引言
在上篇博客中Android进阶之自定义View原理(一)View的Measure过程中，我们研究了View的测量原理，得到View的大小之后，接下来的工作就是如何放置View，本文就从源码角度一探究竟，下一篇博客将基于这两个知识点自定义一个静态的ViewGroup。
View的layout方法作用是确定View的位置，ViewGroup的layout方法不仅要确定自身的位置，还有确定子View的位置。首先我们看看View的layout 过程。
（一）View的layout流程
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;
}
//暂存当前位置
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);
//当布局位置发生变化时，onLayout方法被调用
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;
//OnLayoutChangeListener监听处理
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;
if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
notifyEnterOrExitForAutoFillIfNeeded(true);
}
}
layout方法先调用setFrame方法，然后调用onLayout方法。看看setFrame干了些啥：
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 + ")");
}
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
//位置发生变化，则更新位置值
changed = true;
// Remember our drawn bit
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);
//保存位置
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) {
// 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;
//如果尺寸发生变化,则会重新绘制
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;
mDefaultFocusHighlightSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
setFrame主要将传进来的位置分别赋值给mLeft、mTop、mRight、mBottom,这样就确定了View在父容器的位置，如果大小发生变化，这里默认会调用View的重绘制方法，返回的changed表示View的尺寸和位置是否发生变化,当它发生变化时（如第一次执行layout或者View更新位置后执行requestLayout方法）onLayout方法就会被调用。这是一个空方法，和OnMeasure方法类似，确定位置根据不同的需求有不同的实现，View和ViewGroup方法均没有实现它，但是在实际开发过程中，自定义View的位置是由父容器确定的，所以onLayout方法主要用在自定义ViewGroup中。下面我们再继续看ViewGroup的layout方法。
（二）View的layout流程
@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;
}
}
它的layout过程非常简单，调用的是View的layout方法，另外它的onLayout方法是抽象方法,这就意味这自定义View必须实现布局子View的逻辑。为了弄清ViewGroup的布局子View的流程，我们看看LinearLayout的onLayout方法。
（三）LinearLayout的layout流程:
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l, t, r, b);
}
}
layoutVertical方法:
final int paddingLeft = mPaddingLeft;
int childTop;//子View的top
int childLeft;//子View的top的left
...
//遍历可见非GONE的View
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
//取每个child的宽高
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
//结合LP计算childLeft
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
//执行子View的layout方法
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
//更新childTop
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
总结：View的layout流程核心在于覆写ViewGroup的onLayout方法，它的流程是拿到子View的宽高，然后实现自己的布局子View的逻辑，它一般结合onMeasure方法使用,具体案例我会在下一篇博客详细说明流式布局的案例实现。
