Android View 绘制流程 与invalidate 和postInvalidate 分析－－从源码角度

    整个View树的绘制流程是在ViewRootImpl.java类的performTraversals()函数展开的，该函数做的执行过程可简单概况为

 根据之前设置的状态，判断是否需要重新计算视图大小(measure)、是否重新需要布局视图的位置(layout)、以及是否需要重绘

 (draw)，所以整个View的绘制过程，总结为三步：

1、  Measure：测量View大小

2 、 Layout：对View进行布局

3、  Draw：绘制View：View背景、View内容、View边线、绘制子View(如果有)；

 一、performTraversals() 函数触发时机

  在这里提问下，为什么View的绘制是从performTraversals() 函数开始，这个函数是在什么时候触发的呢？

  来通过源码来分析下到底是什么原因，本文的源码是基于最新的源码6.0.1

  首先我们知道是通过 PhoneWindow的 setContentView 将View 加进来的，分析其源码如下：

@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
    // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
    // decor, when theme attributes and the like are crystalized. Do not check the feature
    // before this happens.
    if (mContentParent == null) {
        installDecor();
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        mContentParent.removeAllViews();
    }

    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        view.setLayoutParams(params);
        final Scene newScene = new Scene(mContentParent, view);
        transitionTo(newScene);
    } else {

        mContentParent.addView(view, params);
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
}

然后调用到ViewGroup 的addView 方法：

/**
 * Adds a child view with the specified layout parameters.
 *
 * <p><strong>Note:</strong> do not invoke this method from
 * {@link #draw(android.graphics.Canvas)}, {@link #onDraw(android.graphics.Canvas)},
 * {@link #dispatchDraw(android.graphics.Canvas)} or any related method.</p>
 *
 * @param child the child view to add
 * @param params the layout parameters to set on the child
 */
public void addView(View child, LayoutParams params) {
    addView(child, -1, params);
}

/**
 * Adds a child view with the specified layout parameters.
 *
 * <p><strong>Note:</strong> do not invoke this method from
 * {@link #draw(android.graphics.Canvas)}, {@link #onDraw(android.graphics.Canvas)},
 * {@link #dispatchDraw(android.graphics.Canvas)} or any related method.</p>
 *
 * @param child the child view to add
 * @param index the position at which to add the child or -1 to add last
 * @param params the layout parameters to set on the child
 */
public void addView(View child, int index, LayoutParams params) {
    if (DBG) {
        System.out.println(this + " addView");
    }

    if (child == null) {
        throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
    }

    // addViewInner() will call child.requestLayout() when setting the new LayoutParams
    // therefore, we call requestLayout() on ourselves before, so that the child's request
    // will be blocked at our level
    requestLayout();
    invalidate(true);
    addViewInner(child, index, params, false);
}

  看见addView 的方法中调运invalidate方法，这不就真相大白了。

   当我们写一个Activity时，我们一定会通过setContentView方法将我们要展示的界面传入该方法，该方法会讲通过addView追加到id为content的一个FrameLayout（ViewGroup）中，然后addView方法中通过调运invalidate(true)去通知触发ViewRootImpl类的performTraversals()方法，至此递归绘制我们自定义的所有布局。

最终会调用到ViewRootImpl的invalidate方法，从而调用 scheduleTraversals

void invalidate() {
    mDirty.set(0, 0, mWidth, mHeight);
    if (!mWillDrawSoon) {
        scheduleTraversals();
    }
}

  

 进入到scheduleTraversals 放法中：

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

  来看看  mTraversalRunnable实现了什么，其代码如下：

final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

        if (mProfile) {
            Debug.startMethodTracing("ViewAncestor");
        }

        performTraversals();

        if (mProfile) {
            Debug.stopMethodTracing();
            mProfile = false;
        }
    }
}

   从上面代码可以看出 ， scheduleTraversals 通过Handler的Runnable发送一个异步消息，然后调用doTraversal方法，然后最终调用performTraversals()执行重绘。文章开头背景知识介绍说过的，performTraversals就是整个View树开始绘制的调用入口，所以说View调运invalidate方法的实质是层层上传到父级，直到传递到ViewRootImpl后触发了scheduleTraversals方法，从而执行performTraversals 进行重绘；

至此，对于为什么View的绘制入口是在performTraversals 本文分析完成；

二、  View绘制三大过程的具体分析

接下来，来具体分析下View绘制的三大过程，即performTraversals 做了具体什么事情，其源码如下：

private void performTravelsals(){
....
 int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
 int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height)
 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

