FrameLayout是一个ViewGroup。在ViewGroup最重要的两步方法是测量和布局：onMeasure()、onLayout()方法。所以这里只分析FrameLayout的onMeasure()、onLayout()方法。
FrameLayout的特征：
FrameLayout的所有子View一般都是在容器的左上方，除非子View有设置layout_gravity这个属性。
FrameLayout的onMeasure()源码：
流程：FrameLayout的onMeasure()，会先遍历所有子View，并为每个子View确定MeasureSpec，计算出子view中最大的宽度和高度，并把宽或高是match_parent的子view放进一个List集合中。把子view的最大宽高度+padding,然后调用setMeasuredDimension()方法，根据frameLayout的MeasureSpec和最大宽高度确认FrameLayout的大小。之后把刚刚match_parent的子view集合遍历，根据已经确定了FrameLayout的大小再重新计算设置这个子View的MeasureSpec
private final ArrayList<View> mMatchParentChildren = new ArrayList<>(1);
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
//是否大小已经确定，不需要子view的和大小来确定
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
//父类可以会多次调用onMeasure，为了避免数据叠加，每次先清空数据
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
//测量所有子View，并记录最大的宽高
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//这里是设置子view的measureSpec，并调用子view的onmeasure方法，与getChildMeasureSpec()不同的是，这个measureChildMargin()方法把margin也去除了。
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);
}
}
}
}
//最大宽高加上padding
// 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());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//主要处理viewGroup的at_most、unspecified两种模式的大小，at_most:大小不能超过父viewGroup默认大小，unspecified默认是子view所需的大小
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
//将所有子view的宽高的是match_parent的记录下来，然后重新赋值这些子view的宽高
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();
//宽度
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);
}
//给match_parent的子view重新设置大小
//这里又调用了子View的measure（）方法，在上面的确定子view的measureSpec已经调用了一次，所以子view可能会被父view调用多次onMeasure(),要记得数据清楚。
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
FrameLayout的match_parent的子view会被调用两次onMeasure，所以再onMeasure()要注意数据的清楚，FrameLayout这里onMeasure也是每调用一次就会清除List的集合。大概实现思路是：先记录子View的最大宽高->确定FrameLayout自己的大小->重新设置width、height是match_parent的大小。
FrameLayout的onLayout()源码：
onLayout布局再FrameLayout的大小确定之后，因为其子View不用涉及到换行等问题，一般子view如果不设置gravity的属性，默认是在FrameLayout左上显示。所以这里的布局，只需要判断gravity的属性，然后根据属性值来动态确认left和top的位置
//frameLayout所有的子View都是从容器左上方开始，除了一个gravity
@Override
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();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
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;
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;
}
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);
}
}
}
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