android 【MeasureSpec】 introduction

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: The parent has not imposed any constraint on the child. It can be whatever size it wants.
EXACTLY: 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.
AT_MOST: The child can be as large as it wants up to the specified size.

MeasureSpecs are implemented as ints to reduce object allocation. This class is provided to pack and unpack the <size, mode> tuple into the int.

　一个MeasureSpec封装了父布局传递给子布局的布局要求，每个MeasureSpec代表了一组宽度和高度的要求。一个MeasureSpec由大小和模式组成。

它有三种模式：

UNSPECIFIED(未指定),

父元素不对子元素施加任何束缚，子元素可以得到任意想要的大小；

EXACTLY(完全)，

父元素决定子元素的确切大小，子元素将被限定在给定的边界里而忽略它本身大小；

AT_MOST(至多)，

子元素至多达到指定大小的值。

MeasureSpecs通过实现int 来减少对象分配。此类提供了打包和解包<size，model>元组到int中。

　　它常用的三个函数：

　　1.static int getMode(int measureSpec):根据提供的测量值(格式)提取模式(上述三个模式之一)

　　2.static int getSize(int measureSpec):根据提供的测量值(格式)提取大小值(这个大小也就是我们通常所说的大小)

　　3.static int makeMeasureSpec(int size,int mode):根据提供的大小值和模式创建一个测量值(格式)

对于此类的应用，常常写这样一个方法，在onMeasure方法里调用：

public static int resolveSize(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:            result = Math.min(size, specSize);            break;        case MeasureSpec.EXACTLY:            result = specSize;            break;        }        return result;    }

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