Android放大镜效果的简单实现

package com.example.myapi.pictobig;
import com.example.myapi.R;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapShader;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.graphics.Shader.TileMode;
import android.graphics.drawable.ShapeDrawable;
import android.graphics.drawable.shapes.OvalShape;
import android.view.MotionEvent;
import android.view.View;
import android.widget.ImageView;

/**
 * 图片放大镜工具类
 * @author yw-tony
 *
 */
public class PicToBigTools extends ImageView{
    private Bitmap bitmap;
    private ShapeDrawable drawable;
    /*放大镜的半径*/
    private static final int RADIUS = 80;
    /*放大倍数*/
    private static final int FACTOR = 3;
    private Matrix matrix = new Matrix();
    /**
     * 构造方法
     * @param context
     */
    public PicToBigTools(Context context) {
        super(context);
        Bitmap bmp = BitmapFactory.decodeResource(getResources(), R.drawable.pictobig_icon);
        bitmap = bmp;
        BitmapShader shader = new BitmapShader(
                Bitmap.createScaledBitmap(bmp, bmp.getWidth()*FACTOR,
                        bmp.getHeight()*FACTOR, true), TileMode.CLAMP, TileMode.CLAMP);
        //圆形的drawable
        drawable = new ShapeDrawable(new OvalShape());  

        drawable.getPaint().setShader(shader);
        /*设置放大的区域*/
        drawable.setBounds(0, 0, RADIUS*2, RADIUS*2);
    }     

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        final int x = (int) event.getX();
        final int y = (int) event.getY();  

        /**设置图片等比例缩放*/
        // 获得图片的宽高
        int width = bitmap.getWidth();
        int height = bitmap.getHeight();
         // 计算缩放比例
        float scaleWidth = ((float) RADIUS*2) / width;
        float scaleHeight = ((float) RADIUS*2) / height;
        matrix.postScale(scaleWidth,scaleHeight);

        //这个位置表示的是，画shader的起始位置
        matrix.setTranslate(RADIUS-x*FACTOR, RADIUS-y*FACTOR);
        drawable.getPaint().getShader().setLocalMatrix(matrix);
        //bounds，就是那个圆的外切矩形
        drawable.setBounds(x-RADIUS, y-RADIUS, x+RADIUS, y+RADIUS);
        invalidate();//更新UI
        return true;
    }  

    @Override
    public void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawBitmap(bitmap, 0, 0, null);
        drawable.draw(canvas);//绘制图片
    }
}