...
 int desiredWindowWidth;
 int desiredWindowHeight;
 performLayout(lp, desiredWindowWidth, desiredWindowHeight);

...

 performDraw();



}

从上述源码分析，也验证来了文章开头分析的View 绘制过程的三大步的正确性；

2.1  Measure 过程分析

先来看看performMeasure  做了什么事情，首先看代码中，该函数会去调用 View .measure 函数；

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

measure 具体实现如下:

/**
 * <p>
 * This is called to find out how big a view should be. The parent
 * supplies constraint information in the width and height parameters.
 * </p>
 *
 * <p>
 * The actual measurement work of a view is performed in
 * {@link #onMeasure(int, int)}, called by this method. Therefore, only
 * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
 * </p>
 *
 *
 * @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) {
    boolean optical = isLayoutModeOptical(this);
    if (optical != isLayoutModeOptical(mParent)) {
        Insets insets = getOpticalInsets();
        int oWidth  = insets.left + insets.right;
        int oHeight = insets.top  + insets.bottom;
        widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
        heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
    }

    // Suppress sign extension for the low bytes
    long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
    if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

    if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
            widthMeasureSpec != mOldWidthMeasureSpec ||
            heightMeasureSpec != mOldHeightMeasureSpec) {

        // first clears the measured dimension flag
        mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

        resolveRtlPropertiesIfNeeded();

        int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                mMeasureCache.indexOfKey(key);
        if (cacheIndex < 0 || sIgnoreMeasureCache) {
            // measure ourselves, this should set the measured dimension flag back
            onMeasure(widthMeasureSpec, heightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        } else {
            long value = mMeasureCache.valueAt(cacheIndex);
            // Casting a long to int drops the high 32 bits, no mask needed
            setMeasuredDimensionRaw((int) (value >> 32), (int) value);
            mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        // flag not set, setMeasuredDimension() was not invoked, we raise
        // an exception to warn the developer
        if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
            throw new IllegalStateException("View with id " + getId() + ": "
                    + getClass().getName() + "#onMeasure() did not set the"
                    + " measured dimension by calling"
                    + " setMeasuredDimension()");
        }

        mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
    }

    mOldWidthMeasureSpec = widthMeasureSpec;
    mOldHeightMeasureSpec = heightMeasureSpec;

    mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
            (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}

    从注释信息可以得出很多重要的信息，告诉Measure方法为整个View树计算实际的大小，然后设置实际的高和宽，每个View控件的实际宽高都是由父视图和自身决定的，实际的测量是在onMeasure方法进行，所以在View的子类需要重写onMeasure方法，这是因为measure方法是final的，不允许重载，所以View子类只能通过重载onMeasure来实现自己的测量逻辑。

   measure的两个参数都是父View传递过来的，也就是代表了父view的规格，它由两部分组成，高16位表示MODE，定义在MeasureSpec类（View的内部类）中，有三种类型，MeasureSpec.EXACTLY表示确定大小， MeasureSpec.AT_MOST表示最大大小， MeasureSpec.UNSPECIFIED不确定。低16位表示size，也就是父View的大小。对于系统Window类的DecorVIew对象Mode一般都为MeasureSpec.EXACTLY ，而size分别对应屏幕宽高，对于子View来说大小是由父View和子View共同决定的。

从代码可以看出measure方法最终回调了View的onMeasure方法，我们来看下View的onMeasure源码，如下：

/**
 * <p>
 * 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 overridden by subclasses to provide accurate and efficient
 * measurement of their contents.
 * </p>
 *
 * <p>
 * <strong>CONTRACT:</strong> When overriding this method, you
 * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
 * measured width and height of this view. Failure to do so will trigger an
 * <code>IllegalStateException</code>, thrown by
 * {@link #measure(int, int)}. Calling the superclass'
 * {@link #onMeasure(int, int)} is a valid use.
 * </p>
 *
 * <p>
 * The base class implementation of measure defaults to the background size,
 * unless a larger size is allowed by the MeasureSpec. Subclasses should
 * override {@link #onMeasure(int, int)} to provide better measurements of
 * their content.
 * </p>
 *
 * <p>
 * If this method is overridden, it is the subclass's responsibility to make
 * sure the measured height and width are at least the view's minimum height
 * and width ({@link #getSuggestedMinimumHeight()} and
 * {@link #getSuggestedMinimumWidth()}).
 * </p>
 *
 * @param widthMeasureSpec horizontal space requirements as imposed by the parent.
 *                         The requirements are encoded with
 *                         {@link android.view.View.MeasureSpec}.
 * @param heightMeasureSpec vertical space requirements as imposed by the parent.
 *                         The requirements are encoded with
 *                         {@link android.view.View.MeasureSpec}.
 *
 * @see #getMeasuredWidth()
 * @see #getMeasuredHeight()
 * @see #setMeasuredDimension(int, int)
 * @see #getSuggestedMinimumHeight()
 * @see #getSuggestedMinimumWidth()
 * @see android.view.View.MeasureSpec#getMode(int)
 * @see android.view.View.MeasureSpec#getSize(int)
 */
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

