片的大小值
   public static int getDefaultSize(int size, int measureSpec) {
       int result = size;  
       int specMode = MeasureSpec.getMode(measureSpec);
       int specSize =  MeasureSpec.getSize(measureSpec);

       //根据不同的mode值，取得宽和高的实际值。
       switch (specMode) {
       case MeasureSpec.UNSPECIFIED:  //表示该View的大小父视图未定，设置为默认值
           result = size;
           break;
       case MeasureSpec.AT_MOST:      //表示该View的大小由父视图指定了
       case MeasureSpec.EXACTLY:
           result = specSize;
           break;
       }
       return result;
   }
   //获得设置了android:minHeight属性或者该View背景图片的大小值， 最为该View的参考值
   protected int getSuggestedMinimumWidth() {
       int suggestedMinWidth = mMinWidth;  //  android:minHeight

       if (mBGDrawable != null) { // 背景图片对应地Width。
           final int bgMinWidth = mBGDrawable.getMinimumWidth();
           if (suggestedMinWidth < bgMinWidth) {
               suggestedMinWidth = bgMinWidth;
           }
       }

       return suggestedMinWidth;
   }
   //设置View在measure过程中宽和高
   protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
       mMeasuredWidth = measuredWidth;
       mMeasuredHeight = measuredHeight;

       mPrivateFlags |= MEASURED_DIMENSION_SET;  //设置了MEASURED_DIMENSION_SET标记
   }

主要功能就是根据该View属性(android:minWidth和背景图片大小)和父View对该子View的"测量要求"，设置该 View的 mMeasuredWidth 和 mMeasuredHeight 值。

这儿只是一般的View类型地实现方法。一般来说，父View，也就是ViewGroup类型，都需要在重写onMeasure() 方法，遍历所有子View，设置每个子View的大小。基本思想如下：遍历所有子View，设置每个子View的大小。伪

代码表示为：

[java] view plain copy print ?

//某个ViewGroup类型的视图
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//必须调用super.ononMeasure()或者直接调用setMeasuredDimension()方法设置该View大小，否则会报异常。
super.onMeasure(widthMeasureSpec , heightMeasureSpec)
//setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
// getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
//遍历每个子View
for(int i = 0 ; i < getChildCount() ; i++){
View child = getChildAt(i);
//调用子View的onMeasure，设置他们的大小。childWidthMeasureSpec ， childHeightMeasureSpec ?
child.onMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}

   //某个ViewGroup类型的视图
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
	    //必须调用super.ononMeasure()或者直接调用setMeasuredDimension()方法设置该View大小，否则会报异常。
	    super.onMeasure(widthMeasureSpec , heightMeasureSpec)
        //setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
        //        getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
        
	    //遍历每个子View
	    for(int i = 0 ; i < getChildCount() ; i++){
	    	View child = getChildAt(i);
	    	//调用子View的onMeasure，设置他们的大小。childWidthMeasureSpec ， childHeightMeasureSpec ?
	    	child.onMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
	    }
   }

Step 2、Step 3 代码也比较好理解，但问题是我们示例代码中widthMeasureSpec、heightMeasureSpec是如何

确定的呢？父View是如何设定其值的？

要想回答这个问题，我们看是去源代码里找找答案吧。在ViewGroup.java类中，为我们提供了三个方法，去设置

每个子View的大小，基本思想也如同我们之前描述的思想：遍历所有子View，设置每个子View的大小。

主要有如下方法：

[java] view plain copy print ?

/**
* Ask all of the children of this view to measure themselves, taking into
* account both the MeasureSpec requirements for this view and its padding.
* We skip children that are in the GONE state The heavy lifting is done in
* getChildMeasureSpec.
*/
//widthMeasureSpec 和 heightMeasureSpec 表示该父View的布局要求
//遍历每个子View，然后调用measureChild()方法去实现每个子View大小
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) { // 不处于 “GONE” 状态
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding.
* The heavy lifting is done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param parentHeightMeasureSpec The height requirements for this view
*/
//测量每个子View高宽时，清楚了该View本身的边距大小，即android:padding属性或android:paddingLeft等属性标记
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams(); // LayoutParams属性
//设置子View的childWidthMeasureSpec属性，去除了该父View的边距值 mPaddingLeft + mPaddingRight
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
//设置子View的childHeightMeasureSpec属性，去除了该父View的边距值 mPaddingTop + mPaddingBottom
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

/**
 * Ask all of the children of this view to measure themselves, taking into
 * account both the MeasureSpec requirements for this view and its padding.
 * We skip children that are in the GONE state The heavy lifting is done in
 * getChildMeasureSpec.
 */
//widthMeasureSpec 和  heightMeasureSpec 表示该父View的布局要求
//遍历每个子View，然后调用measureChild()方法去实现每个子View大小
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
    final int size = mChildrenCount;
    final View[] children = mChildren;
    for (int i = 0; i < size; ++i) {
        final View child = children[i];
        if ((child.mViewFlags & VISIBILITY_MASK) != GONE) { // 不处于 “GONE” 状态
            measureChild(child, widthMeasureSpec, heightMeasureSpec);
        }
    }
}
   
/**
 * Ask one of the children of this view to measure itself, taking into
 * account both the MeasureSpec requirements for this view and its padding.
 * The heavy lifting is done in getChildMeasureSpec.
 *
 * @param child The child to measure
 * @param parentWidthMeasureSpec The width requirements for this view
 * @param parentHeightMeasureSpec The height requirements for this view
 */
//测量每个子View高宽时，清楚了该View本身的边距大小，即android:padding属性 或android:paddingLeft等属性标记
protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams(); // LayoutParams属性
    //设置子View的childWidthMeasureSpec属性，去除了该父View的边距值  mPaddingLeft + mPaddingRight
    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    //设置子View的childHeightMeasureSpec属性，去除了该父View的边距值  mPaddingTop + mPaddingBottom
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

measureChildren()方法：遍历所有子View，调用measureChild()方法去设置该子View的属性值。

measureChild() 方法：获取特定子View的widthMeasureSpec、heightMeasureSpec，调用measure()方法

设置子View的实际宽高值。

getChildMeasureSpec()就是获取子View的widthMeasureSpec、heightMeasureSpec值。

[java] view plain copy print ?

