Android.异步加载图片.使用LruCache和SD卡或手机缓存

技术概述:

1.使用了线程池来管理下载任务
2.使用LruCache来缓存图片
3.使用手机来缓存图片
4.GridView滑动的时候取消下载任务，静止的时候进行下载，GridView滑动更加的流畅
5.降低了代码的耦合性，结构更加的清晰，便于以后重用

接下来我们先来看看项目的结构:

FileUtils 文件操作的工具类，提供保存图片，获取图片，判断图片是否存在，删除图片的一些方法，这个类比较简单

package com.example.asyncimageloader;import java.io.File;import java.io.FileOutputStream;import java.io.IOException;import android.content.Context;import android.graphics.Bitmap;import android.graphics.Bitmap.CompressFormat;import android.graphics.BitmapFactory;import android.os.Environment;public class FileUtils {    /**     * sd卡的根目录     */    private static String mSdRootPath = Environment.getExternalStorageDirectory().getPath();    /**     * 手机的缓存根目录     */    private static String mDataRootPath = null;    /**     * 保存Image的目录名     */    private final static String FOLDER_NAME = "/AndroidImage";    public FileUtils(Context context){        mDataRootPath = context.getCacheDir().getPath();    }    /**     * 获取储存Image的目录     * @return     */    private String getStorageDirectory(){        return Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED) ?                mSdRootPath + FOLDER_NAME : mDataRootPath + FOLDER_NAME;    }    /**     * 保存Image的方法，有sd卡存储到sd卡，没有就存储到手机目录     * @param fileName      * @param bitmap        * @throws IOException     */    public void savaBitmap(String fileName, Bitmap bitmap) throws IOException{        if(bitmap == null){            return;        }        String path = getStorageDirectory();        File folderFile = new File(path);        if(!folderFile.exists()){            folderFile.mkdir();        }        File file = new File(path + File.separator + fileName);        file.createNewFile();        FileOutputStream fos = new FileOutputStream(file);        bitmap.compress(CompressFormat.JPEG, 100, fos);        fos.flush();        fos.close();    }    /**     * 从手机或者sd卡获取Bitmap     * @param fileName     * @return     */    public Bitmap getBitmap(String fileName){        return BitmapFactory.decodeFile(getStorageDirectory() + File.separator + fileName);    }    /**     * 判断文件是否存在     * @param fileName     * @return     */    public boolean isFileExists(String fileName){        return new File(getStorageDirectory() + File.separator + fileName).exists();    }    /**     * 获取文件的大小     * @param fileName     * @return     */    public long getFileSize(String fileName) {        return new File(getStorageDirectory() + File.separator + fileName).length();    }    /**     * 删除SD卡或者手机的缓存图片和目录     */    public void deleteFile() {        File dirFile = new File(getStorageDirectory());        if(! dirFile.exists()){            return;        }        if (dirFile.isDirectory()) {            String[] children = dirFile.list();            for (int i = 0; i < children.length; i++) {                new File(dirFile, children[i]).delete();            }        }        dirFile.delete();    }}

Images类，提供图片资源的类，来自Android文档中BitmapFun中http://developer.android.com/training/displaying-bitmaps/index.html，里面一个静态的图片Url数组，这里面比较坑爹，因为是图片是google官网提供的，有时候会下载超时