  可以看见onMeasure默认的实现仅仅调用了setMeasuredDimension，setMeasuredDimension函数是一个很关键的函数，它对View的成员变量mMeasuredWidth和mMeasuredHeight变量赋值，measure的主要目的就是对View树中的每个View的mMeasuredWidth和mMeasuredHeight进行赋值，所以一旦这两个变量被赋值意味着该View的测量工作结束。

     View的默认大小，从代码可以分析得出：setMeasuredDimension传入的参数都是通过getDefaultSize返回的，所以再来看下getDefaultSize方法源码，如下：

/**
 * Utility to return a default size. Uses the supplied size if the
 * MeasureSpec imposed no constraints. 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.
 */
public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);

    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result;
}

   从上面得出：如果specMode等于AT_MOST或EXACTLY就返回specSize，这就是系统默认的规格；

   继续看上面onMeasure方法，其中getDefaultSize参数的widthMeasureSpec和heightMeasureSpec都是由父View传递进来的；

   getSuggestedMinimumWidth与getSuggestedMinimumHeight都是View的方法，具体如下：

/**
 * Returns the suggested minimum width that the view should use. This
 * returns the maximum of the view's minimum width)
 * and the background's minimum width
 *  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
 * <p>
 * When being used in {@link #onMeasure(int, int)}, the caller should still
 * ensure the returned width is within the requirements of the parent.
 *
 * @return The suggested minimum width of the view.
 */
protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

/**
 * Returns the suggested minimum height that the view should use. This
 * returns the maximum of the view's minimum height
 * and the background's minimum height
 * ({@link android.graphics.drawable.Drawable#getMinimumHeight()}).
 * <p>
 * When being used in {@link #onMeasure(int, int)}, the caller should still
 * ensure the returned height is within the requirements of the parent.
 *
 * @return The suggested minimum height of the view.
 */
protected int getSuggestedMinimumHeight() {
    return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

}

 通过上面代码得出：建议的最小宽度和高度都是由View的Background尺寸与通过设置View的miniXXX属性共同决定的。

   至此，最基础的元素View的measure过程就完成了。

   但是View实际是嵌套的，所以measure是递归传递的，从而每个View都需要measure。实际能够嵌套的View一般都是ViewGroup的子类，所以在ViewGroup中定义了measureChildren, measureChild, measureChildWithMargins方法来对子视图进行测量，measureChildren内部实质只是循环调用measureChild，measureChild和measureChildWithMargins的区别就是是否把margin和padding也作为子视图的大小。

  以ViewGroup中稍微复杂的measureChildWithMargins方法来分析：

/**
 * 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.
 *
 * @param child The child to measure
 * @param parentWidthMeasureSpec The width requirements for this view
 * @param widthUsed Extra space that has been used up by the parent
 *        horizontally (possibly by other children of the parent)
 * @param parentHeightMeasureSpec The height requirements for this view
 * @param heightUsed Extra space that has been used up by the parent
 *        vertically (possibly by other children of the parent)
 */
protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

    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);
}

/**
 * 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. For example,
 * if the this view knows its size (because its MeasureSpec has a mode of
 * EXACTLY), and the child has indicated in its LayoutParams that it wants
 * to be the same size as the parent, the parent should ask the child to
 * layout given an exact size.
 *
 * @param spec The requirements for this view
 * @param padding The padding of this view for the current dimension and
 *        margins, if applicable
 * @param childDimension How big the child wants to be in the current
 *        dimension
 * @return a MeasureSpec integer for the child
 */
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);

    int size = Math.max(0, specSize - padding);

    int resultSize = 0;
    int resultMode = 0;

    switch (specMode) {
    // Parent has imposed an exact size on us
    case MeasureSpec.EXACTLY:
        if (childDimension >= 0) {
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size. So be it.
            resultSize = size;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent has imposed a maximum size on us
    case MeasureSpec.AT_MOST:
        if (childDimension >= 0) {
            // Child wants a specific size... so be it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } 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;
            resultMode = MeasureSpec.AT_MOST;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent asked to see how big we want to be
    case MeasureSpec.UNSPECIFIED:
        if (childDimension >= 0) {
            // Child wants a specific size... let him have it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size... find out how big it should
            // be
            resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        }
        break;
    }
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

