Android 先进先出延迟加载

Question

参考延迟加载方法 http://open-pim.com/tmp/LazyList.zip< /a>，在 Lazy 中讨论Fedor 在 ListView 中加载图像

我想知道在显示从网络下载的图像时，以下代码是否可以从后进先出（LIFO）更改为先进先出（FIFO）

public class ImageLoader {

//the simplest in-memory cache implementation. This should be replaced with something like SoftReference or BitmapOptions.inPurgeable(since 1.6)
private HashMap<String, Bitmap> cache=new HashMap<String, Bitmap>();

private File cacheDir;

public ImageLoader(Context context){


    photoLoaderThread.setPriority(Thread.NORM_PRIORITY-1);

    //Find the dir to save cached images
    if (android.os.Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED))
        cacheDir=new File(android.os.Environment.getExternalStorageDirectory(),"More");
    else
        cacheDir=context.getCacheDir();
    if(!cacheDir.exists())
        cacheDir.mkdirs();
}

final int stub_id=R.drawable.stub;
public void DisplayImage(String url, Activity activity, ImageView imageView)
{

    if(cache.containsKey(url))
    {           
        imageView.setImageBitmap(cache.get(url));
    }
    else
    {           
        queuePhoto(url, activity, imageView);     
        imageView.setImageResource(stub_id);
    }    
}

private void queuePhoto(String url, Activity activity, ImageView imageView)
{
    //This ImageView may be used for other images before. So there may be some old tasks in the queue. We need to discard them. 
    photosQueue.Clean(imageView);
    PhotoToLoad p=new PhotoToLoad(url, imageView);
    synchronized(photosQueue.photosToLoad){
        photosQueue.photosToLoad.push(p);

        photosQueue.photosToLoad.notifyAll();
    }

    //start thread if it's not started yet
    if(photoLoaderThread.getState()==Thread.State.NEW)
        photoLoaderThread.start();
}

private Bitmap getBitmap(String url) 
{
    //I identify images by hashcode. Not a perfect solution, good for the demo.
    String filename=String.valueOf(url.hashCode());
    File f=new File(cacheDir, filename);

    //from SD cache
    Bitmap b = decodeFile(f);
    if(b!=null)
        return b;


    //from web
    try {
        Bitmap bitmap=null;

        InputStream instream = null;   
        HttpGet httpRequest = new HttpGet(new URI(url));   
        HttpClient httpclient = new DefaultHttpClient();   

        HttpResponse response = (HttpResponse) httpclient.execute(httpRequest);   
        HttpEntity entity = response.getEntity();   
        BufferedHttpEntity bufHttpEntity = new BufferedHttpEntity(entity);   
        try   
        {   
        instream = bufHttpEntity.getContent();         
        } catch (Exception e)   
        {   

        } finally   
        {   
        if (instream != null)   
        {   
        instream.close();   
        }   
        }






        OutputStream os = new FileOutputStream(f);

        Utils.CopyStream(instream, os);
        os.close();
        bitmap = decodeFile(f);
        return bitmap;
    } catch (Exception ex){
       ex.printStackTrace();
       return null;
    }
}

//decodes image and scales it to reduce memory consumption
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=70;
        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;
    }
}

PhotosQueue photosQueue=new PhotosQueue();

public void stopThread()
{
    photoLoaderThread.interrupt();
}

//stores list of photos to download
class PhotosQueue
{
    private Stack<PhotoToLoad> photosToLoad=new Stack<PhotoToLoad>();


    //removes all instances of this ImageView
    public void Clean(ImageView image)
    {
        for(int j=0 ;j<photosToLoad.size();){
            if(photosToLoad.get(j).imageView==image)
                  photosToLoad.remove(j);
            else
                ++j;
        }
    }
}

class PhotosLoader extends Thread {
    public void run() {
        try {
            while(true)
            {
                //thread waits until there are any images to load in the queue
                if(photosQueue.photosToLoad.size()==0)
                    synchronized(photosQueue.photosToLoad){
                        photosQueue.photosToLoad.wait();
                    }
                if(photosQueue.photosToLoad.size()!=0)
                {
                    PhotoToLoad photoToLoad;
                    synchronized(photosQueue.photosToLoad){
                        photoToLoad=photosQueue.photosToLoad.pop();

                    }
                    Bitmap bmp=getBitmap(photoToLoad.url);
                    cache.put(photoToLoad.url, bmp);
                    Object tag=photoToLoad.imageView.getTag();
                    if(tag!=null && ((String)tag).equals(photoToLoad.url)){
                        BitmapDisplayer bd=new BitmapDisplayer(bmp, photoToLoad.imageView);
                        Activity a=(Activity)photoToLoad.imageView.getContext();
                        a.runOnUiThread(bd);
                    }
                }
                if(Thread.interrupted())
                    break;
            }
        } catch (InterruptedException e) {

            //allow thread to exit
        }
    }
}

PhotosLoader photoLoaderThread=new PhotosLoader();

//Used to display bitmap in the UI thread
class BitmapDisplayer implements Runnable
{
    Bitmap bitmap;
    ImageView imageView;
    public BitmapDisplayer(Bitmap b, ImageView i){bitmap=b;imageView=i;}
    public void run()
    {
        if(bitmap!=null)
            imageView.setImageBitmap(bitmap);
        else
            imageView.setImageResource(stub_id);
    }
}

public void clearCache() {
    //clear memory cache
    cache.clear();

