Listview异步加载图片之优化篇

在APP应用中，listview的异步加载图片方式能够带来很好的用户体验，同时也是考量程序性能的一个重要指标。关于listview的异步加载，网上其实很多示例了，中心思想都差不多，不过很多版本或是有bug，或是有性能问题有待优化。有鉴于此，本人在网上找了个相对理想的版本并在此基础上进行改造，下面就让在下阐述其原理以探索个中奥秘，与诸君共赏…

贴张效果图先：

2013-2-1 17:25 上传

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异步加载图片基本思想：

1.      先从内存缓存中获取图片显示（内存缓冲）

2.      获取不到的话从SD卡里获取（SD卡缓冲）

3.      都获取不到的话从网络下载图片并保存到SD卡同时加入内存并显示（视情况看是否要显示）

OK，先上adapter的代码：

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public class LoaderAdapter extends BaseAdapter{            private static final String TAG = "LoaderAdapter";         private boolean mBusy = false;            public void setFlagBusy(boolean busy) {                 this.mBusy = busy;         }                       private ImageLoader mImageLoader;         private int mCount;         private Context mContext;         private String[] urlArrays;                               public LoaderAdapter(int count, Context context, String []url) {                 this.mCount = count;                 this.mContext = context;                 urlArrays = url;                 mImageLoader = new ImageLoader(context);         }                    public ImageLoader getImageLoader(){                 return mImageLoader;         }            @Override         public int getCount() {                 return mCount;         }            @Override         public Object getItem(int position) {                 return position;         }            @Override         public long getItemId(int position) {                 return position;         }            @Override         public View getView(int position, View convertView, ViewGroup parent) {                    ViewHolder viewHolder = null;                 if (convertView == null) {                         convertView = LayoutInflater.from(mContext).inflate(                                         R.layout.list_item, null);                         viewHolder = new ViewHolder();                         viewHolder.mTextView = (TextView) convertView                                         .findViewById(R.id.tv_tips);                         viewHolder.mImageView = (ImageView) convertView                                         .findViewById(R.id.iv_image);                         convertView.setTag(viewHolder);                 } else {                         viewHolder = (ViewHolder) convertView.getTag();                 }                 String url = "";                 url = urlArrays[position % urlArrays.length];                                    viewHolder.mImageView.setImageResource(R.drawable.ic_launcher);                                       if (!mBusy) {                         mImageLoader.DisplayImage(url, viewHolder.mImageView, false);                         viewHolder.mTextView.setText("--" + position                                         + "--IDLE ||TOUCH_SCROLL");                 } else {                         mImageLoader.DisplayImage(url, viewHolder.mImageView, true);                                         viewHolder.mTextView.setText("--" + position + "--FLING");                 }                 return convertView;         }            static class ViewHolder {                 TextView mTextView;                 ImageView mImageView;         } }