    从上面分析得出，getChildMeasureSpec的逻辑是通过其父View提供的MeasureSpec参数得到specMode和specSize，然后根据计算出来的specMode以及子View的childDimension（layout_width或layout_height）来计算自身的measureSpec，如果其本身包含子视图，则计算出来的measureSpec将作为调用其子视图measure函数的参数，同时也作为自身调用setMeasuredDimension的参数，如果其不包含子视图则默认情况下最终会调用onMeasure的默认实现，并最终调用到setMeasuredDimension。

    所以可以看见onMeasure的参数其实就是这么计算出来的。同时从上面的分析可以看出来，最终决定View的measure大小是View的setMeasuredDimension方法，所以我们可以通过setMeasuredDimension设定具体值来设置View的mMeasuredWidth和mMeasuredHeight的大小，但是一个好的自定义View应该要根据子视图的measureSpec来设置mMeasuredWidth和mMeasuredHeight的大小，这样的灵活性更大；

    当通过setMeasuredDimension方法最终设置完成View的measure之后View的mMeasuredWidth和mMeasuredHeight成员才会有具体的数值，所以如果我们自定义的View或者使用现成的View想通过getMeasuredWidth()和getMeasuredHeight()方法来获取View测量的宽高，必须保证这两个方法在onMeasure流程之后被调用才能返回有效值。

至此，View的Measure 过程分析完成，是不是有点小复杂，来总结一下Measure的具体实现原理：

    通过上面分析可以看出measure过程主要就是从顶层父View向子View递归调用view.measure方法（measure中又回调onMeasure方法）的过程。具体measure核心主要有如下几点：

• MeasureSpec（View的内部类）测量规格为int型，值由高16位规格模式specMode和低16位具体尺寸specSize组成。其中specMode只有三种值：
• MeasureSpec.EXACTLY //确定模式，父View希望子View的大小是确定的，由specSize决定；
  MeasureSpec.AT_MOST //最多模式，父View希望子View的大小最多是specSize指定的值；
  MeasureSpec.UNSPECIFIED //未指定模式，父View完全依据子View的设计值来决定； 
• /**

•  * Measure specification mode: The parent has not imposed any constraint
   * on the child. It can be whatever size it wants.
   */
  public static final int UNSPECIFIED = 0 << MODE_SHIFT;
  
  /**
   * Measure specification mode: The parent has determined an exact size
   * for the child. The child is going to be given those bounds regardless
   * of how big it wants to be.
   */
  public static final int EXACTLY     = 1 << MODE_SHIFT;
  
  /**
   * Measure specification mode: The child can be as large as it wants up
   * to the specified size.
   */
  public static final int AT_MOST     = 2 << MODE_SHIFT;

• View的measure方法是final的，不允许重载，View子类只能重载onMeasure来完成自己的测量逻辑。

• 最顶层DecorView测量时的MeasureSpec是由ViewRootImpl中getRootMeasureSpec方法确定的（LayoutParams宽高参数均为MATCH_PARENT，specMode是EXACTLY，specSize为物理屏幕大小）。

• ViewGroup类提供了measureChild，measureChild和measureChildWithMargins方法，简化了父子View的尺寸计算。

• 只要是ViewGroup的子类就必须要求LayoutParams继承子MarginLayoutParams，否则无法使用layout_margin参数。

• View的布局大小由父View和子View共同决定。

• 使用View的getMeasuredWidth()和getMeasuredHeight()方法来获取View测量的宽高，必须保证这两个方法在onMeasure流程之后被调用才能返回有效值。

2.2 Layout  过程分析

2.2.1 View Layout 相关方法

peromLayout 具体实现如下：

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    mLayoutRequested = false;
    mScrollMayChange = true;
    mInLayout = true;

    final View host = mView;
    if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
        Log.v(TAG, "Laying out " + host + " to (" +
                host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
    }

