上面就是FrameLayout的测量过程，结合注释能看得很明白。首先FrameLayout会测量所有的子view，如果FrameLayout的大小是确定的，那么一轮测量就可以确定所有子view的大小。如果FrameLayout的大小不确定，比如设置为wrap_content，那么此时那些宽或高参数为match_parent的子view是无法被确切测量大小的，因为此时parent的大小都还不知道呢，并且这些子view会存到mMatchParentChildren里。一轮测量下来，此时FrameLayout的宽和高分别都是测量过的子view的最大的宽和最大的高（最大的宽和最大的高不一定会出现在同一个子view上）。为防止出现极端情况，比如所有的子view宽高参数都是match_parent，那么此时测量出来的宽和高都是0（为什么是0后面会解释）。因此还需要对比一下最小的建议值以及前景的宽和高。

最后一轮是测量那些宽或高参数为match_parent的子view，此时FrameLayout的大小已经确定了，然后使用FrameLayout的SpecMode和第一轮刚测量出来的宽高重新构造子view的MeasureSpec，然后再重新测量。

至于父layout怎么测量子view的，其实从onMeasure的第二轮测量中就可以看到，首先父layout会依据自己的MeasureSpec和要测量的子view的LayoutParams.width、LayoutParams.height来生成子view的MeasureSpec，然后将这个MeasureSpec传给子view的measure方法，子view再根据自己的情况来测量自己大小。在父layout生成子view的MeasureSpec的过程中，主要是getChildMeasureSpec方法，浏览一下这个方法的源代码。

/** * 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 = 0; resultMode = MeasureSpec.UNSPECIFIED; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size.... find out how // big it should be resultSize = 0; resultMode = MeasureSpec.UNSPECIFIED; } break; } return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }

方法并不难，我们可以很清晰地看到父layout是如何生产子view的MeasureSpec的，各项的注释也写得很清楚，总结起来如下表：

这个很简单，可以看到父layout是如何根据自己的MeasureSpec和子view的LayoutParams来创建子view的MeasureSpec的。