源码的注解:
A MeasureSpec encapsulates the layout requirements passed from parent to child.
Each MeasureSpec represents a requirement for either the width or the height.
A MeasureSpec is comprised of a size and a mode. There are three possible modes:UNSPECIFIED;EXACTLY;AT_MOST
MeasureSpecs are implemented as ints to reduce object allocation. This class is provided to pack and unpack the <size, mode> tuple into the int.
大意：该MeasureSpec封装了布局所需的有父传给子；每个MeasureSpec都代表宽度和高度的要求；一个MeasureSpec包含size和mode。mode有三种：UNSPECIFIED;EXACTLY;AT_MOST；MeasureSpec的实现为了降低对象的开销。该类提供了分解和组合size和mode的方法。
源码：
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/**
* Measure specification mode: The parent has not imposed any constraint
* on the child. It can be whatever size it wants.
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* Measure specification mode: The parent has determined an exact size
* for the child. The child is going to be given those bounds regardless
* of how big it wants to be.
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* Measure specification mode: The child can be as large as it wants up
* to the specified size.
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
/**
* Creates a measure specification based on the supplied size and mode.
*
* The mode must always be one of the following:
* <ul>
* <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>
* <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>
* <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>
* </ul>
*
* <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's
* implementation was such that the order of arguments did not matter
* and overflow in either value could impact the resulting MeasureSpec.
* {@link android.widget.RelativeLayout} was affected by this bug.
* Apps targeting API levels greater than 17 will get the fixed, more strict
* behavior.</p>
*
* @param size the size of the measure specification
* @param mode the mode of the measure specification
* @return the measure specification based on size and mode
*/
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
/**
* Extracts the mode from the supplied measure specification.
*
* @param measureSpec the measure specification to extract the mode from
* @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
* {@link android.view.View.MeasureSpec#AT_MOST} or
* {@link android.view.View.MeasureSpec#EXACTLY}
*/
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
/**
* Extracts the size from the supplied measure specification.
*
* @param measureSpec the measure specification to extract the size from
* @return the size in pixels defined in the supplied measure specification
*/
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
static int adjust(int measureSpec, int delta) {
final int mode = getMode(measureSpec);
if (mode == UNSPECIFIED) {
// No need to adjust size for UNSPECIFIED mode.
return makeMeasureSpec(0, UNSPECIFIED);
}
int size = getSize(measureSpec) + delta;
if (size < 0) {
Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size +
") spec: " + toString(measureSpec) + " delta: " + delta);
size = 0;
}
return makeMeasureSpec(size, mode);
}
/**
* Returns a String representation of the specified measure
* specification.
*
* @param measureSpec the measure specification to convert to a String
* @return a String with the following format: "MeasureSpec: MODE SIZE"
*/
public static String toString(int measureSpec) {
int mode = getMode(measureSpec);
int size = getSize(measureSpec);
StringBuilder sb = new StringBuilder("MeasureSpec: ");
if (mode == UNSPECIFIED)
sb.append("UNSPECIFIED ");
else if (mode == EXACTLY)
sb.append("EXACTLY ");
else if (mode == AT_MOST)
sb.append("AT_MOST ");
else
sb.append(mode).append(" ");
sb.append(size);
return sb.toString();
}
}
其实没有多少内容；看看关键的几个变量和方法：
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
MODE_SHIFT 就是10进制的30；
MODE_MASK 是16进制的 0x3 加上 向左移 30个位
0x3是即十进制的 3 二进制 11 左移30位，即
11 后面紧跟30个0:11 000000 00000000 00000000 00000000
MODE_MASK：11 000000 00000000 00000000 00000000
UNSPECIFIED 根据上面的计算即是：
00 000000 00000000 00000000 00000000
public static final int EXACTLY = 1 << MODE_SHIFT;
public static final int AT_MOST = 2 << MODE_SHIFT;
EXACTLY 即是1左移30位：
01 000000 00000000 00000000 00000000
AT_MOST 即是2左移30位：
10 000000 00000000 00000000 00000000
makeMeasureSpec方法
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
sUseBrokenMakeMeasureSpec就是一个boolean值，其实不管是true或者false，返回的结果是一样的。我们看看：
size + mode ： MeasureSpec 是一个32位的二进制值，前面两位存mode值，后面30位存size值；所以
举例子：mode为AT_MOST
10 000000 00000000 00000000 00000000
size 为 00 000000 00000000 00000000 00000011
两个数相加得出：
10 000000 00000000 00000000 00000011
下面看 (size & ~MODE_MASK) | (mode & MODE_MASK)
也是用上面的值计算：
size & ~MODE_MASK ：
size：00 000000 00000000 00000000 00000011
~MODE_MASK（取反） ： 00 111111 11111111 11111111 11111111
& 与运算：00 000000 00000000 00000000 00000011
size & ~MODE_MASK ：00 000000 00000000 00000000 00000011
mode & MODE_MASK ：10 000000 00000000 00000000 00000000
最后 | 或运算：10 000000 00000000 00000000 00000011
结果是相同的，所以旧版本是else判断那个，而新版本google开发人员将简单问题简单化了。
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
意思就是取mode值，即measureSpec的31和32位；
因为MODE_MASK的31和32位是1，1与运算会取回本身值。而后面30位都为0.这样返回结果就只有mode了，而除去size值。
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
getSize()其实与getMode()原理一样。由于是取反“~”所以MODE_MASK取反为 00 111111 11111111 11111111 11111111
与运算就可以除掉31位和32位的值。
总结
MeasureSpec这个View的内部类其实就是做一件事：
保存size和mode值到一个32位的int值上。google开发人员十分巧妙地将两个值存放在一个int值上。这点我们在日常开发中也可以借鉴。
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