    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
    try {
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

        mInLayout = false;
        int numViewsRequestingLayout = mLayoutRequesters.size();
        if (numViewsRequestingLayout > 0) {
            // requestLayout() was called during layout.
            // If no layout-request flags are set on the requesting views, there is no problem.
            // If some requests are still pending, then we need to clear those flags and do
            // a full request/measure/layout pass to handle this situation.
            ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
                    false);
            if (validLayoutRequesters != null) {
                // Set this flag to indicate that any further requests are happening during
                // the second pass, which may result in posting those requests to the next
                // frame instead
                mHandlingLayoutInLayoutRequest = true;

                // Process fresh layout requests, then measure and layout
                int numValidRequests = validLayoutRequesters.size();
                for (int i = 0; i < numValidRequests; ++i) {
                    final View view = validLayoutRequesters.get(i);
                    Log.w("View", "requestLayout() improperly called by " + view +
                            " during layout: running second layout pass");
                    view.requestLayout();
                }
                measureHierarchy(host, lp, mView.getContext().getResources(),
                        desiredWindowWidth, desiredWindowHeight);
                mInLayout = true;
                host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

                mHandlingLayoutInLayoutRequest = false;

                // Check the valid requests again, this time without checking/clearing the
                // layout flags, since requests happening during the second pass get noop'd
                validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
                if (validLayoutRequesters != null) {
                    final ArrayList<View> finalRequesters = validLayoutRequesters;
                    // Post second-pass requests to the next frame
                    getRunQueue().post(new Runnable() {
                        @Override
                        public void run() {
                            int numValidRequests = finalRequesters.size();
                            for (int i = 0; i < numValidRequests; ++i) {
                                final View view = finalRequesters.get(i);
                                Log.w("View", "requestLayout() improperly called by " + view +
                                        " during second layout pass: posting in next frame");
                                view.requestLayout();
                            }
                        }
                    });
                }
            }

        }
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}

 最终会调用到

  host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

     可以看见layout方法接收四个参数，这四个参数分别代表相对Parent的左、上、右、下坐标。而且还可以看见左上都为0，右下分别为上面 Measure 刚刚测量的width和height。

/**
 * Assign a size and position to a view and all of its
 * descendants
 *
 * <p>This is the second phase of the layout mechanism.
 * (The first is measuring). In this phase, each parent calls
 * layout on all of its children to position them.
 * This is typically done using the child measurements
 * that were stored in the measure pass().</p>
 *
 * <p>Derived classes should not override this method.
 * Derived classes with children should override
 * onLayout. In that method, they should
 * call layout on each of their children.</p>
 *
 * @param l Left position, relative to parent
 * @param t Top position, relative to parent
 * @param r Right position, relative to parent
 * @param b Bottom position, relative to parent
 */
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
    if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
        onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
        mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
    }

    int oldL = mLeft;
    int oldT = mTop;
    int oldB = mBottom;
    int oldR = mRight;

    boolean changed = isLayoutModeOptical(mParent) ?
            setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        onLayout(changed, l, t, r, b);
        mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnLayoutChangeListeners != null) {
            ArrayList<OnLayoutChangeListener> listenersCopy =
                    (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
            int numListeners = listenersCopy.size();
            for (int i = 0; i < numListeners; ++i) {
                listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
            }
        }
    }

    mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
    mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}

  

 从上面方法中调用

 onLayout(changed, l, t, r, b);

 

/**
 * Called from layout when this view should
 * assign a size and position to each of its children.
 *
 * Derived classes with children should override
 * this method and call layout on each of
 * their children.
 * @param changed This is a new size or position for this view
 * @param left Left position, relative to parent
 * @param top Top position, relative to parent
 * @param right Right position, relative to parent
 * @param bottom Bottom position, relative to parent
 */
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

这是个是空方法，没啥好看的；

2.2.2 ViewGroup layout 相关方法

接下来，来看看ViewGroup的相关方法

/**
 * {@inheritDoc}
 */
@Override
public final void layout(int l, int t, int r, int b) {
    if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
        if (mTransition != null) {
            mTransition.layoutChange(this);
        }
        super.layout(l, t, r, b);
    } else {
        // record the fact that we noop'd it; request layout when transition finishes
        mLayoutCalledWhileSuppressed = true;
    }
}

 从上面得出ViewGroup  的 layout 方法 最终还是调用到父类View 的layout ,然后类似measure 调用View onLayout方法

   对比上面View的layout和ViewGroup的layout方法可以发现，View的layout方法是可以在子类重写的，而ViewGroup的layout是不能在子类重写的，言外之意就是说ViewGroup中只能通过重写onLayout方法。那我们接下来看下ViewGroup的onLayout方法，如下

