转载请注明出处
异步加载图片的例子，网上也比较多，大部分用了HashMap<String, SoftReference<Drawable>> imageCache ，但是现在已经不再推荐使用这种方式了，因为从 Android 2.3 (API Level 9)开始，垃圾回收器会更倾向于回收持有软引用或弱引用的对象，这让软引用和弱引用变得不再可靠。另外，Android 3.0 (API Level 11)中，图片的数据会存储在本地的内存当中，因而无法用一种可预见的方式将其释放，这就有潜在的风险造成应用程序的内存溢出并崩溃，所以我这里用得是LruCache来缓存图片，当存储Image的大小大于LruCache设定的值，系统自动释放内存，这个类是3.1版本中提供的，如果你是在更早的Android版本中开发，则需要导入android-support-v4的jar包（这里要注意咯）
为什么写这篇文章呢？
因为我之前做的项目中，也有异步加载图片，那时候用得是Thread去下载图片，每次下载图片都要new Thread去下载，而且还是并发去下载，每次都new 一个线程浪费内存，老板说服务器承受不起这么多的连接，叫我改成先获取一张图片之后再去获取下一张，这样子保存与服务器的连接为一个，服务器压力小了，然后楼主就想到线程池，线程池很好的帮我们管理并发的问题，并发的问题解决了，可是后面又出问题了，图片多了就出现OOM(OutOfMemory)异常，之后用了SoftReference，先用SoftReference中获取图片，SoftReference没有就开线程去下载，老板说你为什么不把图片在手机上做个缓存呢，于是我用了手机缓存，大概思路就是先从SoftReference中获取图片，如果SoftReference没有就去手机缓存中获取，手机缓存中没有就开启先从去下载，然后成功的解决了OOM的问题，前些天老板要我重构下代码，我也觉得之前写的代码耦合性太强，早就想改，然后之前看到guolin的的这篇文章，LruCache和滑动过程中取消下载任务，停下来的时候才去下载这2点比较好，值得我学习，然后我就将我的项目异步加载这一块改了下，发到这里做个记录吧，以后类似的异步加载图片直接拷贝代码，提交开发的效率
这篇文章做了哪些方面的优化
- 使用了线程池来管理下载任务
- 使用LruCache来缓存图片
- 使用手机来缓存图片
- 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
- 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，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，有时候会下载超时或者需要FQ
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", 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"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 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 (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保存 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
- 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
- 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
- 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
- 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
- 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
好了，代码就全部介绍完了，我们运行下程序看看效果，这里的效果是用一个线程下载图片的效果，可能有一些不完整，想看看效果自己动手试试，郁闷，家里的网络卡，gif的图片出不来效果，只能明天上传图片啦,公司也上传不了动画效果，我去，那我就贴一张普通图片，哈哈
好了，今天的讲解到此结束，有疑问的朋友请在下面留言,源码里面GridView的布局文件 columnWedth设置了120dip,有点丑，大家可以改成90dip