用GestureDetector实现ViewPager功能

本节目的：用android手势事件做一个图片浏览器。
简述：可能一见到图片浏览器，大家脑海里首先出现的是ViewPager吧。但其实用GestureDetector也可以实现，
优点是：
1.简单。
2.消耗内存少。经过笔者测试，这种方式在24张图片下依然不会出现OOM异常。而ViewPager呢，笔者手机里放7张就不行了。
（这种方法其实替换掉了原来的图片，本身就只会有一张图片在内存）
3.图片可以随意拉伸
缺点是：
没有滑动的动画效果。

至于以后选择哪种方式实现，读者自行选择。

GestureDetector的详细用法参见：
http://blog.csdn.net/getchance/article/details/8478993

步骤：
1.视图文件中：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <ImageView
        android:id="@+id/imageview"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        />

</RelativeLayout>

2.控制文件中：

package com.example.imagebrowser;

import android.app.Activity;
import android.content.res.Resources;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.view.GestureDetector;
import android.view.MotionEvent;
import android.widget.ImageView;

public class MainActivity extends Activity {

    //Data field
    private int[] mImageId;
    private ImageView mImageView;
    private int mCurrentPosition=0;
    final int mRightFlag = 0;  
    final int mLeftFlag = 1;  
    private GestureDetector gestureDetector;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initComponent();
        setImage();
        processGestureDetector();
    }

    //The method is used to init UI components
    private void initComponent(){
        mImageView=(ImageView)findViewById(R.id.imageview);
    }

    //The method is used to set image
    private void setImage(){
        mImageId=new int[]{R.drawable.pic1,R.drawable.pic2,R.drawable.pic3,R.drawable.pic4,
                R.drawable.pic5,R.drawable.pic6,R.drawable.pic7,R.drawable.pic8};
        Resources resources = getBaseContext().getResources();   
        Drawable imageDrawable = resources.getDrawable(mImageId[mCurrentPosition]);
        mImageView.setBackgroundDrawable(imageDrawable);
    }

    //The method is used to process GestureDetector 
    private void processGestureDetector(){
        GestureDetector.OnGestureListener onGestureListener =new GestureDetector.SimpleOnGestureListener() {//实例化监听器 

            @Override  
            public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,  
                    float velocityY) {  
                float x = e2.getX() - e1.getX();  
                float y = e2.getY() - e1.getY();  

                if (x > 0) {  
                    doResult(mRightFlag);  
                } else if (x < 0) {  
                    doResult(mLeftFlag);  
                }  
                return true;  
            }  
        };
        gestureDetector = new GestureDetector(MainActivity.this,onGestureListener);//用此监听器生成GestureDetector实例
    }

    //The method override onTouchEvent method
    public boolean onTouchEvent(MotionEvent event) {

        return gestureDetector.onTouchEvent(event);  
    }  

    //The method to handle switch behavior :switch image
    public void doResult(int action) {  

        switch (action) {  
        case mRightFlag:  
            mCurrentPosition=(mCurrentPosition+mImageId.length+1)%mImageId.length;
            setImage();
            break;  

        case mLeftFlag:  
            mCurrentPosition=(mCurrentPosition+mImageId.length-1)%mImageId.length;
            setImage();
            break;  

        }  
    }  

}

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