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。