关键代码是ImageLoader的DisplayImage方法，再看ImageLoader的实现

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public class ImageLoader {            private MemoryCache memoryCache = new MemoryCache();         private AbstractFileCache fileCache;         private Map imageViews = Collections                         .synchronizedMap(new WeakHashMap());         // 线程池         private ExecutorService executorService;            public ImageLoader(Context context) {                 fileCache = new FileCache(context);                 executorService = Executors.newFixedThreadPool(5);         }            // 最主要的方法         public void DisplayImage(String url, ImageView imageView, boolean isLoadOnlyFromCache) {                 imageViews.put(imageView, url);                 // 先从内存缓存中查找                    Bitmap bitmap = memoryCache.get(url);                 if (bitmap != null)                         imageView.setImageBitmap(bitmap);                 else if (!isLoadOnlyFromCache){                                                    // 若没有的话则开启新线程加载图片                         queuePhoto(url, imageView);                 }         }            private void queuePhoto(String url, ImageView imageView) {                 PhotoToLoad p = new PhotoToLoad(url, imageView);                 executorService.submit(new PhotosLoader(p));         }            private Bitmap getBitmap(String url) {                 File f = fileCache.getFile(url);                                    // 先从文件缓存中查找是否有                 Bitmap b = null;                 if (f != null && f.exists()){                         b = decodeFile(f);                 }                 if (b != null){                         return b;                 }                 // 最后从指定的url中下载图片                 try {                         Bitmap bitmap = null;                         URL imageUrl = new URL(url);                         HttpURLConnection conn = (HttpURLConnection) imageUrl                                         .openConnection();                         conn.setConnectTimeout(30000);                         conn.setReadTimeout(30000);                         conn.setInstanceFollowRedirects(true);                         InputStream is = conn.getInputStream();                         OutputStream os = new FileOutputStream(f);                         CopyStream(is, os);                         os.close();                         bitmap = decodeFile(f);                         return bitmap;                 } catch (Exception ex) {                         Log.e("", "getBitmap catch Exception...\nmessage = " + ex.getMessage());                         return null;                 }         }            // decode这个图片并且按比例缩放以减少内存消耗，虚拟机对每张图片的缓存大小也是有限制的         private Bitmap decodeFile(File f) {                 try {                         // decode image size                         BitmapFactory.Options o = new BitmapFactory.Options();                         o.inJustDecodeBounds = true;                         BitmapFactory.decodeStream(new FileInputStream(f), null, o);                            // Find the correct scale value. It should be the power of 2.                         final int REQUIRED_SIZE = 100;                         int width_tmp = o.outWidth, height_tmp = o.outHeight;                         int scale = 1;                         while (true) {                                 if (width_tmp / 2 < REQUIRED_SIZE                                                 || height_tmp / 2 < REQUIRED_SIZE)                                         break;                                 width_tmp /= 2;                                 height_tmp /= 2;                                 scale *= 2;                         }                            // decode with inSampleSize                         BitmapFactory.Options o2 = new BitmapFactory.Options();                         o2.inSampleSize = scale;                         return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);                 } catch (FileNotFoundException e) {                 }                 return null;         }            // Task for the queue         private class PhotoToLoad {                 public String url;                 public ImageView imageView;                    public PhotoToLoad(String u, ImageView i) {                         url = u;                         imageView = i;                 }         }            class PhotosLoader implements Runnable {                 PhotoToLoad photoToLoad;                    PhotosLoader(PhotoToLoad photoToLoad) {                         this.photoToLoad = photoToLoad;                 }                    @Override                 public void run() {                         if (imageViewReused(photoToLoad))                                 return;                         Bitmap bmp = getBitmap(photoToLoad.url);                         memoryCache.put(photoToLoad.url, bmp);                         if (imageViewReused(photoToLoad))                                 return;                         BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad);                         // 更新的操作放在UI线程中                         Activity a = (Activity) photoToLoad.imageView.getContext();                         a.runOnUiThread(bd);                 }         }                     boolean imageViewReused(PhotoToLoad photoToLoad) {                 String tag = imageViews.get(photoToLoad.imageView);                 if (tag == null || !tag.equals(photoToLoad.url))                         return true;                 return false;         }            // 用于在UI线程中更新界面         class BitmapDisplayer implements Runnable {                 Bitmap bitmap;                 PhotoToLoad photoToLoad;                    public BitmapDisplayer(Bitmap b, PhotoToLoad p) {                         bitmap = b;                         photoToLoad = p;                 }                    public void run() {                         if (imageViewReused(photoToLoad))                                 return;                         if (bitmap != null)                                 photoToLoad.imageView.setImageBitmap(bitmap);                            }         }            public void clearCache() {                 memoryCache.clear();                 fileCache.clear();         }            public static void CopyStream(InputStream is, OutputStream os) {                 final int buffer_size = 1024;                 try {                         byte[] bytes = new byte[buffer_size];                         for (;;) {                                 int count = is.read(bytes, 0, buffer_size);                                 if (count == -1)                                         break;                                 os.write(bytes, 0, count);                         }                 } catch (Exception ex) {                         Log.e("", "CopyStream catch Exception...");                 }         } }

