Android之ScaleGestureDetector（缩放手势检测）

一、概述

ScaleGestureDetector这个类是专门用来检测两个手指在屏幕上做缩放的手势用的，最简单的应用就是用来缩放图片或者缩放网页。

二、要求

利用ScaleGestureDetector这个类实现图片缩放。

三、实现

新建工程MyScale，修改main.xml文件，在里面添加一个Button和一个SurfaceView，如下：

1 <?xml version="1.0" encoding="utf-8"?>
2 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
3     android:layout_width="fill_parent"
4     android:layout_height="fill_parent"
5     android:orientation="vertical" >
6
7     <Button
8         android:id="@+id/button"
9         android:layout_width="fill_parent"
10         android:layout_height="wrap_content"
11         android:text="显示图片"
12         />
13
14     <SurfaceView
15         android:id="@+id/surfaceview"
16         android:layout_width="fill_parent"
17         android:layout_height="fill_parent"
18         />
19
20 </LinearLayout>

修改MyScaleActivity.java文件，主要是实现按钮的监听和定义一个类实现ScaleGestureDetector.OnScaleGestureListener接口：

1 package com.nan.scale;
2
3 import android.app.Activity;
4 import android.graphics.Bitmap;
5 import android.graphics.BitmapFactory;
6 import android.graphics.Canvas;
7 import android.graphics.Color;
8 import android.graphics.Matrix;
9 import android.graphics.Rect;
10 import android.os.Bundle;
11 import android.view.MotionEvent;
12 import android.view.ScaleGestureDetector;
13 import android.view.SurfaceHolder;
14 import android.view.SurfaceView;
15 import android.view.View;
16 import android.widget.Button;
17
18
19
20 public class MyScaleActivity extends Activity
21 {
22     private Button mButton = null;
23     private SurfaceView mSurfaceView = null;
24     private SurfaceHolder mSurfaceHolder = null;
25     private ScaleGestureDetector mScaleGestureDetector = null;
26     private Bitmap mBitmap = null;
27
28     /** Called when the activity is first created. */
29     @Override
30     public void onCreate(Bundle savedInstanceState)
31     {
32         super.onCreate(savedInstanceState);
33         setContentView(R.layout.main);
34
35         mSurfaceView = (SurfaceView)this.findViewById(R.id.surfaceview);
36         mSurfaceHolder = mSurfaceView.getHolder();
37         mScaleGestureDetector = new ScaleGestureDetector(this,new ScaleGestureListener());
38         mButton = (Button)this.findViewById(R.id.button);
39         //按钮监听
40         mButton.setOnClickListener(new View.OnClickListener()
41         {
42
43             @Override
44             public void onClick(View v)
45             {
46                 // TODO Auto-generated method stub
47                 mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.mmm);
48                 //锁定整个SurfaceView
49                 Canvas mCanvas = mSurfaceHolder.lockCanvas();
50                 //画图
51                 mCanvas.drawBitmap(mBitmap, 0f, 0f, null);
52                 //绘制完成，提交修改
53                 mSurfaceHolder.unlockCanvasAndPost(mCanvas);
54                 //重新锁一次
55                 mSurfaceHolder.lockCanvas(new Rect(0, 0, 0, 0));
56                 mSurfaceHolder.unlockCanvasAndPost(mCanvas);
57             }
58         });
59
60     }
61
62     @Override
63     public boolean onTouchEvent(MotionEvent event)
64     {
65         //返回给ScaleGestureDetector来处理
66         return mScaleGestureDetector.onTouchEvent(event);
67     }
68
69
70     public class ScaleGestureListener implements ScaleGestureDetector.OnScaleGestureListener
71     {
72
73         @Override
74         public boolean onScale(ScaleGestureDetector detector)
75         {
76             // TODO Auto-generated method stub
77
78             Matrix mMatrix = new Matrix();
79             //缩放比例
80             float scale = detector.getScaleFactor()/3;
81             mMatrix.setScale(scale, scale);
82
83             //锁定整个SurfaceView
84             Canvas mCanvas = mSurfaceHolder.lockCanvas();
85             //清屏
86             mCanvas.drawColor(Color.BLACK);
87             //画缩放后的图
88             mCanvas.drawBitmap(mBitmap, mMatrix, null);
89             //绘制完成，提交修改
90             mSurfaceHolder.unlockCanvasAndPost(mCanvas);
91             //重新锁一次
92             mSurfaceHolder.lockCanvas(new Rect(0, 0, 0, 0));
93             mSurfaceHolder.unlockCanvasAndPost(mCanvas);
94
95             return false;
96         }
97
98         @Override
99         public boolean onScaleBegin(ScaleGestureDetector detector)
100         {
101             // TODO Auto-generated method stub
102             //一定要返回true才会进入onScale()这个函数
103             return true;
104         }
105
106         @Override
107         public void onScaleEnd(ScaleGestureDetector detector)
108         {
109             // TODO Auto-generated method stub
110
111         }
112
113     }
114
115 }

运行该程序：



点击一下“显示图片”按钮：



然后用两个手指缩小图片：



放大图片：



好了。可以发现，每次一开始缩放图片的时候都有一个突变，如果用来做应用的话这个程序还需要改善。