Android GUI之View测量

在上篇文章（ http://www.cnblogs.com/jerehedu/p/4607599.html#gui ）中，根据源码探索了View的绘制过程，过程有三个主要步骤，分别为测量、布局、绘制。系统对绘制已经做了很好的封装，我们主要对测量和布局过程进行分析，看一看android是如何对view进行测量和布局的。

根据上篇文章的分析，我们知道在ViewRootImpl的performMeasure方法中，实际上调用了mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);方法。根据源码我们找到了该方法的原型，此方法在View类中，并且是final方法，不可被子类重写，方法的具体源码如下：

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {  boolean optical = isLayoutModeOptical(this);  if (optical != isLayoutModeOptical(mParent)) {    ……  }  // Suppress sign extension for the low bytes  long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;  if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);  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;      } else {   long value = mMeasureCache.valueAt(cacheIndex);   // Casting a long to int drops the high 32 bits, no mask needed   setMeasuredDimensionRaw((int) (value >> 32), (int) value);   mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;      } ……      mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;  }  mOldWidthMeasureSpec = widthMeasureSpec;  mOldHeightMeasureSpec = heightMeasureSpec;  mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |   (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension     } 

根据方法内容和说明，可以知道本方法就是用来测量View的大小的，而需要的两个参数是由父View构建的，用于说明父View对子View的测量的规格要求，实际上在这个方法中真正完成测量大小的是方法onMeasure，此方法我们稍后分析。在此之前我们先要明白measure方法中的两个参数的含义，刚才有提到参数是父View对子View的测量规格要求，那么Android是如何描述的呢，这里用到了一个类MeasureSpec，此类为View中的一个内部类，关键源码如下：

public static class MeasureSpec {   private static final int MODE_SHIFT = 30;   private static final int MODE_MASK  = 0x3 << MODE_SHIFT;   public static final int UNSPECIFIED = 0 << MODE_SHIFT;   public static final int EXACTLY  = 1 << MODE_SHIFT;   public static final int AT_MOST  = 2 << MODE_SHIFT;   public static int makeMeasureSpec(int size, int mode) {    if (sUseBrokenMakeMeasureSpec) {     return size + mode;    } else {     return (size & ~MODE_MASK) | (mode & MODE_MASK);    }   }   public static int getMode(int measureSpec) {    return (measureSpec & MODE_MASK);   }   public static int getSize(int measureSpec) {    return (measureSpec & ~MODE_MASK);   }    …… } 

根据SDK，此类封装了父View对子View的布局要求，每个实例都代表了对子View的高度或者宽度的要求，测量要求包含两个部分，分别为尺寸和模式。模式主要由三种，具体如下：

该类中提供了用来计算和生成测量要求的方法，具体如下：

明白了MeasureSpec，我们在回过头来，看一看onMeasure方法，该方法的源码如下：

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

此方法的默认实现非常简单，调用了setMeasuredDimersion方法将测量好的尺寸保存到mMeasuredWidth和mMeasuredHeight。而在setMeasuredDimersion方法中调用了getDefaultSize用来计算，该方法具体如下：

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; } 

很明显，此方法根据提供的默认大小和测量要求计算View的实际大小。到此为止，View完了测量过程。不过大多数情况下，当我们自定义ViewGroup的时候，我们需要重写onMeasure方法，在此方法中，可以遍历所有的子View并要求他们对自己的大小进行测量，同时不要忘记调用setMeasuredDimension进行保存测量结果，在ViewGroup是通过如下三个方法实现的，关键代码如下：

方法mesureChildren，遍历所有的非隐藏的子View，并调用measureChild方法设置子View的测量要求。

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);      }  } } 

方法measureChild，获取子View的测量规格，并调用measure进行测量实际大小。

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); } 

方法getChildMeasureSpec用于获取View的测量规格要求。

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 = 0;    resultMode = MeasureSpec.UNSPECIFIED;   } else if (childDimension == LayoutParams.WRAP_CONTENT) {    // Child wants to determine its own size.... find out how    // big it should be    resultSize = 0;    resultMode = MeasureSpec.UNSPECIFIED;   }   break;  }  return MeasureSpec.makeMeasureSpec(resultSize, resultMode); } 

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