MeasureSpec中的测量模式和match_parent、wrap_content是什么对应关系？

Question

下面是我的测试程序, 重写了 View 中的 getDefaultSize 方法，打印出每次调用 getDefaultSize 的时候传递进来的 measureSpec 的模式，在主 activity 的 xml 中放了 3 个 TestView，分别使用 wrap_content、match_parent、具体 dp 值，来指定 layout_width 和 layout_height 的值，期望通过 log 看到 3 个 view 在调用自己的 onMeasure 方法的时候传递进来的测量约束的模式。但是打印结果让我很糊涂，因为打印的结果和设置的值看起来并不是一一对应的关系？所以我想问，测量模式和 layout_width 的值的类型是到底什么对应关系？

MainActivity 布局文件如下：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.testmeasurespec.MainActivity" >

    <com.example.testmeasurespec.TestView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/ic_launcher"
        android:tag="test1" />

    <com.example.testmeasurespec.TestView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@drawable/ic_launcher"
        android:tag="test2" />

    <com.example.testmeasurespec.TestView
        android:layout_width="20dp"
        android:layout_height="20dp"
        android:background="@drawable/ic_launcher"
        android:tag="test3" />

</RelativeLayout>

测试 TestView 类如下：

public class TestView extends View {

    private static final String TAG = "TestView";

    public TestView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    public TestView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public TestView(Context context) {
        super(context);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // TODO Auto-generated method stub
        Log.i(TAG, "onMeasure:" + getTag());
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

    public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
            case MeasureSpec.UNSPECIFIED:
                Log.i(TAG, "specMode:MeasureSpec.UNSPECIFIED");
                result = size;
                break;
            case MeasureSpec.AT_MOST:
                Log.i(TAG, "specMode:MeasureSpec.AT_MOST");
            case MeasureSpec.EXACTLY:
                Log.i(TAG, "specMode:MeasureSpec.EXACTLY");
                result = specSize;
                break;
        }
        return result;
    }
}

打印结果如下：
11-21 16:13:48.812: I/TestView(21213): onMeasure:test3
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): onMeasure:test2
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): onMeasure:test1
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): onMeasure:test3
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): onMeasure:test2
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): onMeasure:test1
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.812: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.892: I/TestView(21213): onMeasure:test3
11-21 16:13:48.892: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.892: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.892: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.892: I/TestView(21213): onMeasure:test2
11-21 16:13:48.892: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.892: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): onMeasure:test1
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): onMeasure:test3
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): onMeasure:test2
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): onMeasure:test1
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.AT_MOST
11-21 16:13:48.902: I/TestView(21213): specMode:MeasureSpec.EXACTLY

Answer 1

最简单的映射关系是：
* wrap_parent -> MeasureSpec.AT_MOST
* match_parent -> MeasureSpec.EXACTLY
* 具体值 -> MeasureSpec.EXACTLY

但是上面代码由于放在 RelativeLayout 中，RelativeLayout 是一个比较复杂的 ViewGroup，其中子 view 的大小不仅跟 layout_width、layout_height 属性相关，还更很多属性有关系，所以会改变上述映射情况，使结果变得特别复杂。