自定义view的measureSpec是谁的mode和size

一、简单背景介绍

在自定义view（不是ViewGroup）时，需要重写父类的onMeausre(), onLayout(), onDraw()：

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

看了很多书和文章，都介绍了widthMeasureSpec是什么东西。他是一个

MeasureSpec

，32位的int，前两位记录了他的mode，后30位记录了他的size。

mode如下：

Mode	Name	Cool
00	UNSPECIFIED	0
01	EXACTLY	1073741824
10	AT_MOST	-2147483648

但是widthMeasureSpec和heightMeasureSpec是从哪儿来的？他的mode和size是在xml中定义的mode和size？还是他父亲的mode和size？

二、探索onMeasure中的widthMeasureSpec

自定义view的xml文件如下：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent">

<com.example.wangxuan.customviewheight.CustomView
android:layout_width="300px"
android:layout_height="400px"
android:background="@color/colorPrimary" />

</RelativeLayout>

自定义view的java代码：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int widthSpec = MeasureSpec.getMode(widthMeasureSpec);
int width = MeasureSpec.getSize(widthMeasureSpec);
int heightSpec = MeasureSpec.getMode(heightMeasureSpec);
int height = MeasureSpec.getSize(heightMeasureSpec);

Log.e(TAG, "before wspec:" + widthSpec + ",w:" + width + ",hspec:" + heightSpec + ",h:" + height);

if (widthSpec == MeasureSpec.EXACTLY) {
Log.e(TAG, "width EXACTLY");
} else if (widthSpec == MeasureSpec.AT_MOST) {
Log.e(TAG, "width AT_MOST");
} else {
Log.e(TAG, "width UNSPECIFIED");
}
if (heightSpec == MeasureSpec.EXACTLY) {
Log.e(TAG, "height EXACTLY");
} else if (heightSpec == MeasureSpec.AT_MOST) {
Log.e(TAG, "height AT_MOST");
} else {
Log.e(TAG, "height UNSPECIFIED");
}
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
Log.e(TAG, "after wspec:" + widthSpec + ",w:" + width + ",hspec:" + heightSpec + ",h:" + height);

if (widthSpec == MeasureSpec.EXACTLY) {
Log.e(TAG, "width EXACTLY");
} else if (widthSpec == MeasureSpec.AT_MOST) {
Log.e(TAG, "width AT_MOST");
} else {
Log.e(TAG, "width UNSPECIFIED");
}
if (heightSpec == MeasureSpec.EXACTLY) {
Log.e(TAG, "height EXACTLY");
} else if (heightSpec == MeasureSpec.AT_MOST) {
Log.e(TAG, "height AT_MOST");
} else {
Log.e(TAG, "height UNSPECIFIED");
}
}

运行结果：

E/CustomView: before wspec:1073741824,w:300,hspec:-2147483648,h:1038
E/CustomView: width EXACTLY
E/CustomView: height AT_MOST
E/CustomView: after wspec:1073741824,w:300,hspec:-2147483648,h:1038
E/CustomView: width EXACTLY
E/CustomView: height AT_MOST
E/CustomView: before wspec:1073741824,w:300,hspec:1073741824,h:400
E/CustomView: width EXACTLY
E/CustomView: height EXACTLY
E/CustomView: after wspec:1073741824,w:300,hspec:1073741824,h:400
E/CustomView: width EXACTLY
E/CustomView: height EXACTLY

三、解释

在RelativeLayout中的onMeasure中会对其所有的子view进行测量。因为RelativeLayout的水平和竖直方向都是相对的，所以在进行测量其子view的大小时，需要分水平和竖直两个方向分别测量，即先测量width，在测量height。下面只给出了测量width的源代码：

int count = views.length;

for (int i = 0; i < count; i++) {
View child = views[i];
if (child.getVisibility() != GONE) {
LayoutParams params = (LayoutParams) child.getLayoutParams();
int[] rules = params.getRules(layoutDirection);

applyHorizontalSizeRules(params, myWidth, rules);
measureChildHorizontal(child, params, myWidth, myHeight);

if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
offsetHorizontalAxis = true;
}
}
}

可以看到，RelativeLayout在测量子view的时候，是调用了

child.getLayoutParams()

。而myWidth和myHeight则是RelativeLayout的width和height。所以可知自定义view的widthMeasureSpec是自身xml规定的size和mode。那传进去的父view的myWidth和myHeight实际是为了帮助子 view进行一些测量，比如match_parent时，需要通过父亲的大小来界定子 view的大小。