MeasureSpecMode 的三个值都是什么意思？

Section1

查看View源码中的

/** * 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} */@MeasureSpecModepublic static int getMode(int measureSpec) {    //noinspection ResourceType    return (measureSpec & MODE_MASK);}

Section2

继续朝上找，有如下变量，

private static final int MODE_SHIFT = 30;

/** * 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;

这三个值是多少呢？

Section3

public static void main(String args[]){    System.out.println(getFullBinaryString(UNSPECIFIED));    System.out.println(getFullBinaryString(EXACTLY));    System.out.println(getFullBinaryString(AT_MOST));}public static String getFullBinaryString(int num){    char [] mChars = new char [32];    for (int i = 0;i<32;i++){        mChars[31-i]= (char)((num>>i&1)+'0');    }    return new String (mChars);}

打印结果如下：

09-26 18:24:07.975 8120-8120/? I/System.out: 0000000000000000000000000000000009-26 18:24:07.975 8120-8120/? I/System.out: 0100000000000000000000000000000009-26 18:24:07.975 8120-8120/? I/System.out: 10000000000000000000000000000000

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可以看到，这个其实就是利用了32位的int值的左侧两位存了 这三种状态。

顺便说下：

UNSPECIFIED   未指定

EXACTLY      指定

AT_MOST      指定了最大值