match_parent、wrap_parent、具体值 和 MeasureSpec 类中 mode 的对应关系

如果不考虑父 view 的影响，测试结果如下：

 * wrap_parent -> MeasureSpec.AT_MOST
 * match_parent -> MeasureSpec.EXACTLY
 * 具体值 -> MeasureSpec.EXACTLY

这是一个符合我们直觉的结果。

一个 view 的 onMeasure 方法最终得到的测量规格值(测量约束值)中包含的测量模式和上面不一定对的上，这是因为 onMeasure 方法中得到的测量规格值(测量约束值)是 measure 方法传过来的，父 view 在调用 measure 方法的时候可以根据自己的情况不使用 ViewGroup 提供的 measureChildren 方法，而改变上面的映射关系，比如 RelativeLayout 中就结合其他 Layout 属性综合确定 MeasureSpec 中的模式。上面的映射关系可以直接查看 ViewGroup 类的 getChildMeasureSpec 方法，这是最简单的 measure 子 view 的方式.

ViewGroup 中 measureChild 方法如下：

    protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

getChildMeasureSpec 方法如下：

    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

我们父 view 是 match_parent 情况下我们只需要关注 case MeasureSpec.EXACTLY 就行了。view 自带的 geDefaultSize 这个方法如果设置包裹内容，最后得到的 view 不会是包裹内容，而是父 view 指定的大小。正如 geDefaultSize 这个方法中 switch 代码中所展示的逻辑。所以 getDefaultSize 这个方法还是很粗糙的方法，连 wrap_content 这个属性都没有提供支持，当然对 match_parent 和 指定值 还是有支持的。这也就是说 View 中自带的 onMeasure 其实可以用性很低。也难怪 reference 文档中说 子 view 必须重新 onMeasure。ViewGroup 中的 measureChildren 方法没有对 margin 进行处理，所以如果子 view 的属性带 margin，那么他其实是不能给子 view 一个结合 margin 的测量约束值。

测试代码如下

TestView.java

/**
 * 测试 match_parent、wrap_parent、具体值 和 MeasureSpec 中  mode 的对应关系
 */
package com.example.testmeasurespec;

import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;

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) {
        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;
    }
}

TestLayout.java

public class TestLayout extends ViewGroup {
    
    public TestLayout(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

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

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

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {

    }
    
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        measureChildren(widthMeasureSpec, heightMeasureSpec);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }
}

main_activity.xml