散列图返回空值

Question

我正在尝试下载图像并将所有下载的图像保存在哈希映射中。我尝试使用以下代码：

public class ImageAdapter extends BaseAdapter{
    private final Context mContext;
   final String[] mImages;
    private LayoutInflater mInflater;
    private static final String TAG = ImageActivity.class.getSimpleName();
  public ImageAdapter(Context context, String[] images){
      mContext=context;
      mImages=images;
      mInflater=(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
  }


    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return mImages.length;
    }

    @Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        return mImages[position];
    }

    @Override
    public long getItemId(int arg0) {
        // TODO Auto-generated method stub
        return 0;
    }

    private static class ViewHolder {
        ImageView imageView;
    }
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder viewHolder;
        if(convertView==null){
            viewHolder=new ViewHolder();
            convertView=mInflater.inflate(R.layout.image_layout, parent, false);
            viewHolder.imageView=(ImageView) convertView.findViewById(R.id.image_View);
            convertView.setTag(viewHolder);
        }
        else{
            viewHolder=(ViewHolder) convertView.getTag();

        }
        String imageUrl=mImages[position];
        ImageDownloader downloader=new ImageDownloader();
        Bitmap value=downloader.urlMap.get(imageUrl);
        Log.d(TAG, "Print Url:" + value);
        if(value==null){

        downloader.download(imageUrl,viewHolder.imageView);
        }
        else
            viewHolder.imageView.setImageBitmap(value);
        return convertView;

        }

}

但是当我调试并检查值包含空值时。它应该包含位图值。

public class ImageDownloader {
    private static final int IO_BUFFER_SIZE = 8 * 1024;
    private static final String TAG = ImageActivity.class.getSimpleName();
    final HashMap<String, Bitmap> urlMap=new HashMap<String, Bitmap>();
     public void download(String url, ImageView imageView) {
         downloadBitmap(url, imageView);

     }


    private void downloadBitmap(final String url,final ImageView imageView) {

        new AsyncTask<Void, Void, Bitmap>() {

            @Override
            protected Bitmap doInBackground(Void... params) {
                return downloadUrlToStream(url, imageView);
            }

            protected void onPostExecute(Bitmap bitmap) {
                imageView.setImageBitmap(bitmap);
                urlMap.put(url, bitmap);
                 Log.d(TAG, "Print value:" + urlMap);
            };
        }.execute();

    }

为什么它不从哈希映射中检索值？

Answer 1

您为每次下载创建一个新的下载程序：

ImageDownloader downloader=new ImageDownloader();
downloader.download(imageUrl,viewHolder.imageView);

你的urlMap（存储下载的Bitmap）不是静态的：

final HashMap<String, Bitmap> urlMap=new HashMap<String, Bitmap>();

因此您实际上已经松开了下载的位图，因为您不会存储对ImageDownloader的任何引用。

但是，你不应该让它静止。很快就会耗尽内存，因为这样就可以将所有位图存储在内存中。

推荐的解决方案是使用LRUCache。它非常易于使用，唯一重要的是覆盖sizeOf正确。以下是您的文档：

http://developer.android.com/training/displaying-bitmaps/cache-bitmap.html

Answer 2

public class ImageAdapter extends BaseAdapter{

  private final Context mContext;
  final String[] mImages;
  private LayoutInflater mInflater;
  private static final String TAG = ImageActivity.class.getSimpleName();
   ImageDownloader downloader;

  public ImageAdapter(Context context, String[] images){
      mContext=context;
      mImages=images;
      mInflater=(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        downloader=new ImageDownloader();
  }


@Override
public int getCount() {
    // TODO Auto-generated method stub
    return mImages.length;
}

@Override
public Object getItem(int position) {
    // TODO Auto-generated method stub
    return mImages[position];
}

@Override
public long getItemId(int arg0) {
    // TODO Auto-generated method stub
    return 0;
}

private static class ViewHolder {
    ImageView imageView;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder viewHolder;
    if(convertView==null){
        viewHolder=new ViewHolder();
        convertView=mInflater.inflate(R.layout.image_layout, parent, false);
        viewHolder.imageView=(ImageView) convertView.findViewById(R.id.image_View);
        convertView.setTag(viewHolder);
    }
    else{
        viewHolder=(ViewHolder) convertView.getTag();

    }
    String imageUrl=mImages[position];

    Bitmap value=downloader.urlMap.get(imageUrl);
    Log.d(TAG, "Print Url:" + value);
    if(value==null){

    downloader.download(imageUrl,viewHolder.imageView);
    }
    else
        viewHolder.imageView.setImageBitmap(value);
    return convertView;