/**
 * {@inheritDoc}
 */
@Override
protected abstract void onLayout(boolean changed,
        int l, int t, int r, int b);

    ViewGroup的onLayout()方法竟然是一个抽象方法，这就是说所有ViewGroup的子类都必须重写这个方法。

     所以在自定义ViewGroup控件中，onLayout配合onMeasure方法一起使用可以实现自定义View的复杂布局。

     自定义View首先调用onMeasure进行测量，然后调用onLayout方法动态获取子View和View的测量大小，然后进行layout布局。重载onLayout的目的就是安排其children在父View的具体位置，重载onLayout通常做法就是写一个for循环调用每一个子视图的layout(l, t, r, b)函数，传入不同的参数l, t, r, b来确定每个子视图在父视图中的显示位置。

2.2.3  继承ViewGroup的其他Layout: LinearLayout   相关方法分析

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    if (mOrientation == VERTICAL) {
        layoutVertical(l, t, r, b);
    } else {
        layoutHorizontal(l, t, r, b);
    }
}

   如上代码所示，LinearLayout的layout过程是分Vertical和Horizontal的，这个就是xml布局的orientation属性设置的，ViewGroup的onLayout重写一般步骤类似，就只分析VERTICAL模式，如下是layoutVertical方法源码：

/**
 * Position the children during a layout pass if the orientation of this
 * LinearLayout is set to {@link #VERTICAL}.
 *
 * @see #getOrientation()
 * @see #setOrientation(int)
 * @see #onLayout(boolean, int, int, int, int)
 * @param left
 * @param top
 * @param right
 * @param bottom
 */
void layoutVertical(int left, int top, int right, int bottom) {
    final int paddingLeft = mPaddingLeft;

    int childTop;
    int childLeft;
    
    // Where right end of child should go
    final int width = right - left;
    int childRight = width - mPaddingRight;
    
    // Space available for child
    int childSpace = width - paddingLeft - mPaddingRight;
    
    final int count = getVirtualChildCount();

    final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
    final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

    switch (majorGravity) {
       case Gravity.BOTTOM:
           // mTotalLength contains the padding already
           childTop = mPaddingTop + bottom - top - mTotalLength;
           break;

           // mTotalLength contains the padding already
       case Gravity.CENTER_VERTICAL:
           childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
           break;

       case Gravity.TOP:
       default:
           childTop = mPaddingTop;
           break;
    }

    //重点，开始遍历
    for (int i = 0; i < count; i++) {
        final View child = getVirtualChildAt(i);
        if (child == null) {
            childTop += measureNullChild(i);
        } else if (child.getVisibility() != GONE) {

  //LinearLayout中其子视图显示的宽和高由measure过程来决定的
  // 因此measure过程的意义就是为layout过程提供视图显示范围的参考值
            final int childWidth = child.getMeasuredWidth();
            final int childHeight = child.getMeasuredHeight();
            
            final LinearLayout.LayoutParams lp =
                    (LinearLayout.LayoutParams) child.getLayoutParams();
            
            int gravity = lp.gravity;
            if (gravity < 0) {
                gravity = minorGravity;
            }
            final int layoutDirection = getLayoutDirection();
            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
            //依据不同的absoluteGravity计算childLeft位置
            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                case Gravity.CENTER_HORIZONTAL:
                    childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                            + lp.leftMargin - lp.rightMargin;
                    break;

                case Gravity.RIGHT:
                    childLeft = childRight - childWidth - lp.rightMargin;
                    break;

                case Gravity.LEFT:
                default:
                    childLeft = paddingLeft + lp.leftMargin;
                    break;
            }

            if (hasDividerBeforeChildAt(i)) {
                childTop += mDividerHeight;
            }

            childTop += lp.topMargin;
            //通过垂直排列计算调运child的layout设置child的位置
            setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                    childWidth, childHeight);
            childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

            i += getChildrenSkipCount(child, i);
        }
    }
}

    从上面分析的LinearLayout的onLayout实现代码可以看出，一般情况下layout过程会参考measure过程中计算得到的mMeasuredWidth和mMeasuredHeight来安排子View在父View中显示的位置，但这不是必须的，measure过程得到的结果可能完全没有实际用处，特别是对于一些自定义的ViewGroup，其子View的个数、位置和大小都是固定的，这时候我们可以忽略整个measure过程，只在layout函数中传入的4个参数来安排每个子View的具体位置。

