android放大镜效果实现

我相信很多用过英语应用的同学都看多一个放大镜的效果，就是选中一段文字后，会有一个放大镜，这个究竟怎么实现的呢，我们今天来分析分析。

public class ShaderView extends View {  
    private final Bitmap bitmap;  
    private final 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 final Matrix matrix = new Matrix();  
  
    public ShaderView(Context context) {  
        super(context);  
        Bitmap bmp = BitmapFactory.decodeResource(getResources(),R.drawable.demo);  
        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);  
        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();  
        // 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();  
        return true;  
    }  
  
    @Override  
    public void onDraw(Canvas canvas) {  
        super.onDraw(canvas);  
        canvas.drawBitmap(bitmap, 0, 0, null);  
        drawable.draw(canvas);  
    }  
}  

基本原理就是使用ShapeDrawable构造一个圆形的drawable，然后它的paint的shader设置为将要放大的图片，然后就是简单的位置移动问题了。放大镜的半径和放大倍数都可以在代码里面修改，代码都有注释，应该很好理解了。不过，一个问题如果只有一种解决方法的话，那未免有点令人沮丧，想玩点另类的都不行。玩程序就得玩出个性，玩出激情。哈哈，废话太多，切回正题。再来看看放大镜的另外一种实现吧

copy

public class PathView extends View {  
    private final Path mPath = new Path();  
    private final Matrix matrix = new Matrix();  
    private final Bitmap bitmap;  
    // 放大镜的半径  
  
    private static final int RADIUS = 80;  
    // 放大倍数  
  
    private static final int FACTOR = 2;  
    private int mCurrentX, mCurrentY;  
  
    public PathView(Context context) {  
        super(context);  
        mPath.addCircle(RADIUS, RADIUS, RADIUS, Direction.CW);  
        matrix.setScale(FACTOR, FACTOR);  
  
        bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.demo);  
    }  
  
    @Override  
    public boolean onTouchEvent(MotionEvent event) {  
        mCurrentX = (int) event.getX();  
        mCurrentY = (int) event.getY();  
  
        invalidate();  
        return true;  
    }  
  
    @Override  
    public void onDraw(Canvas canvas) {  
        super.onDraw(canvas);  
        // 底图  
  
        canvas.drawBitmap(bitmap, 0, 0, null);  
        // 剪切  
  
        canvas.translate(mCurrentX - RADIUS, mCurrentY - RADIUS);  
        canvas.clipPath(mPath);  
        // 画放大后的图  
  
        canvas.translate(RADIUS - mCurrentX * FACTOR, RADIUS - mCurrentY  
                * FACTOR);  
        canvas.drawBitmap(bitmap, matrix, null);  
    }  
}  

这里使用的是Path类，将canvas剪切出一块圆形区域，在其上绘制放大的部分。

工程下载地址http://download.csdn.net/detail/hustpzb/4523274