View的在屏幕上显示所需的大小确定后，接下来我们需要知道它放屏幕的什么地方。
对于View，根据自己的parent传递的位置信息，来设置自己的位置。layout(int l, int t, int r, int b)，l，t，r，b就是相对父布局的参数。
对于ViewGroup而言知道自己放哪还不行，还需要确定子元素的位置。同理ViewGroup在layout(int l, int t, int r, int b)方法中确定它相在它父布局中的位置，然后调用onLayout(boolean changed, int left, int top, int right, int bottom)确定其子布局相对于它的位置。
页面layout的入口是在ViewRootImpl#performLayout，在这个方法最终会调用到View#layout。
private void performTraversals() {
...
performLayout(lp, mWidth, mHeight);
...
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
...
//mView是DecorView
final View host = mView;
...
//对于DecorView而言就是全屏幕范围
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}
DecorView自己没有实现layout，调用的就是View 的layout()。
View#layout：
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//是否使用了视图边界布局，setFrame中会设置left，right，top， bottom
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
//如果left，right，top，bottom 有变，changed就是true
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
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);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
View#layout中核心的方法是 setFrame(...)和 onLayout(...)。
setFrame(...)传入的参数会分别转化为View的mLeft，mTop，mRight，mBottom这四个值，代表的是View的四个顶点这样可以确定View在父容器的位置。
View#setFrame：
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
int drawn = mPrivateFlags & PFLAG_DRAWN;
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
invalidate(sizeChanged);
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
mPrivateFlags |= PFLAG_DRAWN;
invalidate(sizeChanged);
invalidateParentCaches();
}
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
View#onLayout是一个空的方法，一般是ViewGroup实现这个方法遍历所有的子元素并调用其layout方法。按照流程View#layouot中调用的onLayout是DecorView中的onLayout。
DecorView#onLayout:
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
getOutsets(mOutsets);
if (mOutsets.left > 0) {
offsetLeftAndRight(-mOutsets.left);
}
if (mOutsets.top > 0) {
offsetTopAndBottom(-mOutsets.top);
}
}
DecorView#onLayout他会调用父类FrameLayout的onLayout，如果有OutSets就设置偏移。
我们接着看看FrameLayout#onLayout：
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false);
}
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
//去掉View的pad和前背景的pad，获得实践可用的大小
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 +
break; lp.leftMargin - lp.rightMargin;
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);
}
}
}
方法比较简单，计算出子View在父View中的相对位置，然后调用子View的layout(...)。
所以布局过程是从ViewRootImpl对象调用DecorView的layout()方法开始，接着layout()方法调用根视图的onLayout()方法，onLayout()方法会对所包含的子视图逐一执行layout操作，如果子视图是ViewGroup子类对象，则继续调用子视图的layout()，重复这一过程。如果子视图是View子类对象，其onLayout为空，只需要处理自己的layout。
参考：《Android 开发艺术探索》
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