package com.example.asyncimageloader;public class Images {    public final static String[] imageThumbUrls = new String[] {            "https://lh6.googleusercontent.com/-55osAWw3x0Q/URquUtcFr5I/AAAAAAAAAbs/rWlj1RUKrYI/s160-c/A%252520Photographer.jpg",            "https://lh4.googleusercontent.com/--dq8niRp7W4/URquVgmXvgI/AAAAAAAAAbs/-gnuLQfNnBA/s160-c/A%252520Song%252520of%252520Ice%252520and%252520Fire.jpg",            "https://lh5.googleusercontent.com/-7qZeDtRKFKc/URquWZT1gOI/AAAAAAAAAbs/hqWgteyNXsg/s160-c/Another%252520Rockaway%252520Sunset.jpg",            "https://lh3.googleusercontent.com/--L0Km39l5J8/URquXHGcdNI/AAAAAAAAAbs/3ZrSJNrSomQ/s160-c/Antelope%252520Butte.jpg",            "https://lh6.googleusercontent.com/-8HO-4vIFnlw/URquZnsFgtI/AAAAAAAAAbs/WT8jViTF7vw/s160-c/Antelope%252520Hallway.jpg",            "https://lh4.googleusercontent.com/-WIuWgVcU3Qw/URqubRVcj4I/AAAAAAAAAbs/YvbwgGjwdIQ/s160-c/Antelope%252520Walls.jpg",            "https://lh6.googleusercontent.com/-UBmLbPELvoQ/URqucCdv0kI/AAAAAAAAAbs/IdNhr2VQoQs/s160-c/Apre%2525CC%252580s%252520la%252520Pluie.jpg",            "https://lh3.googleusercontent.com/-s-AFpvgSeew/URquc6dF-JI/AAAAAAAAAbs/Mt3xNGRUd68/s160-c/Backlit%252520Cloud.jpg",            "https://lh5.googleusercontent.com/-bvmif9a9YOQ/URquea3heHI/AAAAAAAAAbs/rcr6wyeQtAo/s160-c/Bee%252520and%252520Flower.jpg",            "https://lh5.googleusercontent.com/-n7mdm7I7FGs/URqueT_BT-I/AAAAAAAAAbs/9MYmXlmpSAo/s160-c/Bonzai%252520Rock%252520Sunset.jpg",            "https://lh6.googleusercontent.com/-4CN4X4t0M1k/URqufPozWzI/AAAAAAAAAbs/8wK41lg1KPs/s160-c/Caterpillar.jpg",            "https://lh3.googleusercontent.com/-rrFnVC8xQEg/URqufdrLBaI/AAAAAAAAAbs/s69WYy_fl1E/s160-c/Chess.jpg",            "https://lh5.googleusercontent.com/-WVpRptWH8Yw/URqugh-QmDI/AAAAAAAAAbs/E-MgBgtlUWU/s160-c/Chihuly.jpg",            "https://lh5.googleusercontent.com/-0BDXkYmckbo/URquhKFW84I/AAAAAAAAAbs/ogQtHCTk2JQ/s160-c/Closed%252520Door.jpg",            "https://lh3.googleusercontent.com/-PyggXXZRykM/URquh-kVvoI/AAAAAAAAAbs/hFtDwhtrHHQ/s160-c/Colorado%252520River%252520Sunset.jpg",            "https://lh3.googleusercontent.com/-ZAs4dNZtALc/URquikvOCWI/AAAAAAAAAbs/DXz4h3dll1Y/s160-c/Colors%252520of%252520Autumn.jpg",            "https://lh4.googleusercontent.com/-GztnWEIiMz8/URqukVCU7bI/AAAAAAAAAbs/jo2Hjv6MZ6M/s160-c/Countryside.jpg",            "https://lh4.googleusercontent.com/-bEg9EZ9QoiM/URquklz3FGI/AAAAAAAAAbs/UUuv8Ac2BaE/s160-c/Death%252520Valley%252520-%252520Dunes.jpg",            "https://lh6.googleusercontent.com/-ijQJ8W68tEE/URqulGkvFEI/AAAAAAAAAbs/zPXvIwi_rFw/s160-c/Delicate%252520Arch.jpg",            "https://lh5.googleusercontent.com/-Oh8mMy2ieng/URqullDwehI/AAAAAAAAAbs/TbdeEfsaIZY/s160-c/Despair.jpg",            "https://lh5.googleusercontent.com/-gl0y4UiAOlk/URqumC_KjBI/AAAAAAAAAbs/PM1eT7dn4oo/s160-c/Eagle%252520Fall%252520Sunrise.jpg",            "https://lh3.googleusercontent.com/-hYYHd2_vXPQ/URqumtJa9eI/AAAAAAAAAbs/wAalXVkbSh0/s160-c/Electric%252520Storm.jpg",            "https://lh5.googleusercontent.com/-PyY_yiyjPTo/URqunUOhHFI/AAAAAAAAAbs/azZoULNuJXc/s160-c/False%252520Kiva.jpg",            "https://lh6.googleusercontent.com/-PYvLVdvXywk/URqunwd8hfI/AAAAAAAAAbs/qiMwgkFvf6I/s160-c/Fitzgerald%252520Streaks.jpg",            "https://lh4.googleusercontent.com/-KIR_UobIIqY/URquoCZ9SlI/AAAAAAAAAbs/Y4d4q8sXu4c/s160-c/Foggy%252520Sunset.jpg",            "https://lh6.googleusercontent.com/-9lzOk_OWZH0/URquoo4xYoI/AAAAAAAAAbs/AwgzHtNVCwU/s160-c/Frantic.jpg",            "https://lh3.googleusercontent.com/-0X3JNaKaz48/URqupH78wpI/AAAAAAAAAbs/lHXxu_zbH8s/s160-c/Golden%252520Gate%252520Afternoon.jpg",            "https://lh6.googleusercontent.com/-95sb5ag7ABc/URqupl95RDI/AAAAAAAAAbs/g73R20iVTRA/s160-c/Golden%252520Gate%252520Fog.jpg",            "https://lh3.googleusercontent.com/-JB9v6rtgHhk/URqup21F-zI/AAAAAAAAAbs/64Fb8qMZWXk/s160-c/Golden%252520Grass.jpg",            "https://lh4.googleusercontent.com/-EIBGfnuLtII/URquqVHwaRI/AAAAAAAAAbs/FA4McV2u8VE/s160-c/Grand%252520Teton.jpg",            "https://lh4.googleusercontent.com/-WoMxZvmN9nY/URquq1v2AoI/AAAAAAAAAbs/grj5uMhL6NA/s160-c/Grass%252520Closeup.jpg",            "https://lh3.googleusercontent.com/-6hZiEHXx64Q/URqurxvNdqI/AAAAAAAAAbs/kWMXM3o5OVI/s160-c/Green%252520Grass.jpg",            "https://lh5.googleusercontent.com/-6LVb9OXtQ60/URquteBFuKI/AAAAAAAAAbs/4F4kRgecwFs/s160-c/Hanging%252520Leaf.jpg",            "https://lh4.googleusercontent.com/-zAvf__52ONk/URqutT_IuxI/AAAAAAAAAbs/D_bcuc0thoU/s160-c/Highway%2525201.jpg",            "https://lh6.googleusercontent.com/-H4SrUg615rA/URquuL27fXI/AAAAAAAAAbs/4aEqJfiMsOU/s160-c/Horseshoe%252520Bend%252520Sunset.jpg",            "https://lh4.googleusercontent.com/-JhFi4fb_Pqw/URquuX-QXbI/AAAAAAAAAbs/IXpYUxuweYM/s160-c/Horseshoe%252520Bend.jpg",            "https://lh5.googleusercontent.com/-UGgssvFRJ7g/URquueyJzGI/AAAAAAAAAbs/yYIBlLT0toM/s160-c/Into%252520the%252520Blue.jpg",            "https://lh3.googleusercontent.com/-CH7KoupI7uI/URquu0FF__I/AAAAAAAAAbs/R7GDmI7v_G0/s160-c/Jelly%252520Fish%2525202.jpg",            "https://lh4.googleusercontent.com/-pwuuw6yhg8U/URquvPxR3FI/AAAAAAAAAbs/VNGk6f-tsGE/s160-c/Jelly%252520Fish%2525203.jpg",            "https://lh5.googleusercontent.com/-GoUQVw1fnFw/URquv6xbC0I/AAAAAAAAAbs/zEUVTQQ43Zc/s160-c/Kauai.jpg",            "https://lh6.googleusercontent.com/-8QdYYQEpYjw/URquwvdh88I/AAAAAAAAAbs/cktDy-ysfHo/s160-c/Kyoto%252520Sunset.jpg",            "https://lh4.googleusercontent.com/-vPeekyDjOE0/URquwzJ28qI/AAAAAAAAAbs/qxcyXULsZrg/s160-c/Lake%252520Tahoe%252520Colors.jpg",            "https://lh4.googleusercontent.com/-xBPxWpD4yxU/URquxWHk8AI/AAAAAAAAAbs/ARDPeDYPiMY/s160-c/Lava%252520from%252520the%252520Sky.jpg",            "https://lh3.googleusercontent.com/-897VXrJB6RE/URquxxxd-5I/AAAAAAAAAbs/j-Cz4T4YvIw/s160-c/Leica%25252050mm%252520Summilux.jpg",            "https://lh5.googleusercontent.com/-qSJ4D4iXzGo/URquyDWiJ1I/AAAAAAAAAbs/k2pBXeWehOA/s160-c/Leica%25252050mm%252520Summilux.jpg",            "https://lh6.googleusercontent.com/-dwlPg83vzLg/URquylTVuFI/AAAAAAAAAbs/G6SyQ8b4YsI/s160-c/Leica%252520M8%252520%252528Front%252529.jpg",            "https://lh3.googleusercontent.com/-R3_EYAyJvfk/URquzQBv8eI/AAAAAAAAAbs/b9xhpUM3pEI/s160-c/Light%252520to%252520Sand.jpg",            "https://lh3.googleusercontent.com/-fHY5h67QPi0/URqu0Cp4J1I/AAAAAAAAAbs/0lG6m94Z6vM/s160-c/Little%252520Bit%252520of%252520Paradise.jpg",            "https://lh5.googleusercontent.com/-TzF_LwrCnRM/URqu0RddPOI/AAAAAAAAAbs/gaj2dLiuX0s/s160-c/Lone%252520Pine%252520Sunset.jpg",            "https://lh3.googleusercontent.com/-4HdpJ4_DXU4/URqu046dJ9I/AAAAAAAAAbs/eBOodtk2_uk/s160-c/Lonely%252520Rock.jpg",            "https://lh6.googleusercontent.com/-erbF--z-W4s/URqu1ajSLkI/AAAAAAAAAbs/xjDCDO1INzM/s160-c/Longue%252520Vue.jpg",            "https://lh6.googleusercontent.com/-0CXJRdJaqvc/URqu1opNZNI/AAAAAAAAAbs/PFB2oPUU7Lk/s160-c/Look%252520Me%252520in%252520the%252520Eye.jpg",            "https://lh3.googleusercontent.com/-D_5lNxnDN6g/URqu2Tk7HVI/AAAAAAAAAbs/p0ddca9W__Y/s160-c/Lost%252520in%252520a%252520Field.jpg",            "https://lh6.googleusercontent.com/-flsqwMrIk2Q/URqu24PcmjI/AAAAAAAAAbs/5ocIH85XofM/s160-c/Marshall%252520Beach%252520Sunset.jpg",            "https://lh4.googleusercontent.com/-Y4lgryEVTmU/URqu28kG3gI/AAAAAAAAAbs/OjXpekqtbJ4/s160-c/Mono%252520Lake%252520Blue.jpg",            "https://lh4.googleusercontent.com/-AaHAJPmcGYA/URqu3PIldHI/AAAAAAAAAbs/lcTqk1SIcRs/s160-c/Monument%252520Valley%252520Overlook.jpg",            "https://lh4.googleusercontent.com/-vKxfdQ83dQA/URqu31Yq_BI/AAAAAAAAAbs/OUoGk_2AyfM/s160-c/Moving%252520Rock.jpg",            "https://lh5.googleusercontent.com/-CG62QiPpWXg/URqu4ia4vRI/AAAAAAAAAbs/0YOdqLAlcAc/s160-c/Napali%252520Coast.jpg",            "https://lh6.googleusercontent.com/-wdGrP5PMmJQ/URqu5PZvn7I/AAAAAAAAAbs/m0abEcdPXe4/s160-c/One%252520Wheel.jpg",            "https://lh6.googleusercontent.com/-6WS5DoCGuOA/URqu5qx1UgI/AAAAAAAAAbs/giMw2ixPvrY/s160-c/Open%252520Sky.jpg",            "https://lh6.googleusercontent.com/-u8EHKj8G8GQ/URqu55sM6yI/AAAAAAAAAbs/lIXX_GlTdmI/s160-c/Orange%252520Sunset.jpg",            "https://lh6.googleusercontent.com/-74Z5qj4bTDE/URqu6LSrJrI/AAAAAAAAAbs/XzmVkw90szQ/s160-c/Orchid.jpg",            "https://lh6.googleusercontent.com/-lEQE4h6TePE/URqu6t_lSkI/AAAAAAAAAbs/zvGYKOea_qY/s160-c/Over%252520there.jpg",            "https://lh5.googleusercontent.com/-cauH-53JH2M/URqu66v_USI/AAAAAAAAAbs/EucwwqclfKQ/s160-c/Plumes.jpg",            "https://lh3.googleusercontent.com/-eDLT2jHDoy4/URqu7axzkAI/AAAAAAAAAbs/iVZE-xJ7lZs/s160-c/Rainbokeh.jpg",            "https://lh5.googleusercontent.com/-j1NLqEFIyco/URqu8L1CGcI/AAAAAAAAAbs/aqZkgX66zlI/s160-c/Rainbow.jpg",            "https://lh5.googleusercontent.com/-DRnqmK0t4VU/URqu8XYN9yI/AAAAAAAAAbs/LgvF_592WLU/s160-c/Rice%252520Fields.jpg",            "https://lh3.googleusercontent.com/-hwh1v3EOGcQ/URqu8qOaKwI/AAAAAAAAAbs/IljRJRnbJGw/s160-c/Rockaway%252520Fire%252520Sky.jpg",            "https://lh5.googleusercontent.com/-wjV6FQk7tlk/URqu9jCQ8sI/AAAAAAAAAbs/RyYUpdo-c9o/s160-c/Rockaway%252520Flow.jpg",            "https://lh6.googleusercontent.com/-6cAXNfo7D20/URqu-BdzgPI/AAAAAAAAAbs/OmsYllzJqwo/s160-c/Rockaway%252520Sunset%252520Sky.jpg",            "https://lh3.googleusercontent.com/-sl8fpGPS-RE/URqu_BOkfgI/AAAAAAAAAbs/Dg2Fv-JxOeg/s160-c/Russian%252520Ridge%252520Sunset.jpg",            "https://lh6.googleusercontent.com/-gVtY36mMBIg/URqu_q91lkI/AAAAAAAAAbs/3CiFMBcy5MA/s160-c/Rust%252520Knot.jpg",            "https://lh6.googleusercontent.com/-GHeImuHqJBE/URqu_FKfVLI/AAAAAAAAAbs/axuEJeqam7Q/s160-c/Sailing%252520Stones.jpg",            "https://lh3.googleusercontent.com/-hBbYZjTOwGc/URqu_ycpIrI/AAAAAAAAAbs/nAdJUXnGJYE/s160-c/Seahorse.jpg",            "https://lh3.googleusercontent.com/-Iwi6-i6IexY/URqvAYZHsVI/AAAAAAAAAbs/5ETWl4qXsFE/s160-c/Shinjuku%252520Street.jpg",            "https://lh6.googleusercontent.com/-amhnySTM_MY/URqvAlb5KoI/AAAAAAAAAbs/pFCFgzlKsn0/s160-c/Sierra%252520Heavens.jpg",            "https://lh5.googleusercontent.com/-dJgjepFrYSo/URqvBVJZrAI/AAAAAAAAAbs/v-F5QWpYO6s/s160-c/Sierra%252520Sunset.jpg",            "https://lh4.googleusercontent.com/-Z4zGiC5nWdc/URqvBdEwivI/AAAAAAAAAbs/ZRZR1VJ84QA/s160-c/Sin%252520Lights.jpg",            "https://lh4.googleusercontent.com/-_0cYiWW8ccY/URqvBz3iM4I/AAAAAAAAAbs/9N_Wq8MhLTY/s160-c/Starry%252520Lake.jpg",            "https://lh3.googleusercontent.com/-A9LMoRyuQUA/URqvCYx_JoI/AAAAAAAAAbs/s7sde1Bz9cI/s160-c/Starry%252520Night.jpg",            "https://lh3.googleusercontent.com/-KtLJ3k858eY/URqvC_2h_bI/AAAAAAAAAbs/zzEBImwDA_g/s160-c/Stream.jpg",            "https://lh5.googleusercontent.com/-dFB7Lad6RcA/URqvDUftwWI/AAAAAAAAAbs/BrhoUtXTN7o/s160-c/Strip%252520Sunset.jpg",            "https://lh5.googleusercontent.com/-at6apgFiN20/URqvDyffUZI/AAAAAAAAAbs/clABCx171bE/s160-c/Sunset%252520Hills.jpg",            "https://lh4.googleusercontent.com/-7-EHhtQthII/URqvEYTk4vI/AAAAAAAAAbs/QSJZoB3YjVg/s160-c/Tenaya%252520Lake%2525202.jpg",            "https://lh6.googleusercontent.com/-8MrjV_a-Pok/URqvFC5repI/AAAAAAAAAbs/9inKTg9fbCE/s160-c/Tenaya%252520Lake.jpg",            "https://lh5.googleusercontent.com/-B1HW-z4zwao/URqvFWYRwUI/AAAAAAAAAbs/8Peli53Bs8I/s160-c/The%252520Cave%252520BW.jpg",            "https://lh3.googleusercontent.com/-PO4E-xZKAnQ/URqvGRqjYkI/AAAAAAAAAbs/42nyADFsXag/s160-c/The%252520Fisherman.jpg",            "https://lh4.googleusercontent.com/-iLyZlzfdy7s/URqvG0YScdI/AAAAAAAAAbs/1J9eDKmkXtk/s160-c/The%252520Night%252520is%252520Coming.jpg",            "https://lh6.googleusercontent.com/-G-k7YkkUco0/URqvHhah6fI/AAAAAAAAAbs/_taQQG7t0vo/s160-c/The%252520Road.jpg",            "https://lh6.googleusercontent.com/-h-ALJt7kSus/URqvIThqYfI/AAAAAAAAAbs/ejiv35olWS8/s160-c/Tokyo%252520Heights.jpg",            "https://lh5.googleusercontent.com/-Hy9k-TbS7xg/URqvIjQMOxI/AAAAAAAAAbs/RSpmmOATSkg/s160-c/Tokyo%252520Highway.jpg",            "https://lh6.googleusercontent.com/-83oOvMb4OZs/URqvJL0T7lI/AAAAAAAAAbs/c5TECZ6RONM/s160-c/Tokyo%252520Smog.jpg",            "https://lh3.googleusercontent.com/-FB-jfgREEfI/URqvJI3EXAI/AAAAAAAAAbs/XfyweiRF4v8/s160-c/Tufa%252520at%252520Night.jpg",            "https://lh4.googleusercontent.com/-vngKD5Z1U8w/URqvJUCEgPI/AAAAAAAAAbs/ulxCMVcU6EU/s160-c/Valley%252520Sunset.jpg",            "https://lh6.googleusercontent.com/-DOz5I2E2oMQ/URqvKMND1kI/AAAAAAAAAbs/Iqf0IsInleo/s160-c/Windmill%252520Sunrise.jpg",            "https://lh5.googleusercontent.com/-biyiyWcJ9MU/URqvKculiAI/AAAAAAAAAbs/jyPsCplJOpE/s160-c/Windmill.jpg",            "https://lh4.googleusercontent.com/-PDT167_xRdA/URqvK36mLcI/AAAAAAAAAbs/oi2ik9QseMI/s160-c/Windmills.jpg",            "https://lh5.googleusercontent.com/-kI_QdYx7VlU/URqvLXCB6gI/AAAAAAAAAbs/N31vlZ6u89o/s160-c/Yet%252520Another%252520Rockaway%252520Sunset.jpg",            "https://lh4.googleusercontent.com/-e9NHZ5k5MSs/URqvMIBZjtI/AAAAAAAAAbs/1fV810rDNfQ/s160-c/Yosemite%252520Tree.jpg", };}

