前言
本文主要分析了View的layout源码，总结了layout与onLayout在职能上的关系。
View layout分析
public void layout(int l, int t, int r, int b) {
//1.位运算，取标志位，判断是否需要执行onMeasure方法，1表示执行
//正常View的绘制流程中是不会执行的,取用测量缓存值，会导致onMeasure调用原因见后续说明
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
//重置标志位为0，表示不执行
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//2.判断View在父容器中的位置是否发生变化
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
//3.如果View的位置发生变化或者PFLAG_LAYOUT_REQUIRED标志位为1将会执行onLayout方法，
//在前面的measure分析中，onMeasure方法执行之后，会将该标志位置位1
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
//监听回调，通知观察者，此处对mOnLayoutChangeListeners进行了一次深拷贝，保证线程安全,
//可以推测存在其他线程操作mOnLayoutChangeListeners变量的可能
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);
}
}
}
//重置PFLAG_FORCE_LAYOUT标志位为0，表示不强制layout，与measure方法相关
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
//标志位，表示至少执行过一次layout
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
注释1：
SDK中对该标志位的注释如下：
/**
* Flag indicating that a call to measure() was skipped and should be done
* instead when layout() is invoked.
*/
static final int PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT = 0x8;
//View的measure方法片段
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
//如果从测量缓存值中查到了本次测量规格，则PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT置为1，
即onMeasure方法会在layout中调用
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
我们知道正常的View的绘制流程，view的measure方法一定会得到调用，那么该标志位必定不会成立，至于在什么情况下会跳过measure方法而直接执行layout？阅读源码要点到为止！好吧，其实在源码中并没有找到答案，知道的朋友请科普，不胜感激。
注释2：
默认情况下，父容器的LayoutMode都为控件边界布局模式，所以默认情况下都会执行setFrame方法，下面进入setFrame方法:
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 + ")");
}
//如果View的位置发生变化，该函数将会返回true
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
//获取PFLAG_DRAWN标志位的值,表示是否需要绘制
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 our old position
invalidate(sizeChanged);
//java层位置属性赋值，这就是为何需要在layout之后才能获得View的宽高的原因
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
//native层位置属性赋值
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
//onSizeChange监听器回调，通常可以在该回调函数中获取View的宽、高，
//因为位置属性在上面已经赋值了
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
//如果View可见或者mGhostView不为空，PFLAG_DRAWN标志位强制置为1
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;
}
//Accessibility（盲人辅助功能）相关
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
可见setFrame方法是layout函数中的关键函数，它判断View的位置属性是否改变，并对其行赋值。
思考：
既然layout已经完成了View的位置属性赋值，那么onLayout是用来干嘛的呢？其实在SDK的注释已经说明了一切
/**
* Called from layout when this view should
* assign a size and position to each of its children.
*
* Derived classes with children should override
* this method and call layout on each of
* their children.
*/
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
一般而言，onLayout函数就是为ViewGroup而生，在onLayout中调用child的layout方法，达到摆放child的目的；正常的View是没有child的，所以View的onLayout方法为空实现。
ViewGroup onLayout分析
ViewGroup中覆写了layout方法，并将其定义为final，这样实现类将不能继承layout方法，只能继承onLayout，ViewGroup就应该专注于摆放child，不得不佩服google工程师的设计。下面就来简单看看FrameLayout的onLayout方法：
@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();
//获取ViewGroup的padding值
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
//遍历摆放child
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//获取的测量宽、高
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
//默认gravity为左上
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;
............截取部分分析
//根据摆放规则计算childTop
switch (verticalGravity) {
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:
//可以将margin值设为负数来调整child在父容器中的layout位置
childTop = parentTop + lp.topMargin;
}
//关键点，摆放child
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
总结：
View的layout与onLayout之间的关系不同于measure与onMeasure：
1.layout方法通过setFrame方法用于确定自身的摆放位置，而onLayout方法则是用于摆放child，适用于ViewGroup，onLayout方法根据摆放规则计算出child的左上右下属性，通过调用child.layout方法实现child的摆放。有时候，我们可以通过设置child的margin值为负数，来达到调整child在容器中的位置，如LinearLayout设置为负数可以实现FrameLayout的叠层效果。
2.measure方法扮演着一个测量优化的角色，真正的测量是放在onMeasure方法中，该方法不仅测量自身的宽高，还会测量child的宽高。
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