Android -容器- FrameLayout

• 目录
  Android -容器- FrameLayout
  Android -容器- LinearLayout
  Android -容器- RelativeLayout

onMeasure 流程

1. 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。

2. 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);