ImageDownLoader类，异步下载的核心类，保存图片到手机缓存，将图片加入LruCache中等等

package com.example.asyncimageloader;import java.io.IOException;import java.net.HttpURLConnection;import java.net.URL;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import android.content.Context;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Handler;import android.os.Message;import android.support.v4.util.LruCache;public class ImageDownLoader {    /**     * 缓存Image的类，当存储Image的大小大于LruCache设定的值，系统自动释放内存     */    private LruCache<String, Bitmap> mMemoryCache;    /**     * 操作文件相关类对象的引用     */    private FileUtils fileUtils;    /**     * 下载Image的线程池     */    private ExecutorService mImageThreadPool = null;    public ImageDownLoader(Context context){        //获取系统分配给每个应用程序的最大内存，每个应用系统分配32M        int maxMemory = (int) Runtime.getRuntime().maxMemory();          int mCacheSize = maxMemory / 8;        //给LruCache分配1/8 4M        mMemoryCache = new LruCache<String, Bitmap>(mCacheSize){            //必须重写此方法，来测量Bitmap的大小            @Override            protected int sizeOf(String key, Bitmap value) {                return value.getRowBytes() * value.getHeight();            }        };        fileUtils = new FileUtils(context);    }    /**     * 获取线程池的方法，因为涉及到并发的问题，我们加上同步锁     * @return     */    public ExecutorService getThreadPool(){        if(mImageThreadPool == null){            synchronized(ExecutorService.class){                if(mImageThreadPool == null){                    //为了下载图片更加的流畅，我们用了2个线程来下载图片                    mImageThreadPool = Executors.newFixedThreadPool(2);                }            }        }        return mImageThreadPool;    }    /**     * 添加Bitmap到内存缓存     * @param key     * @param bitmap     */    public void addBitmapToMemoryCache(String key, Bitmap bitmap) {          if (getBitmapFromMemCache(key) == null && bitmap != null) {              mMemoryCache.put(key, bitmap);          }      }      /**     * 从内存缓存中获取一个Bitmap     * @param key     * @return     */    public Bitmap getBitmapFromMemCache(String key) {          return mMemoryCache.get(key);      }     /**     * 先从内存缓存中获取Bitmap,如果没有就从SD卡或者手机缓存中获取，SD卡或者手机缓存     * 没有就去下载     * @param url     * @param listener     * @return     */    public Bitmap downloadImage(final String url, final onImageLoaderListener listener){        //替换Url中非字母和非数字的字符，这里比较重要，因为我们用Url作为文件名，比如我们的Url        //是Http://xiaanming/abc.jpg;用这个作为图片名称，系统会认为xiaanming为一个目录，        //我们没有创建此目录保存文件就会报错        final String subUrl = url.replaceAll("[^\\w]", "");        Bitmap bitmap = showCacheBitmap(subUrl);        if(bitmap != null){            return bitmap;        }else{            final Handler handler = new Handler(){                @Override                public void handleMessage(Message msg) {                    super.handleMessage(msg);                    listener.onImageLoader((Bitmap)msg.obj, url);                }            };            getThreadPool().execute(new Runnable() {                @Override                public void run() {                    Bitmap bitmap = getBitmapFormUrl(url);                    Message msg = handler.obtainMessage();                    msg.obj = bitmap;                    handler.sendMessage(msg);                    try {                        //保存在SD卡或者手机目录                        fileUtils.savaBitmap(subUrl, bitmap);                    } catch (IOException e) {                        e.printStackTrace();                    }                    //将Bitmap 加入内存缓存                    addBitmapToMemoryCache(subUrl, bitmap);                }            });        }        return null;    }    /**     * 获取Bitmap, 内存中没有就去手机或者sd卡中获取，这一步在getView中会调用，比较关键的一步     * @param url     * @return     */    public Bitmap showCacheBitmap(String url){        if(getBitmapFromMemCache(url) != null){            return getBitmapFromMemCache(url);        }else if(fileUtils.isFileExists(url) && fileUtils.getFileSize(url) != 0){            //从SD卡获取手机里面获取Bitmap            Bitmap bitmap = fileUtils.getBitmap(url);            //将Bitmap 加入内存缓存            addBitmapToMemoryCache(url, bitmap);            return bitmap;        }        return null;    }    /**     * 从Url中获取Bitmap     * @param url     * @return     */    private Bitmap getBitmapFormUrl(String url) {        Bitmap bitmap = null;        HttpURLConnection con = null;        try {            URL mImageUrl = new URL(url);            con = (HttpURLConnection) mImageUrl.openConnection();            con.setConnectTimeout(10 * 1000);            con.setReadTimeout(10 * 1000);            con.setDoInput(true);            con.setDoOutput(true);            bitmap = BitmapFactory.decodeStream(con.getInputStream());        } catch (Exception e) {            e.printStackTrace();        } finally {            if (con != null) {                con.disconnect();            }        }        return bitmap;    }    /**     * 取消正在下载的任务     */    public synchronized void cancelTask() {        if(mImageThreadPool != null){            mImageThreadPool.shutdownNow();            mImageThreadPool = null;        }    }    /**     * 异步下载图片的回调接口     * @author len     *     */    public interface onImageLoaderListener{        void onImageLoader(Bitmap bitmap, String url);    }}

