onMeasure 流程
计算出所有的子View中最大的宽，高。因为FrameLayout中的每个View 在不同的层级，所有测量需要计算上，左右上下Margin，加上 FrameLayout自身的上下左右Padding，组成FrameLayout实际需要设置的宽高。
如果FragmeLayout本身宽，高不是MeasureSpec.EXACTLY，需要重新计算 设置了Match_Parent 的子View的大小。
第一次measure：
//确定FrameLayout本身宽高是否为EXACTLY，该变量会导致某些子view再次绘制
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//调用child.measure()
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());
//将高宽属性为MATCH_PARENT的子view进行保存
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
第二次measure：【在FrameLayout高或宽为wrap_content时，只对高或宽属性为match_parent的子view进行再次measure】
count = mMatchParentChildren.size();
//注意只有当count>1时，才会执行
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);
}
}
例子：
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/ll_my_root"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<Button
android:id="@+id/tv_111"
android:layout_width="50dp"
android:layout_height="match_parent"
android:background="@android:color/holo_orange_dark" />
<Button
android:id="@+id/tv_222"
android:layout_width="match_parent"
android:layout_height="50dp"
android:background="@android:color/holo_blue_dark" />
<Button
android:id="@+id/tv_333"
android:layout_width="200dp"
android:layout_height="50dp"
android:layout_marginTop="300dp"
android:background="@android:color/holo_blue_dark" />
</FrameLayout>
Button_111的高度match_parent，会考虑Button_333的marginTop=300dp；Button_222的宽度match_parent，为Button_333的宽度200dp。
onLayout 流程
计算每个子View的layout_gravity，默认的layout_gravity 为Gravity.TOP | Gravity.LEFT，在根据Gravity计算时，通过水平方向的layout_gravity【CENTER_HORIZONTAL、RIGHT、LEFT】计算出childLeft，然后根据垂直方向layout_gravity 【CENTER_VERTICAL、TOP、BOTTOM】计算出childTop。
最后调用child.layout(childLeft, childTop, childLeft + width, childTop + height);