/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results.
*/
// spec参数表示该父View本身所占的widthMeasureSpec 或 heightMeasureSpec值
// padding参数表示该父View的边距大小，见于android:padding属性或android:paddingLeft等属性标记
// childDimension参数表示该子View内部LayoutParams属性的值，可以是wrap_content、match_parent、一个精确指(an exactly size),
// 例如：由android:width指定等。
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec); //获得父View的mode
int specSize = MeasureSpec.getSize(spec); //获得父View的实际值
int size = Math.max(0, specSize - padding); //父View为子View设定的大小，减去边距值，
int resultSize = 0; //子View对应地 size 实际值，由下面的逻辑条件赋值
int resultMode = 0; //子View对应地 mode 值，由下面的逻辑条件赋值
switch (specMode) {
// Parent has imposed an exact size on us
//1、父View是EXACTLY的！
case MeasureSpec.EXACTLY:
//1.1、子View的width或height是个精确值 (an exactly size)
if (childDimension >= 0) {
resultSize = childDimension; //size为精确值
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY 。
}
//1.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY 。
}
//1.3、子View的width或height为 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.AT_MOST; //mode为AT_MOST 。
}
break;
// Parent has imposed a maximum size on us
//2、父View是AT_MOST的！
case MeasureSpec.AT_MOST:
//2.1、子View的width或height是个精确值 (an exactly size)
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension; //size为精确值
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY 。
}
//2.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.AT_MOST; //mode为AT_MOST
}
//2.3、子View的width或height为 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size; //size为父视图大小
resultMode = MeasureSpec.AT_MOST; //mode为AT_MOST
}
break;
// Parent asked to see how big we want to be
//3、父View是UNSPECIFIED的！
case MeasureSpec.UNSPECIFIED:
//3.1、子View的width或height是个精确值 (an exactly size)
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension; //size为精确值
resultMode = MeasureSpec.EXACTLY; //mode为 EXACTLY
}
//3.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0; //size为0！ ,其值未定
resultMode = MeasureSpec.UNSPECIFIED; //mode为 UNSPECIFIED
}
//3.3、子View的width或height为 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0; //size为0! ，其值未定
resultMode = MeasureSpec.UNSPECIFIED; //mode为 UNSPECIFIED
}
break;
}
//根据上面逻辑条件获取的mode和size构建MeasureSpec对象。
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

/**
 * Does the hard part of measureChildren: figuring out the MeasureSpec to
 * pass to a particular child. This method figures out the right MeasureSpec
 * for one dimension (height or width) of one child view.
 *
 * The goal is to combine information from our MeasureSpec with the
 * LayoutParams of the child to get the best possible results.
 */
// spec参数                                    表示该父View本身所占的widthMeasureSpec 或  heightMeasureSpec值
// padding参数                          表示该父View的边距大小，见于android:padding属性 或android:paddingLeft等属性标记
// childDimension参数  表示该子View内部LayoutParams属性的值，可以是wrap_content、match_parent、一个精确指(an exactly size),
//           例如：由android:width指定等。
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
    int specMode = MeasureSpec.getMode(spec);  //获得父View的mode
    int specSize = MeasureSpec.getSize(spec);  //获得父View的实际值

    int size = Math.max(0, specSize - padding); //父View为子View设定的大小，减去边距值，

    int resultSize = 0;    //子View对应地 size 实际值 ，由下面的逻辑条件赋值
    int resultMode = 0;    //子View对应地 mode 值 ， 由下面的逻辑条件赋值

    switch (specMode) {
    // Parent has imposed an exact size on us
    //1、父View是EXACTLY的 ！
    case MeasureSpec.EXACTLY: 
    	//1.1、子View的width或height是个精确值 (an exactly size)
        if (childDimension >= 0) {          
            resultSize = childDimension;         //size为精确值
            resultMode = MeasureSpec.EXACTLY;    //mode为 EXACTLY 。
        } 
        //1.2、子View的width或height为 MATCH_PARENT/FILL_PARENT 
        else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size. So be it.
            resultSize = size;                   //size为父视图大小
            resultMode = MeasureSpec.EXACTLY;    //mode为 EXACTLY 。
        } 
        //1.3、子View的width或height为 WRAP_CONTENT
        else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;                   //size为父视图大小
            resultMode = MeasureSpec.AT_MOST;    //mode为AT_MOST 。
        }
        break;

    // Parent has imposed a maximum size on us
    //2、父View是AT_MOST的 ！    
    case MeasureSpec.AT_MOST:
    	//2.1、子View的width或height是个精确值 (an exactly size)
        if (childDimension >= 0) {
            // Child wants a specific size... so be it
            resultSize = childDimension;        //size为精确值
            resultMode = MeasureSpec.EXACTLY;   //mode为 EXACTLY 。
        }
        //2.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
        else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size, but our size is not fixed.
            // Constrain child to not be bigger than us.
            resultSize = size;                  //size为父视图大小
            resultMode = MeasureSpec.AT_MOST;   //mode为AT_MOST
        }
        //2.3、子View的width或height为 WRAP_CONTENT
        else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;                  //size为父视图大小
            resultMode = MeasureSpec.AT_MOST;   //mode为AT_MOST
        }
        break;

    // Parent asked to see how big we want to be
    //3、父View是UNSPECIFIED的 ！
    case MeasureSpec.UNSPECIFIED:
    	//3.1、子View的width或height是个精确值 (an exactly size)
        if (childDimension >= 0) {
            // Child wants a specific size... let him have it
            resultSize = childDimension;        //size为精确值
            resultMode = MeasureSpec.EXACTLY;   //mode为 EXACTLY
        }
        //3.2、子View的width或height为 MATCH_PARENT/FILL_PARENT
        else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size... find out how big it should
            // be
            resultSize = 0;                        //size为0！ ,其值未定
            resultMode = MeasureSpec.UNSPECIFIED;  //mode为 UNSPECIFIED
        } 
        //3.3、子View的width或height为 WRAP_CONTENT
        else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = 0;                        //size为0! ，其值未定
            resultMode = MeasureSpec.UNSPECIFIED;  //mode为 UNSPECIFIED
        }
        break;
    }
    //根据上面逻辑条件获取的mode和size构建MeasureSpec对象。
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

为了便于分析，我将上面的逻辑判断语句使用列表项进行了说明.

getChildMeasureSpec()方法的主要功能如下：

根据父View的measureSpec值(widthMeasureSpec,heightMeasureSpec)值以及子View的子View内部

LayoutParams属性值，共同决定子View的measureSpec值的大小。主要判断条件主要为MeasureSpec的mode

类型以及LayoutParams的宽高实际值(lp.width,lp.height)，见于以上所贴代码中的列表项： 1、 1.1 ; 1.2 ; 1.3 ;

2、2.1等。

例如，分析列表3：假设当父View为MeasureSpec.UNSPECIFIED类型，即未定义时，只有当子View的width

或height指定时，其mode才为MeasureSpec.EXACTLY，否者该View size为 0 ，mode为MeasureSpec.UNSPECIFIED时

，即处于未指定状态。

由此可以得出，每个View大小的设定都事由其父View以及该View共同决定的。但这只是一个期望的大小，每个

View在测量时最终大小的设定是由setMeasuredDimension()最终决定的。因此，最终确定一个View的“测量长宽“是

由以下几个方面影响：

1、父View的MeasureSpec属性；

2、子View的LayoutParams属性；

3、setMeasuredDimension()或者其它类似设定 mMeasuredWidth 和 mMeasuredHeight 值的方法。

setMeasuredDimension()原型：

[java] view plain copy print ?

