上一篇文章，学习了ViewGroup和View的measure流程。文章最后讲到，本文将会学习ViewGroup和普通View的onMeasure方法的工作。
因为ViewGroup是抽象类，它并没有实现onMeasure方法，而是选择让它的子类实现。之前学习DecorView的measure过程时，说到DecorView的父类继承自FrameLayout，FrameLayout又继承自ViewGroup，并且重写了onMeasure方法。那我们就来看一下FrameLayout的onMeasure方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
//...
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
//...
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}从代码可以看到，child.measure方法的调用参数childWidthMeasureSpec和childHeightMeasureSpec的生成，是与父容器的MeasureSpec值和child的LayoutParams有关的。很明显子元素的测量值，是去除了子元素本身的margin和padding值的。查看LinearLayout和RelativeLayout的源码，可以发现它们的onMeasure方法中，调用child.measure方法的调用参数也是去除了子元素本身的margin和padding值的。由此，我们可以知道，在自定义ViewGroup的时候，我们需要在重写的onMeasure方法中处理子元素的margin和padding值。注意到上述代码中子元素的childWidthMeasureSpec值可以是调用了getChildMeasureSpec获得的（childHeightMeasureSpec同理）。下面我们看一下它的源码：
//VIewGroup.java 6602行
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);
}从上面的getChildMeasureSpec方法源码（这里我们依然不分析UNSPECIFIED模式），结合上面FrameLayout（或者LinearLayout/RelativeLayout）的onMeasure方法，可以知道，getChildMeasureSpec方法的三个参数是：父容器的测量规格值、子元素的LayoutParams中的margin和子元素的padding值的和、子元素的LayoutParams中的width（或者LayoutParams中的height）。
从源码可以总结出子元素MeasureSpec创建的规律，如下表。表格中，第一行表示父容器的测量模式，第一列表示子元素在父容器的约束下，子元素LayoutParams的值：
| x:parentSpecMode y:childLayoutParams |
EXACTLY |
AT_MOST |
UNSPECIFIED |
| dp/px |
EXACTLY childSize |
EXACTLY childSize |
EXACTLY childSize |
| MATCH_PARENT |
EXACTLY parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
| WRAP_CONTENT |
AT_MOST parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
其中：parentSize是值父容器的测量值的大小减去子元素margin和padding占用的值。childSize就是子元素的LayoutParams中的width或者LayoutParams中的height的值。
下面我们看一下，ViewGroup给我们提供的一些关于Measure过程的方法。
//ViewGroup.java 6522行
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) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
//ViewGroup.java 6542行
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}ViewGroup能够拥有子元素，所以它的measure过程除了测量自己的大小以外，还需要遍历去调用所有子元素的measure方法。遍历子元素经行测量这个过程可以通过调用measureChildren方法实现，也可以自己在自定义的ViewGroup的onMeasure方法中实现。
measureChildren方法很简单，就是遍历ViewGroup的子元素，再根据子元素的可见状态调用measureChild方法。在measureChild方法中创建子元素的MeasureSpec，然后调用子元素的measure方法，如果子元素是ViewGroup那么重复这个过程。如果子元素是View，那就回到了View的measure过程了。
//VIewGroup.java 6568行
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);
}自定义ViewGroup的时候，可以自己实现对子元素的margin和padding的处理，也可以调用在这个方法。
普通View的onMeasure方法相对ViewGroup简单些，根据父容器剩余空间，处理普通View的margin和padding，如果有额外的需求，加上这些需求的处理计算出普通View的宽高，调用setMeasuredDimension设置普通View全局宽高值即可。