   PS： 分析到这里，心中有个疑惑 ： getWidth()、getHeight()和getMeasuredWidth()、getMeasuredHeight()这两对方法之间的区别？

  getMeasuredWidth()、getMeasuredHeight()必须在onMeasure之后使用才有效，getWidth()与getHeight()方法必须在layout(int l, int t, int r, int b)执行之后才有效。那我们看下View源码中这些方法的实现吧，如下：

/**
 * Return the width of the your view.
 *
 * @return The width of your view, in pixels.
 */
@ViewDebug.ExportedProperty(category = "layout")
public final int getWidth() {
    return mRight - mLeft;
}

/**
 * Return the height of your view.
 *
 * @return The height of your view, in pixels.
 */
@ViewDebug.ExportedProperty(category = "layout")
public final int getHeight() {
    return mBottom - mTop;
}

/**
 * Return the visible drawing bounds of your view. Fills in the output
 * rectangle with the values from getScrollX(), getScrollY(),
 * getWidth(), and getHeight(). These bounds do not account for any
 * transformation properties currently set on the view, such as
 * {@link #setScaleX(float)} or {@link #setRotation(float)}.
 *
 * @param outRect The (scrolled) drawing bounds of the view.
 */
public void getDrawingRect(Rect outRect) {
    outRect.left = mScrollX;
    outRect.top = mScrollY;
    outRect.right = mScrollX + (mRight - mLeft);
    outRect.bottom = mScrollY + (mBottom - mTop);
}

/**
 * Like {@link #getMeasuredWidthAndState()}, but only returns the
 * raw width component (that is the result is masked by
 * {@link #MEASURED_SIZE_MASK}).
 *
 * @return The raw measured width of this view.
 */
public final int getMeasuredWidth() {
    return mMeasuredWidth & MEASURED_SIZE_MASK;
}


/**
 * Like {@link #getMeasuredHeightAndState()}, but only returns the
 * raw width component (that is the result is masked by
 * {@link #MEASURED_SIZE_MASK}).
 *
 * @return The raw measured height of this view.
 */
public final int getMeasuredHeight() {
    return mMeasuredHeight & MEASURED_SIZE_MASK;
}

至此，Layout 过程已经分析完毕啦，下面对Layout 进行一个小结

2.2.4 Layout 原理小结

    整个layout过程比较容易理解，从上面分析可以看出layout也是从顶层父View向子View的递归调用view.layout方法的过程，即父View根据上一步measure子View所得到的布局大小和布局参数，将子View放在合适的位置上。具体layout核心主要有以下几点：

• View.layout方法可被重载，ViewGroup.layout为final的不可重载，ViewGroup.onLayout为abstract的，子类必须重载实现自己的位置逻辑。

• measure操作完成后得到的是对每个View经测量过的measuredWidth和measuredHeight，layout操作完成之后得到的是对每个View进行位置分配后的mLeft、mTop、mRight、mBottom，这些值都是相对于父View来说的。

• 凡是layout_XXX的布局属性基本都针对的是包含子View的ViewGroup的，当对一个没有父容器的View设置相关layout_XXX属性是没有任何意义；

• 使用View的getWidth()和getHeight()方法来获取View测量的宽高，必须保证这两个方法在onLayout流程之后被调用才能返回有效值。

2.3 Draw 过程分析

      如文章开头中所分析的，draw过程也是在ViewRootImpl的performTraversals内部调用的，其调用顺序在measure()和layout()之后，这里的mView对于Actiity来说就是PhoneWindow.DecorView，ViewRootImpl中的代码会创建一个Canvas对象，然后调用View的draw()方法来执行具体的绘制工。

  

/**
 * Manually render this view (and all of its children) to the given Canvas.
 * The view must have already done a full layout before this function is
 * called.  When implementing a view, implement
 * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
 * If you do need to override this method, call the superclass version.
 *
 * @param canvas The Canvas to which the View is rendered.
 */
@CallSuper
public void draw(Canvas canvas) {
    final int privateFlags = mPrivateFlags;
    final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
            (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
    mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

    /*
     * Draw traversal performs several drawing steps which must be executed
     * in the appropriate order:
     *
     *      1. Draw the background
     *      2. If necessary, save the canvas' layers to prepare for fading
     *      3. Draw view's content
     *      4. Draw children
     *      5. If necessary, draw the fading edges and restore layers
     *      6. Draw decorations (scrollbars for instance)
     */