//设置View在measure过程中宽和高
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= MEASURED_DIMENSION_SET; //设置了MEASURED_DIMENSION_SET标记
}

   //设置View在measure过程中宽和高
   protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
       mMeasuredWidth = measuredWidth;
       mMeasuredHeight = measuredHeight;

       mPrivateFlags |= MEASURED_DIMENSION_SET;  //设置了MEASURED_DIMENSION_SET标记
   }

将上面列表项转换为表格为：

这张表格更能帮助我们分析View的MeasureSpec的确定条件关系。

为了帮助大家理解，下面我们分析某个窗口使用地xml布局文件，我们弄清楚该xml布局文件中每个View的

MeasureSpec值的组成。

[java] view plain copy print ?

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/llayout"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView android:id="@+id/tv"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/hello" />
</LinearLayout>

   <?xml version="1.0" encoding="utf-8"?>
   <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
       android:id="@+id/llayout"
   	   android:orientation="vertical" 
       android:layout_width="match_parent"
   	   android:layout_height="match_parent">
   	
   	
   	<TextView android:id="@+id/tv" 
   	    android:layout_width="match_parent"
   		android:layout_height="wrap_content"
   	    android:text="@string/hello" />

   </LinearLayout>

该布局文件共有两个View： ①、id为llayout的LinearLayout布局控件 ;

②、id为tv的TextView控件。

假设LinearLayout的父View对应地widthSpec和heightSpec值皆为MeasureSpec.EXACTLY类型(Activity窗口

的父View为DecorView，具体原因见第三部分说明)。

对LinearLayout而言比较简单，由于 android:layout_width="match_parent"，因此其width对应地widthSpec

mode值为MeasureSpec.EXACTLY ， size由父视图大小指定 ; 由于android:layout_height = "match_parent"，

因此其height对应地heightSpec mode值为MeasureSpec.EXACTLY，size由父视图大小指定 ;

对TextView而言，其父View为LinearLayout的widthSpec和heightSpec值皆为MeasureSpec.EXACTLY类型，

由于android:layout_width="match_parent" ，因此其width对应地widthSpec mode值为MeasureSpec.EXACTLY，

size由父视图大小指定 ; 由于android:layout_width="wrap_content" ，因此其height对应地widthSpec mode值为

MeasureSpec.AT_MOST，size由父视图大小指定。

我们继续窥测下LinearLayout类是如何进行measure过程的：

[java] view plain copy print ?

public class LinearLayout extends ViewGroup {
...
@Override //onMeasure方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//判断是垂直方向还是水平方向，这儿我们假设是VERTICAL垂直方向，
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
//垂直方向布局
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
mTotalLength = 0; //该LinearLayout测量子View时的总高度。
float totalWeight = 0; //所有子View的权重和 , android:layout_weight
int maxWidth = 0; //保存子View中最大width值
...
final int count = getVirtualChildCount(); //子View的个数
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
...
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
...
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
totalWeight += lp.weight;
//满足该条件地View会在该LinearLayout有剩余高度时，才真正调用measure()
if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
...
} else {
int oldHeight = Integer.MIN_VALUE;
//如果View的hight值为0，并且设置了android:layout_weight属性，重新纠正其height值为WRAP_CONTENT
if (lp.height == 0 && lp.weight > 0) {
oldHeight = 0;
lp.height = LayoutParams.WRAP_CONTENT;
}
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
//对每个子View调用measure()方法
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
//这三行代码做了如下两件事情：
//1、获得该View的measuredHeight值，每个View都会根据他们地属性正确设置值 > 0 ;
//2、更新mTotalLength值：取当前高度mTotalLength值与mTotalLength + childHeight 的最大值
// 于是对于android:layout_height="wrap_height"属性地LinearLayout控件也就知道了它的确切高度值了。
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
...
}
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);
...
}
//后续还有很多处理，包括继续measure()某些符合条件地子View
...
}
void measureChildBeforeLayout(View child, int childIndex,
int widthMeasureSpec, int totalWidth, int heightMeasureSpec,
int totalHeight) {
//调用measureChildWithMargins()方法去设置子View大小
measureChildWithMargins(child, widthMeasureSpec, totalWidth,
heightMeasureSpec, totalHeight);
}
...

   public class LinearLayout extends ViewGroup {
		...
		@Override  //onMeasure方法。
		protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
		    //判断是垂直方向还是水平方向，这儿我们假设是VERTICAL垂直方向，
			if (mOrientation == VERTICAL) {
		        measureVertical(widthMeasureSpec, heightMeasureSpec);
		    } else {
		        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
		    }
		}
		//垂直方向布局
	    void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
	        mTotalLength = 0;         //该LinearLayout测量子View时的总高度。
	    	float totalWeight = 0;    //所有子View的权重和 , android:layout_weight
	    	int maxWidth = 0;         //保存子View中最大width值
	        ...
	        final int count = getVirtualChildCount();  //子View的个数
	        
	        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
	        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
            ...
	        // See how tall everyone is. Also remember max width.
	        for (int i = 0; i < count; ++i) {
	            final View child = getVirtualChildAt(i);
                ...
	            LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();

	            totalWeight += lp.weight;  
	            //满足该条件地View会在该LinearLayout有剩余高度时，才真正调用measure()
	            if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
	                ...
	            } else {
	                int oldHeight = Integer.MIN_VALUE;
	                //如果View的hight值为0，并且设置了android:layout_weight属性，重新纠正其height值为WRAP_CONTENT
	                if (lp.height == 0 && lp.weight > 0) {
	                    oldHeight = 0;
	                    lp.height = LayoutParams.WRAP_CONTENT;
	                }
	                // Determine how big this child would like to be. If this or
	                // previous children have given a weight, then we allow it to
	                // use all available space (and we will shrink things later
	                // if needed).
	                //对每个子View调用measure()方法
	                measureChildBeforeLayout(
	                       child, i, widthMeasureSpec, 0, heightMeasureSpec,
	                       totalWeight == 0 ? mTotalLength : 0);
	                
