Android 用户界面---定制组件（Custom Components）

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;
 
public class CrossView extends View {
   private float mRotation;
    private Paint mPaint;
    public CrossView( Context context, AttributeSet attrs ) {//AttributeSet对象在实例化时被系统传给了视图
        super( context, attrs );
        //start 创建paint对象
        mPaint = new Paint();
        mPaint.setAntiAlias( true );
        mPaint.setColor( 0xFFFFFFFF );
        //end 创建paint对象
       
        //使用obtainStyledAttributes方法来创建一个TypedArray，这是访问存储于AttributeSet中的值的一个方便类，这类执行内部缓存，
        //所以当你结束使用它之后随时调用回收函数。注意：需同时使用<declare-styleable>名称和<arr>名称的访问自定义属性
        TypedArray arr = getContext().obtainStyledAttributes( attrs,R.styleable.cross );
        int color = arr.getColor( R.styleable.cross_android_color, Color.WHITE );
        float rotation = arr.getFloat( R.styleable.cross_rotation, 0f );
        //remember to call this when finished
        arr.recycle();
        setColor(color);
        setRotation(rotation);
    }
    private void setRotation( float rotation ) {
        // TODO Auto-generated method stub
        mRotation = rotation;
    }
    private void setColor( int color ) {
        // TODO Auto-generated method stub
        mPaint.setColor( color );
    }
    //重写onMeasure方法
    @Override
    protected void onMeasure( int widthMeasureSpec, int heightMeasureSpec ) {
        // TODO Auto-generated method stub
        super.onMeasure( widthMeasureSpec, heightMeasureSpec );
        //需要使用计算好的宽和高的值作为该方法的实参
        setMeasuredDimension( calculateMeasure(widthMeasureSpec), calculateMeasure(heightMeasureSpec) );
    }
    float[] mPoints = {0.5f,0f,0.5f,1f,0f,0.5f,1f,0.5f};
    @Override
    protected void onDraw( Canvas canvas ) {
        // TODO Auto-generated method stub
        super.onDraw( canvas );
        canvas.save();//所有的在画布上绘图的调用都应当受对应的sava（）和restore（）的约束
        int scale = getWidth();
        canvas.scale( scale, scale );
        canvas.rotate( mRotation );
        canvas.drawLines( mPoints, mPaint );//绘制十字的两条线
        canvas.restore();//所有的在画布上绘图的调用都应当受对应的sava（）和restore（）的约束
    }
    
    private static final int DEFAULT_SIZE = 100;//默认的试图尺寸
    //实现计算测量值的代码
    private int calculateMeasure(int measureSpec){
        int result = ( int ) ( DEFAULT_SIZE*getResources().getDisplayMetrics().density );
        int specMode = MeasureSpec.getMode( measureSpec );//在MeasureSpec中检索模式
        int specSize = MeasureSpec.getSize( measureSpec );//在MeasureSpec中检索尺寸
        //基于模式选择尺寸
        if(specMode == MeasureSpec.EXACTLY){
            result = specSize;
        }else if(specMode == MeasureSpec.AT_MOST){
            result = Math.min( result, specSize );
        }
        return result;
    }
}
（二）自定义属性
 <?xml version="1.0" encoding="utf-8"?>
<!-- 该文件被放置在res/values/目录下 -->
<resources>
   <!-- 声明属性 -->
   <declare-styleable name="cross">
     <attr name="android:color"/>
     <attr name="rotation" format="string"/>
   </declare-styleable>
   <!-- 
   <attr name="test" format="string"/>
   <declare-styleable name="foo">
       <attr name="test"/>
   </declare-styleable>
   <declare-styleable name="bar">
       <attr name="test"/>
   </declare-styleable> -->
</resources>
（三）在XML中使用自定义View
<?xml version="1.0" encoding="utf-8"?>
 <!-- 要使用在XML中的自定义属性，首先必须为视图声明命名空间 -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              xmlns:example="http://schemas.android.com/apk/res/com.example"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <!-- 添加CrossView -->
    <com.example.CrossView
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
    />
    <com.example.CrossView
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       example.rotation="30" 
       android:color="#0000FF"/>
    />
    <com.example.CrossView
        android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       example.rotation="40" 
       android:color="#FFFF00"
    />
</LinearLayout>