Android應用開發基礎篇（16）—–ScaleGestureDetector（縮放手勢檢測）

一、概述

      ScaleGestureDetector這個類是專門用來檢測兩個手指在螢幕上做縮放的手勢用的，最簡單的應用就是用來縮放圖片或者縮放網頁。

二、要求

     利用ScaleGestureDetector這個類實現圖片縮放。

三、實現

     建立工程MyScale，修改main.xml檔案，在裡面添加一個Button和一個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="顯示圖片"
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檔案，主要是實現按鈕的監聽和定義一個類實現ScaleGestureDetector.OnScaleGestureListener介面：

 

  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         //按鈕監聽
 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                 //鎖定整個SurfaceView
 49                 Canvas mCanvas = mSurfaceHolder.lockCanvas();
 50                 //畫圖
 51                 mCanvas.drawBitmap(mBitmap, 0f, 0f, null);
 52                 //繪製完成，提交修改
 53                 mSurfaceHolder.unlockCanvasAndPost(mCanvas);
 54                 //重新鎖一次
 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         //返回給ScaleGestureDetector來處理
 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             //縮放比例
 80             float scale = detector.getScaleFactor()/3;
 81             mMatrix.setScale(scale, scale);
 82             
 83             //鎖定整個SurfaceView
 84             Canvas mCanvas = mSurfaceHolder.lockCanvas();
 85             //清屏
 86             mCanvas.drawColor(Color.BLACK);
 87             //畫縮放後的圖
 88             mCanvas.drawBitmap(mBitmap, mMatrix, null);
 89             //繪製完成，提交修改
 90             mSurfaceHolder.unlockCanvasAndPost(mCanvas);
 91             //重新鎖一次
 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才會進入onScale()這個函數
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 }

運行該程式：

 

點擊一下“顯示圖片”按鈕：

 

然後用兩個手指縮小圖片：

 

放大圖片：

 

好了。可以發現，每次一開始縮放圖片的時候都有一個突變，如果用來做應用的話這個程式還需要改善。

PS：圖片來源於互連網