	                //这三行代码做了如下两件事情：
	                //1、获得该View的measuredHeight值，每个View都会根据他们地属性正确设置值  > 0 ;
	                //2、更新mTotalLength值：取当前高度mTotalLength值与mTotalLength + childHeight 的最大值
	                // 于是对于android:layout_height="wrap_height"属性地LinearLayout控件也就知道了它的确切高度值了。
	                final int childHeight = child.getMeasuredHeight();
	                final int totalLength = mTotalLength;
	                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
	                       lp.bottomMargin + getNextLocationOffset(child));
	                ...
	            }
	            final int margin = lp.leftMargin + lp.rightMargin;
	            final int measuredWidth = child.getMeasuredWidth() + margin;
	            maxWidth = Math.max(maxWidth, measuredWidth);
	            ...
	        }
            //后续还有很多处理，包括继续measure()某些符合条件地子View
	        ...
	    }
	    void measureChildBeforeLayout(View child, int childIndex,
	            int widthMeasureSpec, int totalWidth, int heightMeasureSpec,
	            int totalHeight) {
	    	//调用measureChildWithMargins()方法去设置子View大小
	        measureChildWithMargins(child, widthMeasureSpec, totalWidth,
	                heightMeasureSpec, totalHeight);
	    }
		...
	}

继续看看measureChildWithMargins()方法，该方法定义在ViewGroup.java内，基本流程同于measureChild()方法，但添加了对子View Margin的处理，即：android:margin属性或者android:marginLeft等属性的处理。

measureChildWithMargins@ViewGroup.java

[java] view plain copy print ?

/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*/
//基本流程同于measureChild()方法，但添加了对子View Margin的处理，即：android:margin属性或者android:marginLeft等属性的处理
//widthUsed参数表示该父View已经使用的宽度
//heightUsed参数表示该父View已经使用的高度
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//获得子View的childWidthMeasureSpec和childHeightMeasureSpec值
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

	/**
	 * Ask one of the children of this view to measure itself, taking into
	 * account both the MeasureSpec requirements for this view and its padding
	 * and margins. The child must have MarginLayoutParams The heavy lifting is
	 * done in getChildMeasureSpec.
	 */
	//基本流程同于measureChild()方法，但添加了对子View Margin的处理，即：android:margin属性或者android:marginLeft等属性的处理
	//widthUsed参数  表示该父View已经使用的宽度
	//heightUsed参数  表示该父View已经使用的高度
	protected void measureChildWithMargins(View child,
	        int parentWidthMeasureSpec, int widthUsed,
	        int parentHeightMeasureSpec, int heightUsed) {
	    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

	    //获得子View的childWidthMeasureSpec和childHeightMeasureSpec值
	    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
	            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
	                    + widthUsed, lp.width);
	    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
	            mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
	                    + heightUsed, lp.height);

	    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
	}

measure()过程时，LinearLayout类做了如下事情：

1、遍历每个子View，对其调用measure()方法；

2、子View measure()完成后，需要取得该子View地宽高实际值，继而做处理(例如：LinearLayout属性为

android:widht="wrap_content"时，LinearLayout的实际width值则是每个子View的width值的累加值)。

2.2 WRAP_CONTENT、MATCH_PARENT以及measure动机揭秘

子View地宽高实际值，即child.getMeasuredWidth()值得返回最终会是一个确定值？难道WRAP_CONTENT(

其值为-2) 、MATCH_PARENT(值为-1)或者说一个具体值(an exactly size > 0)。前面我们说过,View最终“测量”值的

确定是有三个部分组成地：

①、父View的MeasureSpec属性；

②、子View的LayoutParams属性；

③、setMeasuredDimension()或者其它类似设定 mMeasuredWidth 和 mMeasuredHeight 值的方法。

因此，一个View必须以某种合适地方法确定它地最终大小。例如，如下自定义View：

[java] view plain copy print ?

//自定义View
public Class MyView extends View {
//针对不同地mode值，设置本View地大小
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec){
//获得父View传递给我们地测量需求
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int width = 0 ;
int height = 0 ;
//对UNSPECIFIED 则抛出异常
if(widthMode == MeasureSpec.UNSPECIFIED || heightMode == MeasureSpec.UNSPECIFIED)
throw new RuntimeException("widthMode or heightMode cannot be UNSPECIFIED");
//精确指定
if(widthMode == MeasureSpec.EXACTLY){
width = 100 ;
}
//模糊指定
else if(widthMode == MeasureSpec.AT_MOST )
width = 50 ;
//精确指定
if(heightMode == MeasureSpec.EXACTLY){
height = 100 ;
}
//模糊指定
else if(heightMode == MeasureSpec.AT_MOST )
height = 50 ;
setMeasuredDimension(width , height) ;
}
}

//自定义View	
public Class MyView extends View {
	
	 //针对不同地mode值，设置本View地大小
	 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec){
		 //获得父View传递给我们地测量需求
		 int widthMode = MeasureSpec.getMode(widthMeasureSpec);
	     int heightMode = MeasureSpec.getMode(heightMeasureSpec);
		 
	     int width = 0 ;
	     int height = 0 ;
	     //对UNSPECIFIED 则抛出异常
	     if(widthMode == MeasureSpec.UNSPECIFIED || heightMode == MeasureSpec.UNSPECIFIED)
	         throw new RuntimeException("widthMode or heightMode cannot be UNSPECIFIED");
	    
	     //精确指定
	     if(widthMode == MeasureSpec.EXACTLY){
	    	 width = 100 ;
	     }
	     //模糊指定
	     else if(widthMode == MeasureSpec.AT_MOST )
	    	 width = 50 ; 
	     
	      //精确指定
	     if(heightMode == MeasureSpec.EXACTLY){
	    	 height = 100 ;
	     }
	     //模糊指定
	     else if(heightMode == MeasureSpec.AT_MOST )
	    	 height = 50 ;
	     
	     setMeasuredDimension(width , height) ;
	 }
}

该自定义View重写了onMeasure()方法，根据传递过来的widthMeasureSpec和heightMeasureSpec简单设置了

该View的mMeasuredWidth 和 mMeasuredHeight值。

对于TextView而言，如果它地mode不是Exactly类型，它会根据一些属性，例如：android:textStyle

、android:textSizeandroid:typeface等去确定TextView类地需要占用地长和宽。

因此，如果你地自定义View必须手动对不同mode做出处理。否则，则是mode对你而言是无效的。

Android框架中提供地一系列View/ViewGroup都需要去进行这个measure()过程地，因为在layout()过程中，父

View需要调用getMeasuredWidth()或getMeasuredHeight()去为每个子View设置他们地布局坐标，只有确定布局

坐标后，才能真正地将该View 绘制(draw)出来，否则该View的layout大小为0，得不到期望效果。我们继续看看

LinearLayout的layout布局过程：

[java] view plain copy print ?

