简介
我们在自定义控件的时候经常要重写View的onMeasure方法，onMeasure方法有两个int类型的参数，这两个参数就是MeasureSpec，这两个参数可不是普通的int数值，里面包含了mode和size两个信息，一个int有32位二进制，用高2位表示mode，低30位表示size，这样在一个int里面包含两个数值信息的设计，是为了减少对象的创建和内存的分配
onMeasure()方法用来测量空间的宽高，方法中的MeasureSpec参数由控件的父布局传入，是父View对子View的宽高的约束。
要注意onMeasure()方法的参数类型是int而不是MeasureSpec，MeasureSpec只是一个工具类，用来打包和拆包mode和size
源码
在讲解前，我建议先看下MeasureSpec的源码和源码的注释，源码很简单，注释里也讲的很清楚
为了更便于阅读，以下源码是比较早的安卓版本Android 4.1(Jelly Bean)，API16
/**
* 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:
* <dl>
* <dt>UNSPECIFIED</dt>
* <dd>
* The parent has not imposed any constraint on the child. It can be whatever size
* it wants.
* </dd>
*
* <dt>EXACTLY</dt>
* <dd>
* 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.
* </dd>
*
* <dt>AT_MOST</dt>
* <dd>
* The child can be as large as it wants up to the specified size.
* </dd>
* </dl>
*
* MeasureSpecs are implemented as ints to reduce object allocation. This class
* is provided to pack and unpack the <size, mode> tuple into the int.
*/
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>
*
* @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) {
return size + mode;
}
/**
* 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);
}
/**
* 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();
}
}mode
上面说过MeasureSpec的高两位表示mode，mode有三种类型
UNSPECIFIED：父View对子View没有任何限制，子View的宽高想多大就多大
EXACTLY：父View已经替子View确定了大小，不管子View要多大都不行
AT_MOST：子View要多大就多大，但是不能超过上限
掩码Mask和mode常量
在上面的源码中可以看到三种mode的定义及mask的定义，可以再看一下
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;MODE_SHIFT=30，这个常量下面会用到，做一些位移的操作
MODE_MASK就是掩码，0x3就是二进制0b11，向左位移30位后就是0b11000000000000000000000000000000，这个就可以作为mode的掩码，和int类型的MeasureSpec做位与运算就可以得到mode的值，后面会做详细介绍
UNSPECIFIED模式的值为0，0 << MODE_SHIFT位移运算后其实值还是零，0b00000000000000000000000000000000，高二位为00
EXACTLY模式的值为1 << MODE_SHIFT，也就是1左移30位，用二进制表示为0b01000000000000000000000000000000，高二位为01
int AT_MOST模式的值为2 << MODE_SHIFT，2用二进制表示为0b10，也就是0b10左移30位，用二进制表示为0b10000000000000000000000000000000，高二位为10
组装mode和size
MeasureSpec是一个int类型的数值，由mode和size组成，高二位表示mode，低30位表示size
mode必须为UNSPECIFIED，AT_MOST，EXACTLY三种类型的一种
方法很简单，就是把参数size和mode相加就完事了
比如：
0b01000000000000000000000000000000，mode为EXACTLY
0b00000000000000000000000000000011，size为3
0b01000000000000000000000000000011，这就是相加后的结果measure specification
代码如下所示，非常简单
/**
* 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>
*
* @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) {
return size + mode;
}解析mode
解析MeasureSpec中的mode也是很简单，保留高2位，其余低30位置0就可以了
掩码MODE_MASK值为0b11000000000000000000000000000000，将MeasureSpec和掩码MODE_MASK做位与运算，就可以保留高2位，其余低30位置零，得到mode的值
/**
* 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);
}解析size
解析size和解析mode的方法类似，只是把高2位置零，其余低30位保留
先把掩码MODE_MASK做按位取反，得到0b00111111111111111111111111111111，这个就是size的掩码，再把该掩码和MeasureSpec做位与运算，就得到size的值
size的单位是px
/**
* 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);
}格式化打印输出
这个方法就是把MeasureSpec格式化下打印输出，可以看该MeasureSpec包含的mode和size信息
格式为”MeasureSpec: MODE SIZE”，比如”MeasureSpec: EXACTLY 100″
/**
* 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();
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