Android View 布局流程(Layout)全面解析

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
 int desiredWindowHeight) {
 mLayoutRequested = false;
 mScrollMayChange = true;
 mInLayout = true;

 final View host = mView;
 if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
 Log.v(TAG, "Laying out " + host + " to (" +
  host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
 }

 Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
 try {
 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); // 1

 //省略...
 } finally {
 Trace.traceEnd(Trace.TRACE_TAG_VIEW);
 }
 mInLayout = false;
}

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); // 1

 if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
 onLayout(changed, l, t, r, b);  // 2
 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 boolean setFrame(int left, int top, int right, int bottom) {
 //省略...

 mLeft = left;
 mTop = top;
 mRight = right;
 mBottom = bottom;
 mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);

 //省略...
 return changed;
}

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {

 //把父容器的位置参数传递进去
 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();

 //以下四个值会影响到子View的布局参数
 //parentLeft由父容器的padding和Foreground决定
 final int parentLeft = getPaddingLeftWithForeground();
 //parentRight由父容器的width和padding和Foreground决定
 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();

  //获取子View的测量宽高
  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;

  //当子View设置了水平方向的layout_gravity属性时，根据不同的属性设置不同的childLeft
  //childLeft表示子View的 左上角坐标X值
  switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {

  /* 水平居中，由于子View要在水平中间的位置显示，因此，要先计算出以下：
   * (parentRight - parentLeft -width)/2 此时得出的是父容器减去子View宽度后的
   * 剩余空间的一半，那么再加上parentLeft后，就是子View初始左上角横坐标(此时正好位于中间位置)，
   * 假如子View还受到margin约束，由于leftMargin使子View右偏而rightMargin使子View左偏，所以最后
   * 是 +leftMargin -rightMargin .
   */
  case Gravity.CENTER_HORIZONTAL:
   childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
   lp.leftMargin - lp.rightMargin;
   break;

  //水平居右，子View左上角横坐标等于 parentRight 减去子View的测量宽度 减去 margin
  case Gravity.RIGHT:
   if (!forceLeftGravity) {
   childLeft = parentRight - width - lp.rightMargin;
   break;
   }

  //如果没设置水平方向的layout_gravity，那么它默认是水平居左
  //水平居左，子View的左上角横坐标等于 parentLeft 加上子View的magin值
  case Gravity.LEFT:
  default:
   childLeft = parentLeft + lp.leftMargin;
  }

  //当子View设置了竖直方向的layout_gravity时，根据不同的属性设置同的childTop
  //childTop表示子View的 左上角坐标的Y值
  //分析方法同上
  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;
  }

  //对子元素进行布局，左上角坐标为(childLeft，childTop)，右下角坐标为(childLeft+width,childTop+height)
  child.layout(childLeft, childTop, childLeft + width, childTop + height);
 }
 }
}

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