public class LinearLayout extends ViewGroup {
...
@Override //layout 过程
protected void onLayout(boolean changed, int l, int t, int r, int b) {
//假定是垂直方向布局
if (mOrientation == VERTICAL) {
layoutVertical();
} else {
layoutHorizontal();
}
}
//对每个子View调用layout过程
void layoutVertical() {
...
final int count = getVirtualChildCount();
...
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) { //一般为非null
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
//获得子View测量时的实际宽高值，
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
...
// 封装了child.layout()方法，见如下
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
//width = getMeasuredWidth() ; height = childHeight(); View的大小就是测量大小
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}
...
}

public class LinearLayout extends ViewGroup {
	...
    @Override  //layout 过程
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
		//假定是垂直方向布局
        if (mOrientation == VERTICAL) {
            layoutVertical();
        } else {
            layoutHorizontal();
        }
    }
	//对每个子View调用layout过程
    void layoutVertical() {
        ...
        final int count = getVirtualChildCount();
        ...
        for (int i = 0; i < count; i++) {
            final View child = getVirtualChildAt(i);
            if (child == null) {  //一般为非null
                childTop += measureNullChild(i);
            } else if (child.getVisibility() != GONE) {
            	//获得子View测量时的实际宽高值，
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();
                
                ...
                //  封装了child.layout()方法，见如下
                setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                        childWidth, childHeight); 
                childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

                i += getChildrenSkipCount(child, i);
            }
        }
    }
    //width = getMeasuredWidth() ; height = childHeight(); View的大小就是测量大小
    private void setChildFrame(View child, int left, int top, int width, int height) {
    	
        child.layout(left, top, left + width, top + height);
    }
	...
}

对一个View进行measure操作地主要目的就是为了确定该View地布局大小，见上面所示代码。但measure操作

通常是耗时的，因此对自定义ViewGroup而言，我们可以自由控制measure、layout过程，如果我们知道如何layout

一个View，我们可以跳过该ViewGroup地measure操作(onMeasure()方法中measure所有子View地)，直接去layout

在前面一篇博客<<Android中滑屏初探 ---- scrollTo 以及 scrollBy方法使用说明>>中，我们自定义了一个 ViewGroup, 并且重写了onMeasure()和onLayout()方法去分别操作每个View。就该ViewGroup而言，我们只需要

重写onLayout()操作即可，因为我们知道如何layout每个子View。如下代码所示：

[java] view plain copy print ?

//自定义ViewGroup ，包含了三个LinearLayout控件，存放在不同的布局位置
public class MultiViewGroup extends ViewGroup {
private void init() {
// 初始化3个 LinearLayout控件
LinearLayout oneLL = new LinearLayout(mContext);
oneLL.setBackgroundColor(Color.RED);
addView(oneLL);
...
}
@Override
// 我们知晓每个子View的layout布局大小，因此我们不需要为每个子View进行measure()操作了。
// protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// setMeasuredDimension(width, height);
// // 设置该ViewGroup的大小
// int width = MeasureSpec.getSize(widthMeasureSpec);
// int height = MeasureSpec.getSize(heightMeasureSpec);
// int childCount = getChildCount();
// for (int i = 0; i < childCount; i++) {
// View child = getChildAt(i);
// // 设置每个子视图的大小，即全屏
// child.measure(MultiScreenActivity.screenWidth, MultiScreenActivity.scrrenHeight);
// }
}
// layout过程
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
// TODO Auto-generated method stub
Log.i(TAG, "--- start onLayout --");
int startLeft = 0; // 每个子视图的起始布局坐标
int startTop = 10; // 间距设置为10px 相当于 android：marginTop= "10px"
int childCount = getChildCount();
Log.i(TAG, "--- onLayout childCount is -->" + childCount);
for (int i = 0; i < childCount; i++) {
View child = getChildAt(i);
child.layout(startLeft, startTop,
startLeft + MultiScreenActivity.screenWidth,
startTop + MultiScreenActivity.scrrenHeight);
startLeft = startLeft + MultiScreenActivity.screenWidth ; //校准每个子View的起始布局位置
//三个子视图的在屏幕中的分布如下 [0 , 320] / [320,640] / [640,960]
}
}
}

//自定义ViewGroup ， 包含了三个LinearLayout控件，存放在不同的布局位置
public class MultiViewGroup extends ViewGroup {
	private void init() {
		// 初始化3个 LinearLayout控件
		LinearLayout oneLL = new LinearLayout(mContext);
		oneLL.setBackgroundColor(Color.RED);
        addView(oneLL);
        ...
	}
	@Override
	// 我们知晓每个子View的layout布局大小，因此我们不需要为每个子View进行measure()操作了。
//	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//		setMeasuredDimension(width, height);
//		// 设置该ViewGroup的大小
//		int width = MeasureSpec.getSize(widthMeasureSpec);
//		int height = MeasureSpec.getSize(heightMeasureSpec);
//		int childCount = getChildCount();
//		for (int i = 0; i < childCount; i++) {
//			View child = getChildAt(i);
//			// 设置每个子视图的大小 ， 即全屏
//			child.measure(MultiScreenActivity.screenWidth, MultiScreenActivity.scrrenHeight);
//		}
	}

	// layout过程
	@Override
	protected void onLayout(boolean changed, int l, int t, int r, int b) {
		// TODO Auto-generated method stub
		Log.i(TAG, "--- start onLayout --");
		int startLeft = 0; // 每个子视图的起始布局坐标
		int startTop = 10; // 间距设置为10px 相当于 android：marginTop= "10px"
		int childCount = getChildCount();
		Log.i(TAG, "--- onLayout childCount is -->" + childCount);
		for (int i = 0; i < childCount; i++) {
			View child = getChildAt(i);
			child.layout(startLeft, startTop, 
					startLeft + MultiScreenActivity.screenWidth, 
					startTop + MultiScreenActivity.scrrenHeight);
			startLeft = startLeft + MultiScreenActivity.screenWidth ; //校准每个子View的起始布局位置
			//三个子视图的在屏幕中的分布如下 [0 , 320] / [320,640] / [640,960]
		}
	}
}

更多关于自定义ViewGroup无须重写measure动作的，可以参考 Android API ：

<<Optimizing the View >>

中文翻译见于：<< Android中View绘制优化之三---- 优化View>>

3、root View被添加至窗口时，UI框架是如何设置其LayoutParams值

老子道德经有言：“道生一，一生二，二生三，三生万物。” UI绘制也就是个递归过程。理解其基本架构后，

也就“掌握了一个中心点”了。在第一节中，我们没有说明开始UI绘制时，没有说明mView.measure()参数地由来，

参数也就是我们本节需要弄懂的“道” --- root View的 widthMeasureSpec和heightMeasureSpec 是如何确定的。

对于如下布局文件： main.xml

[java] view plain copy print ?

