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layout
不像测量过程那样最终在onmeasure完成测量，view位置的放置是在layout函数里执行的，onLayout默认是个空函数。但一般来说，在viewGroup中onlayout用来给子view限制大小和子view layout事件分发，因为viewGroup中的layout是final的，不允许重写，所以只能写在onLayout中了。
但layout的调用过程是怎样的呢？我们看一下debug的线程栈
上面没有截出来的其实就是ViewGroup和View的层级layout调用了。
ActivityThread下面那些是app的启动过程了，可以参考：
我们直接分析ViewGroup的layout
@Override
public final void layout(int l, int t, int r, int b) {
if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
if (mTransition != null) {
mTransition.layoutChange(this);
}
super.layout(l, t, r, b);
} else {
// record the fact that we noop'd it; request layout when transition finishes
mLayoutCalledWhileSuppressed = true;
}
}
最终还是会调用View的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;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
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);
}
}
}
final boolean wasLayoutValid = isLayoutValid();
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
if (!wasLayoutValid && isFocused()) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
if (canTakeFocus()) {
clearParentsWantFocus();
} else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) {
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
clearParentsWantFocus();
} else if (!hasParentWantsFocus()) {
clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
} else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) {
mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
View focused = findFocus();
if (focused != null) {
if (!restoreDefaultFocus() && !hasParentWantsFocus()) {
focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
}
}
}
if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
notifyEnterOrExitForAutoFillIfNeeded(true);
}
notifyAppearedOrDisappearedForContentCaptureIfNeeded(true);
}
View的Layout他自己会根据位置放置自己，然后再调用onLayout 也就是说View onLayout就是一个通知。 但是，作为ViewGroup中的onLayout（默认是空方法，继承ViewGroup一般都会去实现它），一般我们会去实现它来完成对子viwe的位置设置
所以ViewGroup中的作用有两个：1.通知自己的位置已经被放置；2.调用子view的layout去放置子view
拿简单的FrameLayout为例：
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
//这时FrameLayout的位置已经放置好
//开始计算子view的位置该放在哪
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();
//减去padding,让子view都放在padding之内
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();//减去left的目的由于子view的是位置坐标是相对于parent的，而不是绝对值，parent也是相对与它父类的坐标，我们减去left，也就减去了parent距离它的parent的left，这也是作为相对坐标合理的操作。
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
//遍历子view
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//FrameLayout只放置可见的View，对与不可见的View也就不会调用他的layout
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//获得子view的测量值
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;
//子view简单的重叠放置
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
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;
}
//放置子View
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
我们所熟知的getHeight等参数就是在layout的过程的setFrame函数中设置的。
那传进layout的left,right参数又是哪里来的呢？
首先layout的是由于调用viewRootImpl的doTraversal->performTraversals执行的，我们熟知的measure,layout,draw也都是在这个函数里面按顺序执行的。
最初的layout参数就是在performLayout里来的
//host是DecorView
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
