Android View 测量流程(Measure)全面解析

private void performTraversals() {
  ...

 if (!mStopped) {
  int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); // 1
  int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); 
  }
 }

 if (didLayout) {
  performLayout(lp, desiredWindowWidth, desiredWindowHeight);
  ...
 }


 if (!cancelDraw && !newSurface) {
  if (!skipDraw || mReportNextDraw) {
  if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
   for (int i = 0; i < mPendingTransitions.size(); ++i) {
   mPendingTransitions.get(i).startChangingAnimations();
   }
   mPendingTransitions.clear();
  }

  performDraw();
  }
 } 
 ...
}

public static class MeasureSpec {
 private static final int MODE_SHIFT = 30;
 private static final int MODE_MASK = 0x3 << MODE_SHIFT;

 /**
  * UNSPECIFIED 模式：
  * 父View不对子View有任何限制，子View需要多大就多大
  */ 
 public static final int UNSPECIFIED = 0 << MODE_SHIFT;

 /**
  * EXACTYLY 模式：
  * 父View已经测量出子Viwe所需要的精确大小，这时候View的最终大小
  * 就是SpecSize所指定的值。对应于match_parent和精确数值这两种模式
  */ 
 public static final int EXACTLY = 1 << MODE_SHIFT;

 /**
  * AT_MOST 模式：
  * 子View的最终大小是父View指定的SpecSize值，并且子View的大小不能大于这个值，
  * 即对应wrap_content这种模式
  */ 
 public static final int AT_MOST = 2 << MODE_SHIFT;

 //将size和mode打包成一个32位的int型数值
 //高2位表示SpecMode，测量模式，低30位表示SpecSize，某种测量模式下的规格大小
 public static int makeMeasureSpec(int size, int mode) {
  if (sUseBrokenMakeMeasureSpec) {
  return size + mode;
  } else {
  return (size & ~MODE_MASK) | (mode & MODE_MASK);
  }
 }

 //将32位的MeasureSpec解包，返回SpecMode,测量模式
 public static int getMode(int measureSpec) {
  return (measureSpec & MODE_MASK);
 }

 //将32位的MeasureSpec解包，返回SpecSize，某种测量模式下的规格大小
 public static int getSize(int measureSpec) {
  return (measureSpec & ~MODE_MASK);
 }
 //...
 }

/**
 * @param windowSize
 *  The available width or height of the window
 *
 * @param rootDimension
 *  The layout params for one dimension (width or height) of the
 *  window.
 *
 * @return The measure spec to use to measure the root view.
 */
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
 int measureSpec;
 switch (rootDimension) {

 case ViewGroup.LayoutParams.MATCH_PARENT:
 // Window can't resize. Force root view to be windowSize.
 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
 break;
 //省略...

 }
 return measureSpec;
}

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
 Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
 try {
 mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
 } finally {
 Trace.traceEnd(Trace.TRACE_TAG_VIEW);
 }
}

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
 boolean optical = isLayoutModeOptical(this);
 if (optical != isLayoutModeOptical(mParent)) {
 ...
 if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
  widthMeasureSpec != mOldWidthMeasureSpec ||
  heightMeasureSpec != mOldHeightMeasureSpec) {
  ...
  if (cacheIndex < 0 || sIgnoreMeasureCache) {
  // measure ourselves, this should set the measured dimension flag back
  onMeasure(widthMeasureSpec, heightMeasureSpec);
  mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
  } 
 ...
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
 //获取当前布局内的子View数量
 int count = getChildCount();

 //判断当前布局的宽高是否是match_parent模式或者指定一个精确的大小，如果是则置measureMatchParent为false.
 final boolean measureMatchParentChildren =
  MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
  MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
 mMatchParentChildren.clear();

 int maxHeight = 0;
 int maxWidth = 0;
 int childState = 0;

 //遍历所有类型不为GONE的子View
 for (int i = 0; i < count; i++) {
 final View child = getChildAt(i);
 if (mMeasureAllChildren || child.getVisibility() != GONE) {
  //对每一个子View进行测量
  measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
  final LayoutParams lp = (LayoutParams) child.getLayoutParams();
  //寻找子View中宽高的最大者，因为如果FrameLayout是wrap_content属性
  //那么它的大小取决于子View中的最大者
  maxWidth = Math.max(maxWidth,
   child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
  maxHeight = Math.max(maxHeight,
   child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
  childState = combineMeasuredStates(childState, child.getMeasuredState());
  //如果FrameLayout是wrap_content模式，那么往mMatchParentChildren中添加
  //宽或者高为match_parent的子View，因为该子View的最终测量大小会受到FrameLayout的最终测量大小影响
  if (measureMatchParentChildren) {
  if (lp.width == LayoutParams.MATCH_PARENT ||
   lp.height == LayoutParams.MATCH_PARENT) {
   mMatchParentChildren.add(child);
  }
  }
 }
 }

 // Account for padding too
 maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
 maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

