Question

我在listview中使用延迟加载图像。奇怪的是，屏幕上可见的第一行和最后一行似乎具有相同的图像（最后一行）屏幕）。我正在使用以下类进行图像加载：

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){
        //Make the background thead low priority. This way it will not affect the UI performance
        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(),"LazyList");
        else
            cacheDir=context.getCacheDir();
        if(!cacheDir.exists())
            cacheDir.mkdirs();
    }

    final int stub_id=R.drawable.default_user_pic;
    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 is=new URL(url).openStream();
            OutputStream os = new FileOutputStream(f);
            HeaderController.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++;
            }

            //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);
                        if(((String)photoToLoad.imageView.getTag()).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();
    }
}

这是我的Expandable listview适配器代码：

import java.util.List;
import java.util.Map;

import android.app.Activity;
import android.content.Context;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.SimpleExpandableListAdapter;
import android.widget.TextView;



public class NearbyAdapter extends SimpleExpandableListAdapter {
    private List<? extends List<? extends Map<String, ?>>> mChildData;
    private String[] mChildFrom;
    private int[] mChildTo;    
    NearbyActivity currentActivity;
    public ImageLoader imageLoader;

    public NearbyAdapter(Activity activity,Context context,
            List<? extends Map<String, ?>> groupData, int groupLayout,
                    String[] groupFrom, int[] groupTo,
                    List<? extends List<? extends Map<String, ?>>> childData,
                    int childLayout, String[] childFrom, int[] childTo) {
            super(context, groupData, groupLayout, groupFrom, groupTo,
                            childData, childLayout, childFrom, childTo);

            mChildData = childData;
            mChildFrom = childFrom;
            mChildTo = childTo;
            currentActivity = (NearbyActivity)activity;
            imageLoader=new ImageLoader(activity.getApplicationContext());
    }


    public View getChildView(int groupPosition, int childPosition,
            boolean isLastChild, View convertView, ViewGroup parent) {

        View v;
        if (convertView == null) {
                v = newChildView(isLastChild, parent);
        } else {
                v = convertView;
        }
        bindView(v, mChildData.get(groupPosition).get(childPosition),mChildFrom,
                        mChildTo, groupPosition, childPosition);
        return v;
    }

    private void bindView(View view, Map<String, ?> data,
            String[] from, int[] to, int groupPosition, int childPosition) {
                if(from[3] != null)
                    view.setTag(data.get(from[4]));


                ImageView imageView = (ImageView) view.findViewById(to[3]);
                if (imageView != null) {

                    imageView.setTag((String) data.get(from[3]));
                    imageLoader.DisplayImage((String) data.get(from[3]), currentActivity, imageView);
                }
    }

    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }

    public boolean isGroupSelectable(int groupPosition) {
        return true;
    }

    public boolean hasStableIds() {
        return true;
    }



}

Answer 1

我有一个类似的问题。我通过将imageview设置为默认图片来解决它。

    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(R.drawable.icon);
        }    
    }

它不漂亮，但它解决了我的重复图像问题。

Answer 2

我强烈建议您使用http://code.google.com/p/libs-for-android/wiki/ImageLoader库。如果需要，我会提供示例。

使用延迟加载时，在ListView中复制ImageView

2 个答案: