View的测量(一)

onMeasure:测量子view的宽高，设置自己的宽和高

wrap_content 会经过两次测量

View的测量

##ViewRootImpl
private static 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;
}

• EXACTYLY 模式: 父View已经测量出子Viwe所需要的精确大小，这时候View的最终大小 * 就是SpecSize所指定的值。对应于match_parent和精确数值这两种模式

• 子View的最终大小是父View指定的SpecSize值，并且子View的大小不能大于这个值， * 即对应wrap_content这种模式

• UNSPECIFIED 模式：
  父View不对子View有任何限制，子View需要多大就多大

performMeasure -> measure -> onMeasure

ViewGroup的measure过程
ViewGroup的measure比较复杂，要遍历所有子元素的measure过程。ViewGroup没有重写onMeasure方法，但是提供了三种测量子View的方法

然后测量自己 setMeasuredDimension()

/**
  *遍历ViewGroup中所有的子控件，调用measuireChild测量宽高
  */
 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) {
            //测量某一个子控件宽高
            measureChild(child, widthMeasureSpec, heightMeasureSpec);
        }
    }
}

/**
* 测量某一个child的宽高
*/
//ViewGroup中方法。
protected void measureChild (View child, int parentWidthMeasureSpec,
       int parentHeightMeasureSpec) {
   final LayoutParams lp = child.getLayoutParams();
   //获取子控件的宽高约束规则
   final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
           mPaddingLeft + mPaddingRight, lp. width);
   final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
           mPaddingTop + mPaddingBottom, lp. height);

   child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

/**
* 测量某一个child的宽高，考虑margin值
*/
protected void measureChildWithMargins (View child,
       int parentWidthMeasureSpec, int widthUsed,
       int parentHeightMeasureSpec, int heightUsed) {
   final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
   //获取子控件的宽高约束规则,相比于 measureChild方法，这里考虑了 lp.margin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);
   //测量子控件
   child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

子View的测量模式和大小

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;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

MeasureSpec.UNSPECIFIED 先别管,
父View是精确测量，子View是dp的，子View的测量模式（MeasureSpec.EXACTLY,dp）
父View是精确测量，子View是match_parent的,子View的测量模式（MeasureSpec.EXACTLY,控件大小父View 或者父View 剩余）
父View是精确测量，子View是warp_content,子View的测量模式（MeasureSpec.AT_MOST，不能超过父View给出大小）

父View是AT_MOST，子View是dp的，子View的测量模式（MeasureSpec.EXACTLY,dp）
父View是AT_MOST，子View是match_parent的,子View的测量模式（MeasureSpec.AT_MOST,不能超过父View给出大小）
父View是AT_MOST，子View是warp_content,子View的测量模式（MeasureSpec.AT_MOST，不能超过父View给出大小）

子View的测量模式，在默认情况下，AT_MOST和MATCH_PARENT都一样，父View给出的最大值。