本文以FrameLayout的layout过程为例，来分析ViewGroup的layout过程
1. FrameLayout的onLayout代码如下：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
onLayout中直接调用了layoutChildren函数。
2.layoutChildren函数：
@Override
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
// 获取子View的数量
final int count = getChildCount();
// 获取上下左右的有效显示区域
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);
// 忽略可见状态为Gone的View
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
// 获取宽、高
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
// View要布局的相对于父布局的左边距
int childLeft;
// View要布局的相对于父布局的上边距
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
// 默认值为 Gravity.TOP | Gravity.START
gravity = DEFAULT_CHILD_GRAVITY;
}
// 获取布局方向，RTL或LTR，希伯来语等少数语言文化中，视图是从右向左显示的（RTL）
final int layoutDirection = getLayoutDirection();
// 获取水平方向上的布局参数（水平方向上收到RTL和LTR的影响）
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
// 获取垂直方向上的布局参数
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
// 计算View在父布局中的左边距
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL://水平居中，水平方向上View居中显示
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT://靠右显示
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT://默认是靠左显示
default:
childLeft = parentLeft + lp.leftMargin;
}
// 计算View在父布局中的上边距
switch (verticalGravity) {
case Gravity.TOP://靠上对齐
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL://垂直居中，垂直方向上View居中显示
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM://靠底对齐
childTop = parentBottom - height - lp.bottomMargin;
break;
default:// 默认靠上对齐
childTop = parentTop + lp.topMargin;
}
//设置View的上、下、左、右的相对于父布局的坐标
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
在layoutChildren中，分别根据子View水平方向 和垂直方向上的布局布局参数，计算出相对于父布局的上下左右坐标，然后调用View的layout方法保存布局参数。
3.View中的layout方法：
public void layout(int l, int t, int r, int b) {
// 在View的measure函数中，若使用缓存值，则会通过
//mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT设置该标志位，表示在接下来的layout中再进行测量
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// 保存上一次的上下左右坐标
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
// 通过setFrame保存上下左右坐标，changed为true表示坐标有变化
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
// 如果坐标有变化或者要求重新layout
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
// 对于View来说，一般不会重新onLayout；对于ViewGroup，需要重写onLayout安排子View的位置
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
// 调用尺寸变化的回调函数
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
//取消强制layout的标志
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
//设置布局完成的标志
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
3.1在layout中，通过调用setFrame保存上下左右的布局参数，现具体分析：
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
if (DBG) {
Log.d("View", this + " View.setFrame(" + left + "," + top + ","
+ right + "," + bottom + ")");
}
// 只要上下左右有一个位置坐标发生了变化，我们就认为位置发生了变化
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
// 计算上一次的宽高和当前的宽高
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// 老的坐标位置无效
invalidate(sizeChanged);
// 保存当前的坐标值，View的getWidth方法返回的就是mRight - mLeft，实际的坐标值最终决定了View的宽和高
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
//设置渲染区域
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
//宽高发生变化
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
// If we are visible, force the DRAWN bit to on so that
// this invalidate will go through (at least to our parent).
// This is because someone may have invalidated this view
// before this call to setFrame came in, thereby clearing
// the DRAWN bit.
mPrivateFlags |= PFLAG_DRAWN;
invalidate(sizeChanged);
// parent display list may need to be recreated based on a change in the bounds
// of any child
invalidateParentCaches();
}
// Reset drawn bit to original value (invalidate turns it off)
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
4. 至此，ViewGroup的布局过程已完成，简单来说，layout决定了一个ViewGroup中子View的位置摆放以及自身的宽和高。
View的getWidth和getHeight代码如下：
public final int getWidth() {
return mRight - mLeft;
}
public final int getHeight() {
return mBottom - mTop;
}
