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 *** 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[] { 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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; /** * *** 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 *** 作和对sd卡的 *** 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。
@L_404_1@
大图加载
推荐阅读：
Android照片墙应用实现，再多的图片也不怕崩溃
Android高效加载大图、多图解决方案，有效避免程序OOM
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