Android中View绘制过程（五） performlayout

performTraversals()中执行了performmeasure确定了每一个View的width，和height，就会调用performLayout,确定这些View在屏幕的位置。

layout方法也是由decorView自上而下的递归调用，知道调用到叶子节点。就将所有的View位置确定好了，在performlayout()中开始Decor调用layout，代码如下

host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

这里的host就是DecorView根据上一篇的测量，可以知道host.getMeasuredWidth(), host.getMeasuredHeight()分别就是屏幕的宽高。Layout在View和GroupView中差别不大。

在View中代码如下

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);
}
}
}

mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}

注意到setFrame()这个调用就是当前VIew的left，top，right，bottom赋值，确定View的位置。例如decorView当时就确定是在左上角到右下角屏幕这么大了，之后调用onLayout

这个要到具体实现的ViewGroup中去看，各种layout，这里观察一下FramLayout是怎么做的。

@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();

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:

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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;
}

child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}

FrameLayout调用了layoutChildren遍历每一个子View并根据子View测量值来确定子View的位置，之后调用layout子View，这样完成由DecorView到叶子节点的所有View的遍历。