在编写下啦刷新的项目代码的时候，在Listview的HeaderView中的head.xml文件中，根布局为RelativeLayout的时候，在计算headerView.measure的时候，出现空指针异常，当将更布局改为Linearlayout就运行正常了。
在思考为何在RelativeLayout出现异常的问题的时候，在查阅官方网站的时候，我注意到这个段话：
Note: In platform version 17 and lower, RelativeLayout was affected by a measurement bug that could cause child views to be measured with incorrect MeasureSpec values. (See MeasureSpec.makeMeasureSpec for more details.) This was triggered when a RelativeLayout container was placed in a scrolling container, such as a ScrollView or HorizontalScrollView. If a custom view not equipped to properly measure with the MeasureSpec mode UNSPECIFIED was placed in a RelativeLayout, this would silently work anyway as RelativeLayout would pass a very large AT_MOST MeasureSpec instead.
This behavior has been preserved for apps that set android:targetSdkVersion="17" or older in their manifest‘s uses-sdktag for compatibility. Apps targeting SDK version 18 or newer will receive the correct behavior
翻下来就是说：
在android系统版本在17级以下（包含17的时候）。使用measure会出现NULL异常情况，这个是一个BUG。原因是在RelativeLayout的控件使用在含有scrolling的时候，该含有scrolling的控件中计算空间大小的时候，没有使用MeasureSpec mode UNSPECIFIED的布局方式在RelativeLayout。自定义的控件则会尽可能的使用AT_MOST 来替换对齐方式。
如果你想解决这个问题有2个方法：
1.讲SDK的目标版本升级
2.将需要使用RelativeLayout的上层包一个LinearLayout即可、
View
源码路径 frameworks\base\core\java\android\view\View.java
源码中国链接：http://www.oschina.net/code/explore/android-2.2-froyo/android/view/View.java
[java] view plaincopyprint?
- public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
- if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
- widthMeasureSpec != mOldWidthMeasureSpec ||
- heightMeasureSpec != mOldHeightMeasureSpec) {
- // first clears the measured dimension flag
- mPrivateFlags &= ~MEASURED_DIMENSION_SET;
- if (ViewDebug.TRACE_HIERARCHY) {
- ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);
- }
- // measure ourselves, this should set the measured dimension flag back
- onMeasure(widthMeasureSpec, heightMeasureSpec);
- // flag not set, setMeasuredDimension() was not invoked, we raise
- // an exception to warn the developer
- if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
- throw new IllegalStateException("onMeasure() did not set the"
- + " measured dimension by calling"
- + " setMeasuredDimension()");
- }
- mPrivateFlags |= LAYOUT_REQUIRED;
- }
- mOldWidthMeasureSpec = widthMeasureSpec;
- mOldHeightMeasureSpec = heightMeasureSpec;
- }
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
mPrivateFlags &= ~MEASURED_DIMENSION_SET;
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);
}
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
throw new IllegalStateException("onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
}
可以看到measure函数有2个参数，widthMeasureSpec 和 heightMeasureSpec。我最初的疑问是不知道该怎么传这两个参数，于是跟到源码里面看看。这个函数的工作大概如下：
（mPrivateFlags这个还没研究，先跳过了）
1.检查传入的widthMeasureSpec和heightMeasureSpec是否与当前的值是一样的，不一样的话，调用onMeasure函数，并设置mPrivateFlags。
2.保存新值到mOldWidthMeasureSpec和mOldHeightMeasureSpec。这两个变量不用深究了，没有其他地方用到，就只是在这个函数中用来比较值用的。
3.这里判断符合条件后会抛出一个IllegalStateException的异常，它的提示信息很清楚，告诉我们要调用setMeasuredDimension()方法。但到底是怎么回事呢？这是在你需要重写onMeasure函数时需要注意的。
先来看看默认的View的onMeasure函数吧：
[java] view plaincopyprint?
- protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
- setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
- getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
- }
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
当我们需要重写onMeasure时，记得要调用setMeasuredDimension来设置自身的mMeasuredWidth和mMeasuredHeight，否则，就会抛出上面那个异常哦~
继续来看setMeasuredDimension：
[java] view plaincopyprint?
- protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
- mMeasuredWidth = measuredWidth;
- mMeasuredHeight = measuredHeight;
- mPrivateFlags |= MEASURED_DIMENSION_SET;
- }
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= MEASURED_DIMENSION_SET;
}
哦，很简单，就是设置了mMeasuredWidth和mMeasuredHeight，然后给mPrivateFlags设置了MEASURED_DIMENSION_SET标志位。那么计算都是在getDefaultSize函数里实现的：
[java] view plaincopyprint?
- 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;
- }
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;
}
看到了一个MeasureSpec，看来主要工作是在这里，必须得进去看看了。
[java] view plaincopyprint?
- 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) {
- return size + mode;
- }
- public static int getMode(int measureSpec) {
- return (measureSpec & MODE_MASK);
- }
- public static int getSize(int measureSpec) {
- return (measureSpec & ~MODE_MASK);
- }
- }
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) {
return size + mode;
}
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}
类不大，就都贴出来了，为了精简篇幅，去掉了注释和toString函数。
这里MODE_MASK二进制是11000（一共30个0）00，也就是最高2位标识mode，其余位标识size。
接下来回到getDefaultSize函数
通过这个类的方法从参数measureSpec中提取出了specMode和specSize。 specMode的作用在下面的switch语句中可以看出来。
[java] view plaincopyprint?
- case MeasureSpec.UNSPECIFIED:
- result = size;
- break;
case MeasureSpec.UNSPECIFIED:
result = size;
break;
这里的size就是getSuggestedMinimumWidth()或者getSuggestedMinimumHeight()，是一个默认的最小宽或高，可以看到如果specMode为MeasureSpec.UNSPECIFIED时，specSize（即我们希望设置的size）是没有用到的。
[java] view plaincopyprint?
- case MeasureSpec.AT_MOST:
- case MeasureSpec.EXACTLY:
- result = specSize;
- break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
当specMode为MeasureSpec.AT_MOST或MeasureSpec.EXACTLY时，从我们传入的参数measureSpec中提取出来的specSize被采用了。这种情况下上面的size就被废弃了。当result确定后，就是setMeasuredDimension被调用了，在里面将会对mMeasuredWidth和mMeasuredHeight进行设置。 简单示例： OK，现在应该理解了吧，下面是一个调用measure方法的示例：
[java] view plaincopyprint?
- mTextView.measure(MeasureSpec.EXACTLY + mTextView.getWidth(), MeasureSpec.EXACTLY);
- mTextView.layout(0, 0, mTextView.getMeasuredWidth(), mTextView.getMeasuredHeight());
mTextView.measure(MeasureSpec.EXACTLY + mTextView.getWidth(), MeasureSpec.EXACTLY); mTextView.layout(0, 0, mTextView.getMeasuredWidth(), mTextView.getMeasuredHeight());
把mode标志和你想设置的大小相加，传进去就OK啦。这里设置height的时候我是想设0，因此直接传了MeasureSpec.EXACTLY进去。
当然，measure完后，并不会实际改变View的尺寸，需要调用View.layout方法去进行布局。按示例调用layout函数后，View的大小将会变成你想要设置成的大小。
另外关于layout，包括整个布局流程，我将要写另一篇博文介绍。因此在这里就不再赘述了。