 // Check against our minimum height and width
 maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
 maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

 // Check against our foreground's minimum height and width
 final Drawable drawable = getForeground();
 if (drawable != null) {
 maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
 maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
 }

 //保存测量结果
 setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
  resolveSizeAndState(maxHeight, heightMeasureSpec,
   childState << MEASURED_HEIGHT_STATE_SHIFT));

 //子View中设置为match_parent的个数
 count = mMatchParentChildren.size();
 //只有FrameLayout的模式为wrap_content的时候才会执行下列语句
 if (count > 1) {
 for (int i = 0; i < count; i++) {
  final View child = mMatchParentChildren.get(i);
  final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

  //对FrameLayout的宽度规格设置，因为这会影响子View的测量
  final int childWidthMeasureSpec;

  /**
  * 如果子View的宽度是match_parent属性，那么对当前FrameLayout的MeasureSpec修改：
  * 把widthMeasureSpec的宽度规格修改为:总宽度 - padding - margin，这样做的意思是：
  * 对于子Viw来说，如果要match_parent，那么它可以覆盖的范围是FrameLayout的测量宽度
  * 减去padding和margin后剩下的空间。
  *
  * 以下两点的结论，可以查看getChildMeasureSpec()方法：
  *
  * 如果子View的宽度是一个确定的值，比如50dp，那么FrameLayout的widthMeasureSpec的宽度规格修改为：
  * SpecSize为子View的宽度，即50dp，SpecMode为EXACTLY模式
  * 
  * 如果子View的宽度是wrap_content属性，那么FrameLayout的widthMeasureSpec的宽度规格修改为：
  * SpecSize为子View的宽度减去padding减去margin，SpecMode为AT_MOST模式
  */
  if (lp.width == LayoutParams.MATCH_PARENT) {
  final int width = Math.max(0, getMeasuredWidth()
   - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
   - lp.leftMargin - lp.rightMargin);
  childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
   width, MeasureSpec.EXACTLY);
  } else {
  childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
   getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
   lp.leftMargin + lp.rightMargin,
   lp.width);
  }
  //同理对高度进行相同的处理，这里省略...

  //对于这部分的子View需要重新进行measure过程
  child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
 }
 }
}

public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
 final int specMode = MeasureSpec.getMode(measureSpec);
 final int specSize = MeasureSpec.getSize(measureSpec);
 final int result;
 switch (specMode) {
 case MeasureSpec.AT_MOST:
  if (specSize < size) {
  result = specSize | MEASURED_STATE_TOO_SMALL;
  } else {
  result = size;
  }
  break;
 case MeasureSpec.EXACTLY:
  result = specSize;
  break;
 case MeasureSpec.UNSPECIFIED:
 default:
  result = size;
 }
 return result | (childMeasuredState & MEASURED_STATE_MASK);
}

protected void measureChildWithMargins(View child,
 int parentWidthMeasureSpec, int widthUsed,
 int parentHeightMeasureSpec, int heightUsed) {
 final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

 final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
  mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
   + widthUsed, lp.width);
 final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
  mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
   + heightUsed, lp.height);

 child.measure(childWidthMeasureSpec, childHeightMeasureSpec); // 1
}

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
 int specMode = MeasureSpec.getMode(spec);
 int specSize = MeasureSpec.getSize(spec);

 //size表示子View可用空间：父容器尺寸减去padding
 int size = Math.max(0, specSize - padding);

 int resultSize = 0;
 int resultMode = 0;

 switch (specMode) {
 // Parent has imposed an exact size on us
 case MeasureSpec.EXACTLY:
 if (childDimension >= 0) {
  resultSize = childDimension;
  resultMode = MeasureSpec.EXACTLY;
 } else if (childDimension == LayoutParams.MATCH_PARENT) {
  // Child wants to be our size. So be it.
  resultSize = size;
  resultMode = MeasureSpec.EXACTLY;
 } else if (childDimension == LayoutParams.WRAP_CONTENT) {
  // Child wants to determine its own size. It can't be
  // bigger than us.
  resultSize = size;
  resultMode = MeasureSpec.AT_MOST;
 }
 break;

 // Parent has imposed a maximum size on us
 case MeasureSpec.AT_MOST:
 //省略..具体可自行参考源码
 break;

 // Parent asked to see how big we want to be
 case MeasureSpec.UNSPECIFIED:
 //省略...具体可自行参考源码
 break;
 }
 return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
 setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
  getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

public static int getDefaultSize(int size, int measureSpec) {
 int result = size;
 int specMode = MeasureSpec.getMode(measureSpec);
 int specSize = MeasureSpec.getSize(measureSpec);

 switch (specMode) {
 case MeasureSpec.UNSPECIFIED:
 result = size;
 break;
 case MeasureSpec.AT_MOST:
 case MeasureSpec.EXACTLY:
 result = specSize;
 break;
 }
 return result;
}

protected int getSuggestedMinimumWidth() {
 return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}