    //clear SD cache
    File[] files=cacheDir.listFiles();
    for(File f:files)
        f.delete();
}

}

Answer 1

堆栈本质上是后进先出，因此您可能需要：

• 在对象引用 photosToLoad 上将 Stack 更改为 LinkedList
• 在第一个内部的 photosToLoad 上将 push() 替换为 addFirst()
  同步块
• 调用 getFirst() 然后removeFirst() 替换第三个中的 pop()
  同步块

Answer 2

请将您的 ImageLoader 类替换为此 ImageLoader 类。我测试了代码，它按 FIFO 顺序工作。

public class ImageLoader {

    MemoryCache memoryCache=new MemoryCache();
    FileCache fileCache;
    private Map<ImageView, String> imageViews=Collections.synchronizedMap(new WeakHashMap<ImageView, String>());

    public ImageLoader(Context context){
        //Make the background thead low priority. This way it will not affect the UI performance
        photoLoaderThread.setPriority(Thread.NORM_PRIORITY-1);

        fileCache=new FileCache(context);
    }

    final int stub_id=R.drawable.stub;
    public void DisplayImage(String url, Activity activity, ImageView imageView)
    {
        imageViews.put(imageView, url);
        Bitmap bitmap=memoryCache.get(url);
        if(bitmap!=null)
            imageView.setImageBitmap(bitmap);
        else
        {
            queuePhoto(url, activity, imageView);
            imageView.setImageResource(stub_id);
        }    
    }

    private void queuePhoto(String url, Activity activity, ImageView imageView)
    {
        //This ImageView may be used for other images before. So there may be some old tasks in the queue. We need to discard them. 
        photosQueue.Clean(imageView);
        PhotoToLoad p=new PhotoToLoad(url, imageView);
        synchronized(photosQueue.photosToLoad){
            photosQueue.photosToLoad.add(p);
            photosQueue.photosToLoad.notifyAll();
        }

        //start thread if it's not started yet
        if(photoLoaderThread.getState()==Thread.State.NEW)
            photoLoaderThread.start();
    }

    private Bitmap getBitmap(String url) 
    {
        File f=fileCache.getFile(url);

        //from SD cache
        Bitmap b = decodeFile(f);
        if(b!=null)
            return b;

        //from web
        try {
            Bitmap bitmap=null;
            URL imageUrl = new URL(url);
            HttpURLConnection conn = (HttpURLConnection)imageUrl.openConnection();
            conn.setConnectTimeout(30000);
            conn.setReadTimeout(30000);
            InputStream is=conn.getInputStream();
            OutputStream os = new FileOutputStream(f);
            Utils.CopyStream(is, os);
            os.close();
            bitmap = decodeFile(f);
            return bitmap;
        } catch (Exception ex){
           ex.printStackTrace();
           return null;
        }
    }

    //decodes image and scales it to reduce memory consumption
    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=70;
            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;
        }
    }

    PhotosQueue photosQueue=new PhotosQueue();

    public void stopThread()
    {
        photoLoaderThread.interrupt();
    }

    //stores list of photos to download
    class PhotosQueue
    {
        private Queue<PhotoToLoad> photosToLoad=new LinkedList<PhotoToLoad>();


        //removes all instances of this ImageView
        public void Clean(ImageView image)
        {
            for(int j=0 ;j<photosToLoad.size();){
               if(photosToLoad.element().imageView==image)
                    photosToLoad.remove(j);
                else
                    ++j;
            }
        }
    }

    class PhotosLoader extends Thread {
        public void run() {
            try {
                while(true)
                {
                    //thread waits until there are any images to load in the queue
                    if(photosQueue.photosToLoad.size()==0)
                        synchronized(photosQueue.photosToLoad){
                            photosQueue.photosToLoad.wait();
                        }
                    if(photosQueue.photosToLoad.size()!=0)
                    {
                        PhotoToLoad photoToLoad;
                        synchronized(photosQueue.photosToLoad){

                            photoToLoad=photosQueue.photosToLoad.poll();

                        }
                        Bitmap bmp=getBitmap(photoToLoad.url);
                        memoryCache.put(photoToLoad.url, bmp);
                        String tag=imageViews.get(photoToLoad.imageView);
                        if(tag!=null && tag.equals(photoToLoad.url)){
                            BitmapDisplayer bd=new BitmapDisplayer(bmp, photoToLoad.imageView);
                            Activity a=(Activity)photoToLoad.imageView.getContext();
                            a.runOnUiThread(bd);
                        }
                    }
                    if(Thread.interrupted())
                        break;
                }
            } catch (InterruptedException e) {
                //allow thread to exit
            }
        }
    }

    PhotosLoader photoLoaderThread=new PhotosLoader();

    //Used to display bitmap in the UI thread
    class BitmapDisplayer implements Runnable
    {
        Bitmap bitmap;
        ImageView imageView;
        public BitmapDisplayer(Bitmap b, ImageView i){bitmap=b;imageView=i;}
        public void run()
        {
            if(bitmap!=null)
                imageView.setImageBitmap(bitmap);
            else
                imageView.setImageResource(stub_id);
        }
    }

    public void clearCache() {
        memoryCache.clear();
        fileCache.clear();
    }

}