简介
FrameLayout是层级布局，即所有的子View都是从底层向上层开始，默认不指定margin或者layout_gravity的话，每个子view的坐标起始坐标都是Framelayout的起始坐标
public class FrameLayout extends ViewGroup {
...
}
FrameLayout就是直接继承自ViewGroup的
onMeasure
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
//判断FrameLayout 宽 高 是否同时设置了Match_parent或者 设置了指定大小的宽高
// measureMatchParentChildren 为 true，则表示没有设置了Match_parent或者 设置了指定大小的宽高
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
//遍历所有子View计算出所有的子View中的最大宽度 maxWidth 和最大高度 maxHeigh
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//计算子View的宽高，包含了子View的左右上下Margin，直接调用ViewGroup中的方法
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());
// FrameLayout没有设置固定大小的宽高或Match_parent高宽，则收集所有设置了Match_parent的子View
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// 加上padding
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//设置自身的宽高
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
...
}
先看前部分，根据FrameLayout 宽 高 是否同时设置了Match_parent或者 设置了指定大小来设置一个Boolean值，然后遍历所有的子View，计算出最大宽度和最大高度，同时调用ViewGroup中的方法进行子View的测量；如果FrameLayout没有设置为match_parent或者固定的值，则会存储是match_parent的所有子View ，接着就是加上padding值，然后设置自己的宽高。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
...
// 如果FrameLayout没有同时设置固定大小或者Match_parent宽高，且有子View设置了Match_parent宽高，则重新测量 设置了Match_Parent宽高的子View
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);
}
}
}
后面部分就是判断是否需要重新测量，如果FrameLayout没有同时设置固定大小或者Match_parent宽高，且有子View设置了Match_parent宽高，则重新测量 设置了Match_Parent宽高的子View
onLayout
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
//遍历子View
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//同样，View的可见性不为GONE才会计算在内
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
// 根据水平方向Gravity，计算View的左右，坐标
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 + lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
// 根据垂直方向Gravity，计算View的上下，坐标
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 + lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
onLayout方法主要就是对子View进行layout
在对子View进行layout的过程中，主要就是通过遍历所有的子View，然后根据水平和垂直两个方向计算view具体的坐标，最后通过子View本身的layout方法进行放置。因为每个view在不同的层级，所以只需要计算每个View的Layout_gravity，默认的Layout_gravity 为Gravity_left | GravityTop, 在根据Gravity计算时，分别通过水平方向的Gravity计算出 左右 坐标，然后根据垂直方向的Gravity计算出上下坐标。
onDraw
FrameLayout没有自己实现onDraw方法
FrameLayout、LinearLayout和RelativeLayout的性能对比
当RelativeLayout和LinearLayout作为ViewGroup表达相同的布局的时候，谁的绘制更快一些，性能相对更好一些？
通过网上的很多实验结果我们得之，两者绘制同样的界面时layout和draw的过程时间消耗相差无几，关键在于measure过程。
LinearLayout：
在没有权重的情况下，就只会单纯的遍历一个方向，遍历一次所有的View；如果View设置了权重 ，那么在第一次遍历的时候这个View是不会进行测量的，在第二次测量（专门用于测量权weight重的）；所以无权重一次遍历，有权重两次遍历。RelativeLayout：因为依赖关系，所以在进行排序后，分别会对水平、垂直方向进行遍历，所以两次遍历。
FrameLayout：某种情况上来说，FrameLayout也可能导致二次测量，不过FrameLayout的二次测量就只针对View为match_parent的了。
