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Android GUI之View布局

在清楚了View绘制机制中的第一步测量之后，我们继续来了解分析View绘制的第二个过程，那就是布局定位。继续跟踪分析源码，根据之前的流程分析我们知道View的绘制是从RootViewImpl的performTraversals方法开始的，在此方法中依次调用了performMeasure、performLayout、performDraw等方法进行测量、布局、绘制，那么下面我们就看看则方performLayout中都做了哪些事情，该方法的关键源码如下：

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,  int desiredWindowHeight) {         mLayoutRequested = false;         mScrollMayChange = true;         mInLayout = true;         final View host = mView;           ……         try {  host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); ……         } finally {  Trace.traceEnd(Trace.TRACE_TAG_VIEW);         }         mInLayout = false;     }

从中看出，最关键的代码就是调用了host.layout方法，那么大家还记不记得host是个什么东东呢？对了，正是我们之前说的根视图DecorView。那么我们就回到DecorView看看它在layout方法中到底做了什么事情。令人失望的是，我们在DecorView中并没有发现该方法，不要急，根据该类的继承体系，我们最终追踪到layout方法在View中。

Android GUI之View布局

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);    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;  } protected void onLayout(boolean changed, int left, int top, int right, int bottom) {}

该方法中的4个参数代表了当前的View与父View之间4个方向上的距离，同时从说明中可以看出，此方法不应该被子类重写，如果需要重新布局，可以在子类中重写的方法是onLayout，此方法在View中是个空方法，什么都没有写。可实际上layout的方法在View中并没有被标识为final，这就意味是可以被重写的。

继续查看ViewGoup中的相关代码，果然layout被重写了并添加了final标识，同时onLayout被标识为抽象方法，所以继承了ViewGroup的类是，是不能重写layout方法的，并且要实现onLayout方法。从代码可以看出，虽然ViewGroup重写了layout，实际本质上还是调用了View的layout，然后通过调用onLayout方法最终完成布局定位的。

    @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;  } } @Override protected abstract void onLayout(boolean changed,   int l, int t, int r, int b);

在DecorView中，并没有发现onLayout方法，所以它使用的肯定是其父类FrameLayout中的，找到FrameLayout的源码，可以查看到onLayout方法，具体如下：

  @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();  mForegroundBoundsChanged = true;  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 +      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);   }  } }