Android视图的绘制流程(上) View的测量

综述

　　View的绘制流程可以分为三大步，它们分别是measure，layout和draw过程。measure表示View的测量过程，用于测量View的宽度和高度；layout用于确定View在父容器的位置；draw则是负责将View绘制到屏幕中。下面主要来看一下View的Measure过程。

测量过程

　　View的绘制流程是从ViewRoot的performTraversals方法开始的，ViewRoot对应ViewRootImpl类。ViewRoot在performTraversals中会调用performMeasure方法来进行对根View的测量过程。而在performMeasure方法中又会调用View的measure方法。对于View的measure方法它是一个final类型，也就是说这个measure方法不能被子类重写。但是在measure方法中调用了onMeasure方法。所以View的子类可以重写onMeasure方法来实现各自的Measure过程。在这里也就是主要对onMeasure方法进行分析。

MeasureSpec

　　MeasureSpec是View类中的一个静态内部类。一个MeasureSpec封装了父布局传递给子布局的布局要求。每个MeasureSpec都代表着一个高度或宽度的要求。每个MesureSpec都是由specSize和specMode组成，它代表着一个32位的int值，其中高2位代表specSize，低30位代表specMode。
　　MeasureSpec的测量模式有三种，下面介绍一下这三种测量模式：

UNSPECIFIED
父容器对子View没有任何的限制，子View可以是任何的大小。
EXACTLY
父容器为子View大小指定一个具体值，View的最终大小就是specSize。对应View属性match_parent和具体值。
AT_MOST
子View的大小最大只能是specSize，也就是所子View的大小不能超过specSize。对应View属性的wrap_content.

　　在MeasureSpec中可以通过specSize和specMode并使用makeMeasureSpec方法来创建一个MeasureSpec，还可以通过getMode和getSize来获取MeasureSpec的specMode和specSize。

View的测量过程

　　在上面已经说到，View的Measure过程是由measure方法来完成的，而measure方法通过调用onMeasure方法来完成View的Measure过程。那么就来看一下onMeasure方法。

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

　　在View的onMeasure方法中只是调用了setMeasuredDimension方法，setMeasuredDimension方法的作用就是设置View的高和宽的测量值。对于View测量后宽和高的值是通过getDefaultSize方法来获取的。下面就来一下这个getDefaultSize方法。

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;
}

　　对于MeasureSpec的AT_MOST和EXACTLY模式下，直接返回的就是MeasureSpec的specSize，也就是说这个specSize就是View测量后的大小。而对于在UNSPECIFIED模式下，View的测量值则为getDefaultSize方法中的第一个参数size。这个size所对应的宽和高是通过getSuggestedMinimumWidth和getSuggestedMinimumHeight两个方法获取的。下面就来看一下这两个方法。

protected int getSuggestedMinimumHeight() {
  return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

}

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

　　在这里可以看到对于View宽和高的取值是根据View是否存在背景进行设置的。在这里以View的宽度来进行说明。若是View没有背景则是View的宽度mMinWidth。对于mMinWidth值得设置可以在XML布局文件中设置minWidth属性，它的默认值为0。也可以通过调用View的setMinimumWidth()方法其赋值。若是View存在背景的话，则取View本身最小宽度mMinWidth和View背景的最小宽度它们中的最大值。

ViewGroup的测量过程

　　对于ViewGroup的Measure过程，ViewGroup处理Measure自己本身的大小，还需要遍历子View，并调用它们的measure方法，然后各个子元素再去递归执行Measure过程。在ViewGroup中并没有重写onMeasure方法，因为ViewGroup它是一个抽象类，对于不同的具体ViewGroup它的onMeasure方法中所实现的过程不一样。但是在ViewGroup中提供了一个measureChildren方法，对子View进行测量。下面就来看一下这个measureChildren方法。

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
  final int size = mChildrenCount;
  final View[] children = mChildren;
  for (int i = 0; i < size; ++i) {
    final View child = children[i];
    if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
      measureChild(child, widthMeasureSpec, heightMeasureSpec);
    }
  }
}