ImageDownLoader中有几个方法比较重要:
1.首先我们需要重写sizeOf(String key, Bitmap value)来计算图片的大小，默认返回图片的数量

2.downloadImage(final String url, final onImageLoaderListener listener)先去LruCache查看Image,没有再去手机缓存中查看，在没有则开启线程下载，这里我们提供了一个回调接口，回调方法中我们将Bitmap和图片Url作为参数，String subUrl = url.replaceAll(“[^\w]”, “”) 我在代码中注释写的比较清楚

3.showCacheBitmap(String url)方法，此方法在Adapter中的getView()当中调用，如果getView()中不调用此方法试试你就知道效果了

ImageAdapter GridView的适配器类，主要是GridView滑动的时候取消下载任务，静止的时候去下载当前显示的item的图片，其他也没什么不同了

package com.example.asyncimageloader;import android.content.Context;import android.graphics.Bitmap;import android.view.View;import android.view.ViewGroup;import android.widget.AbsListView;import android.widget.AbsListView.OnScrollListener;import android.widget.BaseAdapter;import android.widget.GridView;import android.widget.ImageView;import com.example.asyncimageloader.ImageDownLoader.onImageLoaderListener;public class ImageAdapter extends BaseAdapter implements OnScrollListener{    /**     * 上下文对象的引用     */    private Context context;    /**     * Image Url的数组     */    private String [] imageThumbUrls;    /**     * GridView对象的应用     */    private GridView mGridView;    /**     * Image 下载器     */    private ImageDownLoader mImageDownLoader;    /**     * 记录是否刚打开程序，用于解决进入程序不滚动屏幕，不会下载图片的问题。     * 参考http://blog.csdn.net/guolin_blog/article/details/9526203#comments     */    private boolean isFirstEnter = true;    /**     * 一屏中第一个item的位置     */    private int mFirstVisibleItem;    /**     * 一屏中所有item的个数     */    private int mVisibleItemCount;    public ImageAdapter(Context context, GridView mGridView, String [] imageThumbUrls){        this.context = context;        this.mGridView = mGridView;        this.imageThumbUrls = imageThumbUrls;        mImageDownLoader = new ImageDownLoader(context);        mGridView.setOnScrollListener(this);    }    @Override    public void onScrollStateChanged(AbsListView view, int scrollState) {        //仅当GridView静止时才去下载图片，GridView滑动时取消所有正在下载的任务          if(scrollState == AbsListView.OnScrollListener.SCROLL_STATE_IDLE){            showImage(mFirstVisibleItem, mVisibleItemCount);        }else{            cancelTask();        }    }    /**     * GridView滚动的时候调用的方法，刚开始显示GridView也会调用此方法     */    @Override    public void onScroll(AbsListView view, int firstVisibleItem,            int visibleItemCount, int totalItemCount) {        mFirstVisibleItem = firstVisibleItem;        mVisibleItemCount = visibleItemCount;        // 因此在这里为首次进入程序开启下载任务。         if(isFirstEnter && visibleItemCount > 0){            showImage(mFirstVisibleItem, mVisibleItemCount);            isFirstEnter = false;        }    }    @Override    public int getCount() {        return imageThumbUrls.length;    }    @Override    public Object getItem(int position) {        return imageThumbUrls[position];    }    @Override    public long getItemId(int position) {        return position;    }    @Override    public View getView(int position, View convertView, ViewGroup parent) {        ImageView mImageView;        final String mImageUrl = imageThumbUrls[position];        if(convertView == null){            mImageView = new ImageView(context);        }else{            mImageView = (ImageView) convertView;        }        mImageView.setLayoutParams(new GridView.LayoutParams(150, 150));        mImageView.setScaleType(ImageView.ScaleType.CENTER_INSIDE);        //给ImageView设置Tag,这里已经是司空见惯了        mImageView.setTag(mImageUrl);        /*******************************去掉下面这几行试试是什么效果****************************/        Bitmap bitmap = mImageDownLoader.showCacheBitmap(mImageUrl.replaceAll("[^\\w]", ""));        if(bitmap != null){            mImageView.setImageBitmap(bitmap);        }else{            mImageView.setImageDrawable(context.getResources().getDrawable(R.drawable.ic_empty));        }        /**********************************************************************************/        return mImageView;    }    /**     * 显示当前屏幕的图片，先会去查找LruCache，LruCache没有就去sd卡或者手机目录查找，在没有就开启线程去下载     * @param firstVisibleItem     * @param visibleItemCount     */    private void showImage(int firstVisibleItem, int visibleItemCount){        Bitmap bitmap = null;        for(int i=firstVisibleItem; i<firstVisibleItem + visibleItemCount; i++){            String mImageUrl = imageThumbUrls[i];            final ImageView mImageView = (ImageView) mGridView.findViewWithTag(mImageUrl);            bitmap = mImageDownLoader.downloadImage(mImageUrl, new onImageLoaderListener() {                @Override                public void onImageLoader(Bitmap bitmap, String url) {                    if(mImageView != null && bitmap != null){                        mImageView.setImageBitmap(bitmap);                    }                }            });            //if(bitmap != null){            //  mImageView.setImageBitmap(bitmap);            //}else{            //  mImageView.setImageDrawable(context.getResources().getDrawable(R.drawable.ic_empty));            //}        }    }    /**     * 取消下载任务     */    public void cancelTask(){        mImageDownLoader.cancelTask();    }}