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/hello"
/>
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello"
    />
</LinearLayout>

当使用LayoutInflater类解析成View时，LinearLayout对象的LayoutParams参数为null 。具体原因请参考上篇博文

任何一个View被添加至窗口时，都需要利用WindowManager类去操作。例如，如下代码：

[java] view plain copy print ?

//显示一个悬浮窗吧， just so so
public void showView()
{
//解析布局文件
LayoutInflater layoutInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
//rootView对应地LayoutParams属性值为null，将会在UI绘制时设定其值
View rootView = layoutInflater.inflate(R.layout.main, null);
WindowManager windowManager = (WindowManager)getSystemService(Context.WINDOW_SERVICE);
//设置WindowManager.LayoutParams参数值，作为该窗口的各种属性
WindowManager.LayoutParams winparams = WindowManager.LayoutParams();
// 以屏幕左上角为原点，设置x、y初始值
winparams.x = 0;
winparams.y = 0;
//设置悬浮窗口长宽数据
winparams.width = WindowManager.LayoutParams.WRAP_CONTENT;;
winparams.height = WindowManager.LayoutParams.WRAP_CONTENT;;
windowManager.addView(rootView, winparams);
}

//显示一个悬浮窗吧 ， just so so 
public void showView()
{
    //解析布局文件
	LayoutInflater layoutInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    //rootView对应地LayoutParams属性值为null，将会在UI绘制时设定其值
	View rootView = layoutInflater.inflate(R.layout.main, null);
	
	WindowManager windowManager = (WindowManager)getSystemService(Context.WINDOW_SERVICE);
	//设置WindowManager.LayoutParams参数值，作为该窗口的各种属性
	WindowManager.LayoutParams winparams = WindowManager.LayoutParams();
     // 以屏幕左上角为原点，设置x、y初始值
	winparams.x = 0;
	winparams.y = 0;

    //设置悬浮窗口长宽数据
	winparams.width = WindowManager.LayoutParams.WRAP_CONTENT;;
	winparams.height = WindowManager.LayoutParams.WRAP_CONTENT;;
     
	windowManager.addView(rootView, winparams);
}

[java] view plain copy print ?

</pre></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;">       关于WindowManager的使用请看如下博客 ：</span></span></div><div sizcache="28" sizset="150"><span class="link_title" sizcache="28" sizset="150"><span sizcache="28" sizset="150" style="font-family:'Comic Sans MS';font-size:18px;">                               <<<a target=_blank href="http://blog.sina.com.cn/s/blog_4b3c1f950100qd4d.html" target="_blank">android学习---- WindowManager 接口</a> >></span></span></div><div sizcache="28" sizset="151"><span class="link_title" sizcache="28" sizset="151"><span sizcache="28" sizset="151" style="font-family:'Comic Sans MS';font-size:18px;">                              <<<a target=_blank href="http://blog.mcuol.com/User/farsight2009/Article/79339_1.htm" target="_blank">在Android中使用WindowManager实现悬浮窗口</a>>></span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;">      关于WindowManager.LayoutParams类说明请看如下博客： </span></span></div><div sizcache="28" sizset="152"><span class="link_title" sizcache="28" sizset="152"><span sizcache="28" sizset="152" style="font-family:'Comic Sans MS';font-size:18px;">                              << <a target=_blank href="http://blog.sina.com.cn/s/blog_4b3c1f950100qd9s.html">android学习---- WindowManager.LayoutParams</a>>></span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;"></span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;">       下面，我们从获得WindowManager对象引用开始，一步步观察addView()做了一些什么事情。</span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;"></span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;"></span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;"></span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;">   Step 1 、获得WindowManager对象服务 ，具体实现类在ContextImpl.java内中</span></span></div><div><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;">       <span style="color:#330099;"> </span></span><span style="font-family:'Comic Sans MS';"><span style="color:#330099;"><span style="font-size:14px;">   </span><span style="font-size:14px;">路径： /frameworks/base/core/java/android/app/ContextImpl.java </span></span><span style="font-size:14px;"><span style="color:#330099;">     </span><span style="color:#999900;">  </span></span><span style="font-size:14px;color:#ff9966;"> </span></span></span></div><div sizcache="28" sizset="153"><span class="link_title"><span style="font-family:'Comic Sans MS';font-size:18px;"></span></span><div class="dp-highlighter bg_java" sizcache="28" sizset="153"><div class="bar" sizcache="28" sizset="153"><div class="tools" sizcache="28" sizset="153"><strong>[java]</strong> <a target=_blank title="view plain" class="ViewSource" href="http://blog.youkuaiyun.com/qinjuning/article/details/8074262#">view plain</a><a target=_blank title="copy" class="CopyToClipboard" href="http://blog.youkuaiyun.com/qinjuning/article/details/8074262#">copy</a><a target=_blank title="print" class="PrintSource" href="http://blog.youkuaiyun.com/qinjuning/article/details/8074262#">print</a><a target=_blank title="?" class="About" href="http://blog.youkuaiyun.com/qinjuning/article/details/8074262#">?</a></div></div><ol class="dp-j"><li class="alt"><span><span class="annotation">@Override</span><span>  </span></span></li><li><span></span><span class="keyword">public</span><span> Object getSystemService(String name) {  </span></li><li class="alt"><span>    </span><span class="keyword">if</span><span> (WINDOW_SERVICE.equals(name)) {  </span></li><li><span>        </span><span class="keyword">return</span><span> WindowManagerImpl.getDefault();  </span></li><li class="alt"><span>    }  </span></li><li><span>    ...  </span></li><li class="alt"><span>}  </span></li></ol></div><pre class="java" style="DISPLAY: none" name="code">   @Override
   public Object getSystemService(String name) {
       if (WINDOW_SERVICE.equals(name)) {
           return WindowManagerImpl.getDefault();
       }
       ...
   }

WindowManager是个接口，具体返回对象则是WindowManagerImpl的单例对象。

Step 2 、获得WindowManagerImpl的单例对象，以及部分源码分析

路径： /frameworks/base/core/java/android/view/WindowManagerImpl.java

[java] view plain copy print ?

