首先先看下在一个Activity创建到DecorView的measure函数被调用过程的时序图：
可以看到这些主要在ViewRootImpl的performTraversals()函数中完成。如下是preformTraversals()函数，我们关注一下标红的这几行代码
private void performTraversals() {
// cache mView since it is used so much below...
final View host = mView;
if (DEBUG_LIFECYCLE || DEBUG_DEFAULT) {
Xlog.v(TAG, "ViewRoot performTraversals+ : mFirst = " + mFirst + ", this = " + this);
}
if (DBG) {
System.out.println("======================================");
System.out.println("performTraversals");
host.debug();
}
if (host == null || !mAdded)
return;
mIsInTraversal = true;
mWillDrawSoon = true;
boolean windowSizeMayChange = false;
boolean newSurface = false;
boolean surfaceChanged = false;
<span style="background-color: rgb(255, 0, 0);"> WindowManager.LayoutParams lp = mWindowAttributes;</span>
<span style="white-space:pre"> </span>.............
// Ask host how big it wants to be
<span style="background-color: rgb(255, 0, 0);"> windowSizeMayChange |= measureHierarchy(host, lp, res,
desiredWindowWidth, desiredWindowHeight);</span>
......................
}可以看到View的measure工作最后主要由measureHierarchy（）完成，并将Window的LayoutParameter传入到函数内：
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
int childWidthMeasureSpec;
int childHeightMeasureSpec;
boolean windowSizeMayChange = false;
if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
Xlog.v(TAG, "ViewRoot measure+ " + host + " in display " + desiredWindowWidth + "x"
+ desiredWindowHeight + ", lp = " + lp + ", this = " + this);
}
boolean goodMeasure = false;
if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) {
// On large screens, we don't want to allow dialogs to just
// stretch to fill the entire width of the screen to display
// one line of text. First try doing the layout at a smaller
// size to see if it will fit.
final DisplayMetrics packageMetrics = res.getDisplayMetrics();
res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);
int baseSize = 0;
if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
baseSize = (int)mTmpValue.getDimension(packageMetrics);
}
if (DEBUG_DIALOG) {
Xlog.v(TAG, "Window " + mView + ": baseSize=" + baseSize + ", this = " + this);
}
if (baseSize != 0 && desiredWindowWidth > baseSize) {
<span style="background-color: rgb(255, 0, 0);"> childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);</span>
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
if (DEBUG_DIALOG) {
Xlog.v(TAG, "Window " + mView + ": measured (" + host.getMeasuredWidth() + ","
+ host.getMeasuredHeight() + ") " + this);
}
if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
goodMeasure = true;
} else {
// Didn't fit in that size... try expanding a bit.
baseSize = (baseSize+desiredWindowWidth)/2;
if (DEBUG_DIALOG) {
Xlog.v(TAG, "Window " + mView + ": next baseSize=" + baseSize + " " + this);
}
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
<span style="background-color: rgb(255, 0, 0);"> performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);</span>
if (DEBUG_DIALOG) {
Xlog.v(TAG, "Window " + mView + ": measured (" + host.getMeasuredWidth()
+ "," + host.getMeasuredHeight() + ") " + this);
}
if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
if (DEBUG_DIALOG) {
Xlog.v(TAG, "Good!");
}
goodMeasure = true;
}
}
}
}
if (!goodMeasure) {
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
windowSizeMayChange = true;
}
}
if (DBG) {
System.out.println("======================================");
System.out.println("performTraversals -- after measure");
host.debug();
}
if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
Xlog.v(TAG, "ViewRoot measure-: host measured size = (" + host.getMeasuredWidth()
+ "x" + host.getMeasuredHeight() + "), windowSizeMayChange = "
+ windowSizeMayChange + ", this = " + this);
}
return windowSizeMayChange;
}这里，我们分析一下传进去给DecorView的measure函数的widthMeasureSpec和heightMeasureSpec到底是怎么样的？特别是他的mode到底是怎么确定的：
<span style="white-space:pre"> </span>childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
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;
}从这个函数可以看出如果Window的lp是ViewGroup.LayoutParams.MATCH_PARENT那么传入DecorView的measure函数的MeasureSpec的mode就是EXACTLY。而如果如果Window的lp是ViewGroup.LayoutParams.WRAP_CONTENT那么传入DecorView的measure函数的MeasureSpec的mode就是AT_MOST。
通过分析Framework的源代码，特别是FrameLayout的onMeasure（）函数，还可以得出如果下结论：
ViewGroup的measure（）和onMeasure（）的确可能被多次调用。以FrameLayout的onMeasure为例，首先遍历所有的child，调用measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);测量子View的尺寸。这里的measureSpec采用父View给他的measureSpec。.之后如果child的LayoutParameter的高或者宽有一个为match_parent会调整measureSpec再次重新对对应的子View测量一遍，即调用子View的measure函数重新计算一遍： child.measure(childWidthMeasureSpec, childHeightMeasureSpec);。这个是完全可以理解的，因为child为match_parent,而上次测量的时候还不知道父View的尺寸，自然需要重新测量一遍了。
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