android放大镜效果实现

概述

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。

源码分析

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