public class WindowManagerImpl implements WindowManager{
public static WindowManagerImpl getDefault()
{
return mWindowManager;
}
//以特定Window属性添加一个窗口
public void addView(View view, ViewGroup.LayoutParams params)
{
addView(view, params, false);
}
//参数nest表示该窗口是不是一个字窗口
private void addView(View view, ViewGroup.LayoutParams params, boolean nest)
{ ...
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
ViewRoot root;
View panelParentView = null; //该子窗口对应地父窗口View
synchronized (this) {
...//需要对传递过来地参数进行检测...
//对每个窗口皆构建一个ViewRoot对象
root = new ViewRoot(view.getContext());
root.mAddNesting = 1;
//设置root View 的LayoutParams为wparams，即WindowManager.LayoutParams类型
view.setLayoutParams(wparams);
...//对参数检测，以及拷贝原有数组...
//将窗口对应地view、root、wparams保存在属性集合中
mViews[index] = view;
mRoots[index] = root;
mParams[index] = wparams;
}
// do this last because it fires off messages to start doing things
// 调用ViewRoot对象去通知系统添加一个窗口
root.setView(view, wparams, panelParentView);
}
...
//这三个数组分别保存了一个窗口对应地属性
private View[] mViews; //root View对象， View类型
private ViewRoot[] mRoots; //ViewRoot类型，与WMS通信
private WindowManager.LayoutParams[] mParams; //窗口属性
//WindowManagerImpl实现了单例模式
private static WindowManagerImpl mWindowManager = new WindowManagerImpl();
}

public class WindowManagerImpl implements WindowManager{
	   
   public static WindowManagerImpl getDefault()
   {
       return mWindowManager;
   }
   //以特定Window属性添加一个窗口
   public void addView(View view, ViewGroup.LayoutParams params)
   {
       addView(view, params, false);
   }
   //参数nest表示该窗口是不是一个字窗口
   private void addView(View view, ViewGroup.LayoutParams params, boolean nest)
   {   ...
       final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
       
       ViewRoot root;
       View panelParentView = null;    //该子窗口对应地父窗口View
       
       synchronized (this) {
          
    	   ...//需要对传递过来地参数进行检测...
    	   
           //对每个窗口皆构建一个ViewRoot对象
           root = new ViewRoot(view.getContext());
           root.mAddNesting = 1;
           //设置root View 的LayoutParams为wparams，即WindowManager.LayoutParams类型
           view.setLayoutParams(wparams);
           ...//对参数检测，以及拷贝原有数组...
           
           //将窗口对应地view、root、wparams保存在属性集合中
           mViews[index] = view;
           mRoots[index] = root;
           mParams[index] = wparams;
       }
       // do this last because it fires off messages to start doing things
       // 调用ViewRoot对象去通知系统添加一个窗口
       root.setView(view, wparams, panelParentView);
   }
   ...
   //这三个数组分别保存了一个窗口对应地属性
   private View[] mViews;         //root View对象 ， View类型
   private ViewRoot[] mRoots;     //ViewRoot类型 ， 与WMS通信
   private WindowManager.LayoutParams[] mParams;  //窗口属性
   
   //WindowManagerImpl实现了单例模式
   private static WindowManagerImpl mWindowManager = new WindowManagerImpl();
}

WindowManagerImpl类的三个数组集合保存了每个窗口相关属性，这样我们可以通过这些属性去操作特定的

窗口(例如，可以根据View去更新/销毁该窗口)。当参数检查成功时，构建一个ViewRoot对象，并且设置设置root

View 的LayoutParams为wparams，即WindowManager.LayoutParams类型。最后调用root.setView()方法去通知

系统需要创建该窗口。我们接下来往下看看ViewRoot类相关操作。

Step 3、

[java] view plain copy print ?

public final class ViewRoot extends Handler implements ViewParent,View.AttachInfo.Callbacks {
View mView; //所有窗口地root View
final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();
...
/**
* We have one child
*/
public void setView(View view, WindowManager.LayoutParams attrs,
View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
mWindowAttributes.copyFrom(attrs); //保存WindowManager.LayoutParams属性值
attrs = mWindowAttributes;
...
mAdded = true;
int res; /* = WindowManagerImpl.ADD_OKAY; */
// Schedule the first layout -before- adding to the window
// manager, to make sure we do the relayout before receiving
// any other events from the system.
requestLayout(); //请求UI开始绘制。
mInputChannel = new InputChannel(); //创建一个InputChannel对象，接受消息
try {
//通知WindowManagerService添加一个窗口
res = sWindowSession.add(mWindow, mWindowAttributes,
getHostVisibility(), mAttachInfo.mContentInsets,
mInputChannel);
}
...
view.assignParent(this); //将root View的父View设置为该ViewRoot对象(实现了ViewParent接口)
...
}
}
}
}

public final class ViewRoot extends Handler implements ViewParent,View.AttachInfo.Callbacks {
	   
	View mView;	  //所有窗口地root View   
	final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();  

	...
	 /**
     * We have one child
     */
    public void setView(View view, WindowManager.LayoutParams attrs,
            View panelParentView) {
        synchronized (this) {
            if (mView == null) {
                mView = view;
                mWindowAttributes.copyFrom(attrs); //保存WindowManager.LayoutParams属性值
                attrs = mWindowAttributes;
                ...
                
                mAdded = true;
                int res; /* = WindowManagerImpl.ADD_OKAY; */

                // Schedule the first layout -before- adding to the window
                // manager, to make sure we do the relayout before receiving
                // any other events from the system.
                requestLayout();   //请求UI开始绘制。
                mInputChannel = new InputChannel();  //创建一个InputChannel对象，接受消息
                try {
                    //通知WindowManagerService添加一个窗口
                	res = sWindowSession.add(mWindow, mWindowAttributes,
                            getHostVisibility(), mAttachInfo.mContentInsets,
                            mInputChannel);
                } 
                ...
                view.assignParent(this);  //将root View的父View设置为该ViewRoot对象(实现了ViewParent接口)
                ...
            }
        }
    }
}

说明：ViewRoot类继承了Handler，实现了ViewParent接口

setView()方法地主要功能如下：
1、保存相关属性值，例如：mView、mWindowAttributes等；
2、调用requestLayout()方法请求UI绘制，由于ViewRoot是个Handler对象，异步请求；
3、通知WindowManagerService添加一个窗口；
4、注册一个事件监听管道，用来监听：按键(KeyEvent)和触摸(MotionEvent)事件。

我们这儿重点关注 requestLayout()方法请求UI绘制地流程。

Step 4、异步调用请求UI绘制

[java] view plain copy print ?

