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：

1、 UNSPECIFIED：代表父View对子View没有约束，子View可以为任意大小。
2、 EXACTLY：父View确定子View的大小，子View被限定在给定的边界中，忽咯本身的大小。
3、 AT_MOST：子View最大可以达到指定大小的值。

　　该类中提供了用来计算和生成测量要求的方法，具体如下：

1、 public static int makeMeasureSpec(int size, int mode)，此方法最终生成一个32位二进制数用来表明测量规格要求，其中32和31位用来表明模式，后30位代表了大小。
2、 public static int getMode(int measureSpec)，此方法可以根据测量说明，计算模式。
3、 public static int getSize(int measureSpec)，此方法根据测量说明，计算大小。

　　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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