MainActivity 里面一个GridView，然后提供一个系统菜单来删除手机上的缓存图片，直接上代码，比较简单所以里面也没有注释

package com.example.asyncimageloader;import android.app.Activity;import android.os.Bundle;import android.view.Menu;import android.view.MenuItem;import android.widget.GridView;import android.widget.Toast;public class MainActivity extends Activity {    private GridView mGridView;    private String [] imageThumbUrls = Images.imageThumbUrls;     private ImageAdapter mImageAdapter;    private FileUtils fileUtils;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        fileUtils = new FileUtils(this);        mGridView = (GridView) findViewById(R.id.gridView);        mImageAdapter = new ImageAdapter(this, mGridView, imageThumbUrls);        mGridView.setAdapter(mImageAdapter);    }    @Override    protected void onDestroy() {        mImageAdapter.cancelTask();        super.onDestroy();    }    @Override    public boolean onCreateOptionsMenu(Menu menu) {        super.onCreateOptionsMenu(menu);        menu.add("删除手机中图片缓存");        return super.onCreateOptionsMenu(menu);    }    @Override    public boolean onOptionsItemSelected(MenuItem item) {        switch (item.getItemId()) {        case 0:            fileUtils.deleteFile();            Toast.makeText(getApplication(), "清空缓存成功", Toast.LENGTH_SHORT).show();            break;        }        return super.onOptionsItemSelected(item);    }}

MainActivity的布局文件

<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"    tools:context=".MainActivity" >    <GridView        android:id="@+id/gridView"        android:layout_width="match_parent"        android:layout_height="match_parent"        android:stretchMode="columnWidth"         android:columnWidth="90dip"         android:verticalSpacing="10dip"        android:horizontalSpacing="10dip"        android:cacheColorHint="@android:color/transparent"        android:numColumns="auto_fit" >    </GridView></RelativeLayout>

因为有网络操作和对sd卡的操作，所以我们必须加上相对应的权限

<uses-permission android:name="android.permission.INTERNET" /> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> 

好了，代码就全部介绍完了，我们运行下程序看看效果

可以看到，滚动照片墙，会异步加载图片到相应的ImageView上。随着加载图片的增多，会释放掉一些之前加载过的图片，你多滚动几次就可以看得出了。另外为了能让大家明显看出图片的释放情况，我在这个程序中没有使用本地缓存，所有被释放掉的图片再次显示需要从网络上再下载一遍。在实际的项目中配合适当的本地缓存效果会更好。

打开DDMS，我们可以发现，由于有LruCache帮我们管理图片缓存，不管如何滚动照片墙，程序内存始终会保持在一个合理的范围内。

PhotoWallFallsDemo
大图加载

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Android照片墙应用实现，再多的图片也不怕崩溃
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