先从内存中加载，没有则开启线程从SD卡或网络中获取，这里注意从SD卡获取图片是放在子线程里执行的，否则快速滑屏的话会不够流畅，这是优化一。于此同时，在adapter里有个busy变量，表示listview是否处于滑动状态，如果是滑动状态则仅从内存中获取图片，没有的话无需再开启线程去外存或网络获取图片，这是优化二。ImageLoader里的线程使用了线程池，从而避免了过多线程频繁创建和销毁，有的童鞋每次总是new一个线程去执行这是非常不可取的，好一点的用的AsyncTask类，其实内部也是用到了线程池。在从网络获取图片时，先是将其保存到sd卡，然后再加载到内存，这么做的好处是在加载到内存时可以做个压缩处理，以减少图片所占内存，这是优化三。

而图片错位问题的本质源于我们的listview使用了缓存convertView，假设一种场景，一个listview一屏显示九个item，那么在拉出第十个item的时候，事实上该item是重复使用了第一个item，也就是说在第一个item从网络中下载图片并最终要显示的时候其实该item已经不在当前显示区域内了，此时显示的后果将是在可能在第十个item上输出图像，这就导致了图片错位的问题。所以解决之道在于可见则显示，不可见则不显示。在ImageLoader里有个imageViews的map对象，就是用于保存当前显示区域图像对应的url集，在显示前判断处理一下即可。

下面再说下内存缓冲机制，本例采用的是LRU算法，先看看MemoryCache的实现

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public class MemoryCache {            private static final String TAG = "MemoryCache";         // 放入缓存时是个同步操作         // LinkedHashMap构造方法的最后一个参数true代表这个map里的元素将按照最近使用次数由少到多排列，即LRU         // 这样的好处是如果要将缓存中的元素替换，则先遍历出最近最少使用的元素来替换以提高效率         private Map cache = Collections                         .synchronizedMap(new LinkedHashMap(10, 1.5f, true));         // 缓存中图片所占用的字节，初始0，将通过此变量严格控制缓存所占用的堆内存         private long size = 0;// current allocated size         // 缓存只能占用的最大堆内存         private long limit = 1000000;// max memory in bytes            public MemoryCache() {                 // use 25% of available heap size                 setLimit(Runtime.getRuntime().maxMemory() / 10);         }            public void setLimit(long new_limit) {                 limit = new_limit;                 Log.i(TAG, "MemoryCache will use up to " + limit / 1024. / 1024. + "MB");         }            public Bitmap get(String id) {                 try {                         if (!cache.containsKey(id))                                 return null;                         return cache.get(id);                 } catch (NullPointerException ex) {                         return null;                 }         }            public void put(String id, Bitmap bitmap) {                 try {                         if (cache.containsKey(id))                                 size -= getSizeInBytes(cache.get(id));                         cache.put(id, bitmap);                         size += getSizeInBytes(bitmap);                         checkSize();                 } catch (Throwable th) {                         th.printStackTrace();                 }         }                     private void checkSize() {                 Log.i(TAG, "cache size=" + size + " length=" + cache.size());                 if (size > limit) {                         // 先遍历最近最少使用的元素                         Iterator> iter = cache.entrySet().iterator();                         while (iter.hasNext()) {                                 Entry entry = iter.next();                                 size -= getSizeInBytes(entry.getValue());                                 iter.remove();                                 if (size <= limit)                                         break;                         }                         Log.i(TAG, "Clean cache. New size " + cache.size());                 }         }            public void clear() {                 cache.clear();         }                     long getSizeInBytes(Bitmap bitmap) {                 if (bitmap == null)                         return 0;                 return bitmap.getRowBytes() * bitmap.getHeight();         } }

首先限制内存图片缓冲的堆内存大小，每次有图片往缓存里加时判断是否超过限制大小，超过的话就从中取出最少使用的图片并将其移除，当然这里如果不采用这种方式，换做软引用也是可行的，二者目的皆是最大程度的利用已存在于内存中的图片缓存，避免重复制造垃圾增加GC负担，OOM溢出往往皆因内存瞬时大量增加而垃圾回收不及时造成的。只不过二者区别在于LinkedHashMap里的图片缓存在没有移除出去之前是不会被GC回收的，而SoftReference里的图片缓存在没有其他引用保存时随时都会被GC回收。所以在使用LinkedHashMap这种LRU算法缓存更有利于图片的有效命中，当然二者配合使用的话效果更佳，即从LinkedHashMap里移除出的缓存放到SoftReference里，这就是内存的二级缓存，有兴趣的童鞋不凡一试。