    }

}

Answer 3

如果您想尝试LRU缓存，可以使用此代码

public class GalleryAdapter extends BaseAdapter {

AlertDialog alertDialog = null;
List<GalleryDTO> mDatalist;
Activity context;
private LruCache<String, Bitmap> mMemoryCache;
LocationHandler mLocationHandler = null;
public GalleryAdapter(Activity context, List<GalleryDTO> mDatalist) {
    this.context = context;
    this.mDatalist = mDatalist;
    final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
    final int cacheSize = maxMemory / 8;
    mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
        @Override
        protected int sizeOf(String key, Bitmap bitmap) {
            return (int) (getSizeInBytes(bitmap) / 1024);
        }
    };

}

public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
    if (getBitmapFromMemCache(key) == null) {
        mMemoryCache.put(key, bitmap);
    }
}

public Bitmap getBitmapFromMemCache(String key) {
    return mMemoryCache.get(key);
}

public long getSizeInBytes(Bitmap bitmap) {
    if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.HONEYCOMB) {
        return bitmap.getByteCount();
    } else {
        return bitmap.getRowBytes() * bitmap.getHeight();
    }
}

@Override
public int getCount() {

    return mDatalist.size();
}

@Override
public Object getItem(int position) {

    return mDatalist.get(position);
}

@Override
public long getItemId(int position) {

    return mDatalist.get(position).photoId;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder holder;
    if (convertView == null) {
        convertView = context.getLayoutInflater().inflate(
                R.layout.list_item_gallery, parent, false);
        holder = new ViewHolder();
        holder.mPhoto = (ImageView) convertView
                .findViewById(R.id.img_photo);
        holder.mProgress = (ProgressBar) convertView
                .findViewById(R.id.pg_list_item);
        convertView.setTag(holder);
    } else {
        holder = (ViewHolder) convertView.getTag();
    }
    GalleryDTO data = (GalleryDTO) getItem(position);
    loadBitmap(data.photoName, holder);

    return convertView;
}

private static class ViewHolder {
    protected ImageView mPhoto;
    protected ProgressBar mProgress;
}

public void loadBitmap(String respath, ViewHolder holder) {
    final String imageKey = respath;
    final Bitmap bitmap = getBitmapFromMemCache(imageKey);
    if (bitmap != null) {
        holder.mPhoto.setImageBitmap(bitmap);
        holder.mProgress.setVisibility(View.GONE);
    } else {
        if (cancelPotentialWork(respath, holder.mPhoto, holder.mProgress)) {
            final BitmapWorkerTask task = new BitmapWorkerTask(
                    holder.mPhoto, holder.mProgress);
            final AsyncDrawable asyncDrawable = new AsyncDrawable(
                    context.getResources(), holder.mPlaceHolderBitmap, task);
            holder.mPhoto.setImageDrawable(asyncDrawable);
            task.execute(respath);
        }
    }

}

class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {
    private final WeakReference<ImageView> imageViewReference;
    private final WeakReference<ProgressBar> progressbarReference;
    private String data = "";

    public BitmapWorkerTask(ImageView imageView, ProgressBar progressBar) {
        imageViewReference = new WeakReference<ImageView>(imageView);
        progressbarReference = new WeakReference<ProgressBar>(progressBar);
    }

    @Override
    protected Bitmap doInBackground(String... params) {
        data = params[0];
        final Bitmap bitmap = downloadUrlToStream(url);
        addBitmapToMemoryCache(String.valueOf(params[0]), bitmap);
        return bitmap;

    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (isCancelled()) {
            bitmap = null;
        }

        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            final ProgressBar mProgressBar = progressbarReference.get();
            final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(
                    imageView, mProgressBar);
            if (this == bitmapWorkerTask && imageView != null) {
                imageView.setImageBitmap(bitmap);
                if (mProgressBar != null)
                    mProgressBar.setVisibility(View.GONE);
            }
        }
    }
}

static class AsyncDrawable extends BitmapDrawable {
    private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

    public AsyncDrawable(Resources res, Bitmap bitmap,
            BitmapWorkerTask bitmapWorkerTask) {
        super(res, bitmap);
        bitmapWorkerTaskReference = new WeakReference<BitmapWorkerTask>(
                bitmapWorkerTask);
    }

    public BitmapWorkerTask getBitmapWorkerTask() {
        return bitmapWorkerTaskReference.get();
    }
}

public static boolean cancelPotentialWork(String data, ImageView imageView,
        ProgressBar progressBar) {
    final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(
            imageView, progressBar);

    if (bitmapWorkerTask != null) {
        final String bitmapData = bitmapWorkerTask.data;
        if (bitmapData != data) {
            bitmapWorkerTask.cancel(true);
        } else {
            return false;
        }
    }
    return true;
}

private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView,
        ProgressBar progressBar) {
    if (imageView != null) {
        final Drawable drawable = imageView.getDrawable();
        if (drawable instanceof AsyncDrawable) {
            final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
            return asyncDrawable.getBitmapWorkerTask();
        }
    }
    return null;
}

}