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
<com.example.testmeasurespec.TestLayout 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" >
<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" />
</com.example.testmeasurespec.TestLayout>