FrameLayout：帧布局
布局效果：child通过层叠的效果叠加在布局中，布局的大小由child的大小或者背景图片大小、前景图片大小决定。
FrameLayout对child的操作相对较少，所以代码主要体现在:
- onMeasure
- onLayout
onMeasure分析:
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();//获取Children的数目
final boolean measureMatchParentChildren =//是否需要测量MatchParent属性的Children
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
//条件是宽高都非精准模式下
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) {
//一个是自身处于非GONE状态下，一个是设置了mMeasureAllChildren=true
//测量，最终调用
//child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//获取最大值
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
//合并child的视图状态
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {//把math_parent的View加入集合
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too 加上padding
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width //检测是否大于最小宽高
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) {//检测前景图片的最小宽高
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//通过resolveSizeAndState获取需要的MeasureSpec
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {//重新测量match_parent的View
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
//为match_parent，通过makeMeasureSpec获取measureSpec为
//精准模式+layout宽度减去内边距和外边距
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 {
//通过getChildMeasureSpec获取measureSpec
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);
}
}
}
onMeasure()方法里面主要的设计思路：
- 测量childView的宽高和状态，获取最大宽高
- 调用setMeasuredDimension(..)设置自身的宽高
- 重新测量childView中含有math_parent的view（采用FrameLayout的getMeasureWidht/getMeasureHeigh计算）
- child.measure(..)
FrameLayout的onLayout中的代码：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
可以看到是调用了layoutChildren(..)进行布局的排版
void layoutChildren(int left, int top, int right, int bottom,
boolean forceLeftGravity) {
final int count = getChildCount();//获取child个数
final int parentLeft = getPaddingLeftWithForeground();//布局的左边坐标
//布局的右边坐标
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();//布局方向，水平方向用到（左<->右）
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {//水平
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;
}
//上面通过gravity的值计算出child的4边
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
onLayout(..)主要是对child的排版，主要思路是：
- 获取parent的左上右下。
- 根据gravity的值（水平还需要获取布局方向）计算出child需要的左上右下坐标
- 调用child的layout方法进行排版
##总结
- FrameLayout是一个相对简单的布局，也是自定义ViewGroup继承最多的一个。
- 主要注意的是onMeasure和onLayout方法。
- onMeaure要考虑children把控件撑大的情况
- onLayout要考虑gravity和Direction(方向)
END
– Nowy
– 2017.04.12