Android MeasureSpec详解

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 &lt;size, mode&gt; 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 ");elsesb.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 ");elsesb.append(mode).append(" ");sb.append(size);return sb.toString();}