感觉FrameLayout很神秘，是因为用到它的地方少，一直觉得它鸡肋，原来是自己无知。最近需要实现一些layer的效果，就用到了它。它的用法很简单，这里就不多说了，这里就说说它的原理吧。
FrameLayout类里面没有什么东西，主要说的还是它的自身的布局参数FrameLayout.LayoutParams，布局参数类继承MarginLayoutParams。看名词就知道，就是控制view的外边距的，FrameLayout.LayoutParams本身自己定义的参数只有一个gravity。
好了，既然有了布局参数，那就会有空间的一些属性吧。首先FrameLayout也是一个View，所以他必然有Padding(view的内边距)相关属性。这里我们重点关注它的onMeasure与onLayout：
onMeasure意思就是计算出该ViewGroup的布局大小及孩子View的布局大小。
onMeasure
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
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) {
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);
}
}
}
}
// 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());
}
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++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
int childWidthMeasureSpec;
int childHeightMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
lp.leftMargin - lp.rightMargin,
MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
if (lp.height == LayoutParams.MATCH_PARENT) {
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
lp.topMargin - lp.bottomMargin,
MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
简单的说父容器的width就是所有孩子View的width取最大值加View外边距和它的内边距：child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin+getPaddingLeftWithForeground() + getPaddingRightWithForeground();如果ViewGroup有图片，还有跟图片比较取较大值,容器的height也是同样的道理。FrameLayout孩子View的布局大小跟普通View大小确定是一样的。
onLayout
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
mForegroundBoundsChanged = true;
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 = getResolvedLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.LEFT:
childLeft = parentLeft + lp.leftMargin;
break;
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = parentRight - width - lp.rightMargin;
break;
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);
}
}
}
从代码中可以看出，孩子View左上角坐标的X值就是ViewGroup的左内边距加上view的外边距，Y值同理。
哎，怎么感觉自己写着写着，还把问题说复杂了点，感觉还是看代码能明白。
还有，我在2.2版本的代码上，gravity与margin属性必须同时设置，margin才能发挥作用，如果只有margin属性而没有设置gravity,就没有效果。如果有谁知道，欢迎指教哈。
在上一篇中说了下android里drag and drop 一个View，当时那个imageView放在一个LinearLayout里面，事实上放在FrameLayout同样适用，因为他们的布局参数都继承MarginLayoutParams类，所以通过margin控制他们的布局位置同样在FrameLayout使用。事实上GridLayout,LinearLayout,RelativeLayout,FrameLayout的布局参数都是继承MarginLayoutParams的。