/**
* {@inheritDoc}
*/
public void requestLayout() {
checkThread(); //检查是不是UI线程调用，如果不是UI线程，会报异常
mLayoutRequested = true; //置为真，表示需要进行measure和layout过程
scheduleTraversals();
}
//开始UI绘制流程
public void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true; //防止多次调用
sendEmptyMessage(DO_TRAVERSAL); //异步请求UI绘制
}
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case DO_TRAVERSAL:
performTraversals(); //开始UI绘制
break;
}
}

   /**
    * {@inheritDoc}
    */
   public void requestLayout() {
       checkThread();        //检查是不是UI线程调用，如果不是UI线程，会报异常
       mLayoutRequested = true;   //置为真，表示需要进行measure和layout过程
       scheduleTraversals();  
   }
   //开始UI绘制流程
   public void scheduleTraversals() {
       if (!mTraversalScheduled) {
           mTraversalScheduled = true;       //防止多次调用
           sendEmptyMessage(DO_TRAVERSAL);   //异步请求UI绘制
       }
   }
   @Override
   public void handleMessage(Message msg) {
	   switch (msg.what) {
           case DO_TRAVERSAL:
                performTraversals();  //开始UI绘制
                break;
	   }
   }

由于performTraversals()方法比较复杂，我们侧重于第一次设置root View的widhtSpecSize以及

heightSpecSize值。

[java] view plain copy print ?

private void performTraversals() {
// cache mView since it is used so much below...
final View host = mView;
mTraversalScheduled = false;
boolean surfaceChanged = false;
WindowManager.LayoutParams lp = mWindowAttributes;
int desiredWindowWidth; //表示该窗口期望width值
int desiredWindowHeight; //表示该窗口期望width值
int childWidthMeasureSpec; //保存root View的widthMeasureSpec
int childHeightMeasureSpec; //保存root View的heightMeasureSpec
final View.AttachInfo attachInfo = mAttachInfo;
final int viewVisibility = getHostVisibility();
boolean viewVisibilityChanged = mViewVisibility != viewVisibility
|| mNewSurfaceNeeded;
float appScale = mAttachInfo.mApplicationScale;
WindowManager.LayoutParams params = null;
if (mWindowAttributesChanged) {
mWindowAttributesChanged = false;
surfaceChanged = true;
params = lp;
}
Rect frame = mWinFrame;
if (mFirst) { //mFirst表示是否是第一次绘制该Window
fullRedrawNeeded = true;
mLayoutRequested = true;
DisplayMetrics packageMetrics =
mView.getContext().getResources().getDisplayMetrics();
//第一次绘制时desiredWindowWidth，desiredWindowHeight 值大小为屏幕大小
desiredWindowWidth = packageMetrics.widthPixels;
desiredWindowHeight = packageMetrics.heightPixels;
...
} else { //不是第一次绘制，则desiredWindowWidth值为frame保存大小，frame值会由WMS填充
desiredWindowWidth = frame.width();
desiredWindowHeight = frame.height();
...
}
...
boolean insetsChanged = false;
if (mLayoutRequested) {
...//获得root View的widthMeasureSpec 和 heightMeasureSpec值
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
//开始measure过程
host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
...
final boolean didLayout = mLayoutRequested;
boolean triggerGlobalLayoutListener = didLayout
|| attachInfo.mRecomputeGlobalAttributes;
if (didLayout) {
... //layout过程
host.layout(0, 0, host.mMeasuredWidth, host.mMeasuredHeight);
...
}
...
if (!cancelDraw && !newSurface) {
mFullRedrawNeeded = false;
draw(fullRedrawNeeded);
...
}

   private void performTraversals() {
       // cache mView since it is used so much below...
       final View host = mView;

       mTraversalScheduled = false;         
       boolean surfaceChanged = false;
       WindowManager.LayoutParams lp = mWindowAttributes;  

       int desiredWindowWidth;              //表示该窗口期望width值
       int desiredWindowHeight;             //表示该窗口期望width值
       int childWidthMeasureSpec;           //保存root View的widthMeasureSpec
       int childHeightMeasureSpec;          //保存root View的heightMeasureSpec

       final View.AttachInfo attachInfo = mAttachInfo;

       final int viewVisibility = getHostVisibility();
       boolean viewVisibilityChanged = mViewVisibility != viewVisibility
               || mNewSurfaceNeeded;

       float appScale = mAttachInfo.mApplicationScale;

       WindowManager.LayoutParams params = null;
       if (mWindowAttributesChanged) {
           mWindowAttributesChanged = false;
           surfaceChanged = true;
           params = lp;
       }
       Rect frame = mWinFrame;
       if (mFirst) {   //mFirst表示是否是第一次绘制该Window
           fullRedrawNeeded = true;
           mLayoutRequested = true;

           DisplayMetrics packageMetrics =
               mView.getContext().getResources().getDisplayMetrics();
           //第一次绘制时desiredWindowWidth，desiredWindowHeight 值大小为屏幕大小
           desiredWindowWidth = packageMetrics.widthPixels;
           desiredWindowHeight = packageMetrics.heightPixels;
           ...
       } else {   //不是第一次绘制，则desiredWindowWidth值为frame保存大小，frame值会由WMS填充
           desiredWindowWidth = frame.width();
           desiredWindowHeight = frame.height();
           ...
       }
       ...
       boolean insetsChanged = false;

       if (mLayoutRequested) {
           ...//获得root View的widthMeasureSpec 和 heightMeasureSpec值
           childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
           childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
           //开始measure过程
           host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
       }
       ...
       final boolean didLayout = mLayoutRequested;
       
       boolean triggerGlobalLayoutListener = didLayout
               || attachInfo.mRecomputeGlobalAttributes;
       if (didLayout) {
           ... //layout过程
    	   host.layout(0, 0, host.mMeasuredWidth, host.mMeasuredHeight);
           ...
       }
       ...
       if (!cancelDraw && !newSurface) {
           mFullRedrawNeeded = false;
           draw(fullRedrawNeeded);
           ...
   }

[java] view plain copy print ?

/**
* @param windowSize The available width or height of the window
*
* @param rootDimension The layout params for one dimension (width or height) of the window.
*/
private int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}

  /**
    * @param windowSize  The available width or height of the window
    *
    * @param rootDimension The layout params for one dimension (width or height) of the window.
   */
   private int getRootMeasureSpec(int windowSize, int rootDimension) {
       int measureSpec;
       switch (rootDimension) {
       case ViewGroup.LayoutParams.MATCH_PARENT:
           // Window can't resize. Force root view to be windowSize.
           measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
           break;
       case ViewGroup.LayoutParams.WRAP_CONTENT:
           // Window can resize. Set max size for root view.
           measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
           break;
       default:
           // Window wants to be an exact size. Force root view to be that size.
           measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
           break;
       }
       return measureSpec;
   }

调用root View的measure()方法时，其参数是由getRootMeasureSpec()设置的，基本思路同我们前面描述的

差不多。贴出来的代码只是简简单单列出了measure 、layout 、 draw 过程的调用点，里面有很多逻辑处理，

阅读起来比较费劲，我也只能算是个囫囵吞枣水平。大家有兴趣地可以看看源码，加深理解。

最后，由于小子理解水平有限，可能很多地方让大家“丈二和尚--摸不着头脑”，给大家两个小建议吧：

1、仔细钻研源码；

2、想认真系统性研读UI绘制原理的话，建议详细阅读<<Android内核剖析>>第十三章 <UI绘制原理>