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Listview异步加载图片之优化篇

2024-07-11 06:04:57   225人阅读

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


贴张效果图先：

ss.png

         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 = newImageLoader(context);
        }
           
        public ImageLoader getImageLoader(){
                returnmImageLoader;
        }
   
        @Override
        public int getCount() {
                returnmCount;
        }
   
        @Override
        public Object getItem(int position) {
                returnposition;
        }
   
        @Override
        public long getItemId(int position) {
                returnposition;
        }
   
        @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 = newViewHolder();
                        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");
                }
                returnconvertView;
        }
   
        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 = newMemoryCache();
        private AbstractFileCache fileCache;
        private Map<ImageView, String> imageViews = Collections
                        .synchronizedMap(newWeakHashMap<ImageView, String>());
        // 线程池
        private ExecutorService executorService;
   
        public ImageLoader(Context context) {
                fileCache = newFileCache(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);
                elseif (!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 = newPhotoToLoad(url, imageView);
                executorService.submit(newPhotosLoader(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){
                        returnb;
                }
                // 最后从指定的url中下载图片
                try{
                        Bitmap bitmap = null;
                        URL imageUrl = newURL(url);
                        HttpURLConnection conn = (HttpURLConnection) imageUrl
                                        .openConnection();
                        conn.setConnectTimeout(30000);
                        conn.setReadTimeout(30000);
                        conn.setInstanceFollowRedirects(true);
                        InputStream is = conn.getInputStream();
                        OutputStream os = newFileOutputStream(f);
                        CopyStream(is, os);
                        os.close();
                        bitmap = decodeFile(f);
                        returnbitmap;
                }catch(Exception ex) {
                        Log.e("","getBitmap catch Exception...\nmessage = " + ex.getMessage());
                        returnnull;
                }
        }
   
        // 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 = newBitmapFactory.Options();
                        o.inJustDecodeBounds = true;
                        BitmapFactory.decodeStream(newFileInputStream(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 = newBitmapFactory.Options();
                        o2.inSampleSize = scale;
                        returnBitmapFactory.decodeStream(newFileInputStream(f), null, o2);
                }catch(FileNotFoundException e) {
                }
                returnnull;
        }
   
        // 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 = newBitmapDisplayer(bmp, photoToLoad);
                        // 更新的操作放在UI线程中
                        Activity a = (Activity) photoToLoad.imageView.getContext();
                        a.runOnUiThread(bd);
                }
        }
   
        /**
         * 防止图片错位
         *
         * @param photoToLoad
         * @return
         */
        boolean imageViewReused(PhotoToLoad photoToLoad) {
                String tag = imageViews.get(photoToLoad.imageView);
                if(tag == null|| !tag.equals(photoToLoad.url))
                        returntrue;
                returnfalse;
        }
   
        // 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 = newbyte[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<String, Bitmap> cache = Collections
                        .synchronizedMap(newLinkedHashMap<String, Bitmap>(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))
                                returnnull;
                        returncache.get(id);
                }catch(NullPointerException ex) {
                        returnnull;
                }
        }
   
        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();
                }
        }
   
        /**
         * 严格控制堆内存，如果超过将首先替换最近最少使用的那个图片缓存
         *
         */
        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<Entry<String, Bitmap>> iter = cache.entrySet().iterator();
                        while(iter.hasNext()) {
                                Entry<String, Bitmap> 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();
        }
   
        /**
         * 图片占用的内存
         *
         * <a href=http://www.mamicode.com/"/"http://www.jcodecraeer.com/"" target="/"_blank/"">@Param bitmap
         *
         * @return
         */
        long getSizeInBytes(Bitmap bitmap) {
                if(bitmap == null)
                        return0;
                returnbitmap.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。


下面附上工程链接：

LazyLoaderDemo.zip(937.26 KB)


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