　　在这里获取ViewGroup中所有的子View。然后遍历ViewGroup中子View并调用measureChild方法来完成对子View的测量。下面看一下measureChild方法。

protected void measureChild(View child, int parentWidthMeasureSpec,
    int parentHeightMeasureSpec) {
  final LayoutParams lp = child.getLayoutParams();

  final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
      mPaddingLeft + mPaddingRight, lp.width);
  final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
      mPaddingTop + mPaddingBottom, lp.height);

  child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

　　在这段代码中通过getChildMeasureSpec方法获取子View宽和高的MeasureSpec。然后调用子View的measure方法开始对View进行测量。下面就来看一下是如何通过getChildMeasureSpec方法来获取View的MeasureSpec的。

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

  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:
    if (childDimension >= 0) {
      // Child wants a specific size... so be it
      resultSize = childDimension;
      resultMode = MeasureSpec.EXACTLY;
    } else if (childDimension == LayoutParams.MATCH_PARENT) {
      // Child wants to be our size, but our size is not fixed.
      // Constrain child to not be bigger than us.
      resultSize = size;
      resultMode = MeasureSpec.AT_MOST;
    } 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 asked to see how big we want to be
  case MeasureSpec.UNSPECIFIED:
    if (childDimension >= 0) {
      // Child wants a specific size... let him have it
      resultSize = childDimension;
      resultMode = MeasureSpec.EXACTLY;
    } else if (childDimension == LayoutParams.MATCH_PARENT) {
      // Child wants to be our size... find out how big it should
      // be
      resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
      resultMode = MeasureSpec.UNSPECIFIED;
    } else if (childDimension == LayoutParams.WRAP_CONTENT) {
      // Child wants to determine its own size.... find out how
      // big it should be
      resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
      resultMode = MeasureSpec.UNSPECIFIED;
    }
    break;
  }
  return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

　　在这段代码对于MeasureSpec的获取主要是根据父容器的MeasureSpec和View本身的LayoutParams。下面通过一张表格来看一下它们之间的对应关系。

到这里通过getChildMeasureSpec方法获取到子View的MeasureSpec以后，便调用View的Measure方法，开始对View进行测量。
　　正如刚才说的那样对于ViewGroup它是一个抽象类，并没有重写View的onMeasure方法。但是到具体的ViewGroup时，例如FrameLayout，LinearLayout，RelativeLayout等，它们通过重写onMeasure方法来来完成自身以及子View的Measure过程。下面以FrameLayout为例，看一下的Measure过程。在FrameLayout中，它的Measure过程也算是比较简单，下面就来看一下FrameLayout中的onMeasure方法。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  int count = getChildCount();

  final boolean measureMatchParentChildren =
      MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
      MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
  mMatchParentChildren.clear();

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

  for (int i = 0; i < count; i++) {
    final View child = getChildAt(i);
    if (mMeasureAllChildren || child.getVisibility() != GONE) {
      measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
      final LayoutParams lp = (LayoutParams) child.getLayoutParams();
      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());
      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());
  }

  setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
      resolveSizeAndState(maxHeight, heightMeasureSpec,
          childState << MEASURED_HEIGHT_STATE_SHIFT));

  count = mMatchParentChildren.size();
  if (count > 1) {
    for (int i = 0; i < count; i++) {
      final View child = mMatchParentChildren.get(i);
      final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

      final int childWidthMeasureSpec;
      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);
      }

      final int childHeightMeasureSpec;
      if (lp.height == LayoutParams.MATCH_PARENT) {
        final int height = Math.max(0, getMeasuredHeight()
            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
            - lp.topMargin - lp.bottomMargin);
        childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
            height, MeasureSpec.EXACTLY);
      } else {
        childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
            lp.topMargin + lp.bottomMargin,
            lp.height);
      }

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

　　在这部分代码中逻辑也很简单，主要完成了两件事。首先FrameLayout完成自身的测量过程，然后在遍历子View，执行View的measure方法，完成View的Measure过程。在这里代码比较简单就不在进行详细描述。

总结

　　最后对View和ViewGroup的Measure过程做一下总结。对于View，它的Measure很简单，在获取到View的高和宽的测量值之后，便为其设置高和宽。而对于ViewGroup来说，除了完成自身的Measure过程以外，还需要遍历子View，完成子View的测量过程。