    // Step 1, draw the background, if needed
    int saveCount;

    if (!dirtyOpaque) {
        drawBackground(canvas);
    }

    // skip step 2 & 5 if possible (common case)
    final int viewFlags = mViewFlags;
    boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
    boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
    if (!verticalEdges && !horizontalEdges) {
        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);

        // we're done...
        return;
    }

    /*
     * Here we do the full fledged routine...
     * (this is an uncommon case where speed matters less,
     * this is why we repeat some of the tests that have been
     * done above)
     */

    boolean drawTop = false;
    boolean drawBottom = false;
    boolean drawLeft = false;
    boolean drawRight = false;

    float topFadeStrength = 0.0f;
    float bottomFadeStrength = 0.0f;
    float leftFadeStrength = 0.0f;
    float rightFadeStrength = 0.0f;

    // Step 2, save the canvas' layers
    int paddingLeft = mPaddingLeft;

    final boolean offsetRequired = isPaddingOffsetRequired();
    if (offsetRequired) {
        paddingLeft += getLeftPaddingOffset();
    }

    int left = mScrollX + paddingLeft;
    int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
    int top = mScrollY + getFadeTop(offsetRequired);
    int bottom = top + getFadeHeight(offsetRequired);

    if (offsetRequired) {
        right += getRightPaddingOffset();
        bottom += getBottomPaddingOffset();
    }

    final ScrollabilityCache scrollabilityCache = mScrollCache;
    final float fadeHeight = scrollabilityCache.fadingEdgeLength;
    int length = (int) fadeHeight;

    // clip the fade length if top and bottom fades overlap
    // overlapping fades produce odd-looking artifacts
    if (verticalEdges && (top + length > bottom - length)) {
        length = (bottom - top) / 2;
    }

    // also clip horizontal fades if necessary
    if (horizontalEdges && (left + length > right - length)) {
        length = (right - left) / 2;
    }

    if (verticalEdges) {
        topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
        drawTop = topFadeStrength * fadeHeight > 1.0f;
        bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
        drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
    }

    if (horizontalEdges) {
        leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
        drawLeft = leftFadeStrength * fadeHeight > 1.0f;
        rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
        drawRight = rightFadeStrength * fadeHeight > 1.0f;
    }

    saveCount = canvas.getSaveCount();

    int solidColor = getSolidColor();
    if (solidColor == 0) {
        final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

        if (drawTop) {
            canvas.saveLayer(left, top, right, top + length, null, flags);
        }

        if (drawBottom) {
            canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
        }

        if (drawLeft) {
            canvas.saveLayer(left, top, left + length, bottom, null, flags);
        }

        if (drawRight) {
            canvas.saveLayer(right - length, top, right, bottom, null, flags);
        }
    } else {
        scrollabilityCache.setFadeColor(solidColor);
    }

    // Step 3, draw the content
    if (!dirtyOpaque) onDraw(canvas);

    // Step 4, draw the children
    dispatchDraw(canvas);

    // Step 5, draw the fade effect and restore layers
    final Paint p = scrollabilityCache.paint;
    final Matrix matrix = scrollabilityCache.matrix;
    final Shader fade = scrollabilityCache.shader;

    if (drawTop) {
        matrix.setScale(1, fadeHeight * topFadeStrength);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, top, right, top + length, p);
    }

    if (drawBottom) {
        matrix.setScale(1, fadeHeight * bottomFadeStrength);
        matrix.postRotate(180);
        matrix.postTranslate(left, bottom);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, bottom - length, right, bottom, p);
    }

    if (drawLeft) {
        matrix.setScale(1, fadeHeight * leftFadeStrength);
        matrix.postRotate(-90);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, top, left + length, bottom, p);
    }

    if (drawRight) {
        matrix.setScale(1, fadeHeight * rightFadeStrength);
        matrix.postRotate(90);
        matrix.postTranslate(right, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(right - length, top, right, bottom, p);
    }

    canvas.restoreToCount(saveCount);

    // Overlay is part of the content and draws beneath Foreground
    if (mOverlay != null && !mOverlay.isEmpty()) {
        mOverlay.getOverlayView().dispatchDraw(canvas);
    }

    // Step 6, draw decorations (foreground, scrollbars)
    onDrawForeground(canvas);
}

 

从上面代码分析出结论， Draw的绘制工作有以下几步：

1、对View的背景进行绘制。

2、对View的内容进行绘制。

3、对当前View的所有子View进行绘制，如果当前的View没有子View就不需要进行绘制。

4、对View的滚动条进行绘制。

至此整个View的绘制过程分析完毕 ！