Class Overview

This class represents the basic building block for user interface components. A View occupies a rectangular area on the screen and is responsible for drawing and event handling. View is the base class forwidgets, which are used to create interactive UI components (buttons, text fields, etc.).

View.MeasureSpec

extends Object

View.MeasureSpec

extends Object

Class Overview

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:

MeasureSpec是一个32位的int值，高2位为测量模式（SpecMode），低30位是测量大小（SpecSize）.

View默认的onMeasure()方法只支持EXACTLY模式，因此如果让自定义View支持wrap_content属性，就必须重写onMeasure()方法来指定wrap_content大小（进一步说明，由源码得出，wrap_content下的SpecMode是AT_MOST，此时的specSize是parentSize，即如果不指定wrap_content，在布局中使用wrap_content就相当于使用match_parent）.

解决方法：

protected void onMeasure(int widthMeasureSpec,int heightMeasureSpec){super.onMeasure(widthMeasureSpec, heightMeasureSpec);int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);if(widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST){setMeasureDimension(mWidth,mHeight);}else if(widthSpecMode == MeasureSpec.AT_MOST){setMeasureDimension(mWidth,heightSpecSize);}else if(heightSpecMode == MeasureSpec.AT_MOST){setMeasureDimension(widthSoecSize,mHeight)}
}

在上面代码中指定View的默认宽高（mWidth/mHeight），并在wrap_content时设置此宽高。
Demo：

MeasureView.java

package sunny.example.ahthreeviewmeasure;import android.content.Context;
import android.util.AttributeSet;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;public class MeasureView extends View{public MeasureView(Context context) {super(context);// TODO Auto-generated constructor stub}public MeasureView(Context context, AttributeSet attrs) {super(context, attrs);// TODO Auto-generated constructor stub}public MeasureView(Context context, AttributeSet attrs, int defStyle) {super(context, attrs, defStyle);// TODO Auto-generated constructor stub}@Overrideprotected void onMeasure(int widthMeasureSpec,int heightMeasureSpec){super.onMeasure(widthMeasureSpec, heightMeasureSpec);int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);if(widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST){setMeasuredDimension(400,400);//指定宽高}else if(widthSpecMode == MeasureSpec.AT_MOST){setMeasuredDimension(400,heightSpecSize);}else if(heightSpecMode == MeasureSpec.AT_MOST){setMeasuredDimension(widthSpecSize,400);}}}

activity_main.xml

<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="sunny.example.ahthreeviewmeasure.MainActivity" ><sunny.example.ahthreeviewmeasure.MeasureViewandroid:id="@+id/measureView"android:layout_width="match_parent"android:layout_height="wrap_content"android:background="#09acda" /></RelativeLayout>

MainActivity.java

package sunny.example.ahthreeviewmeasure;import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;public class MainActivity extends ActionBarActivity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);//MeasureView mView = (MeasureView)findViewById(R.id.measureView);}}