Android仅加载ListView的可见元素

Question

我在我的应用程序中使用延迟加载列表，我只需要加载 ListView 的可见元素。我在互联网上搜索，尝试了很多东西，但仍然无法让这些东西工作。你能给我一个建议吗怎么办呢？

这是 ImageLoader 类：

package com.stampii.stampii.tableview;

import java.io.File;
import java.io.InputStream;
import java.util.HashMap;
import java.util.Stack;
import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import com.stampii.stampii.R;
import android.app.Activity;
import android.content.Context;
import android.content.res.AssetManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.util.Log;
import android.widget.ImageView;

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;
    private AssetManager mAssetManager;


    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);
        mAssetManager = context.getAssets();

        //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(),"Stampii");
        else
            cacheDir=context.getCacheDir();
        if(!cacheDir.exists())
            cacheDir.mkdirs();
    }


    final int stub_id=R.drawable.default_img;
    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)  //opashka
    {
        //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 src) {
        Bitmap myBitmap = null;


                //Decryption
                try {
                Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
                SecretKeySpec keySpec = new SecretKeySpec("01234567890abcde".getBytes(), "AES");
                IvParameterSpec ivSpec = new IvParameterSpec("fedcba9876543210".getBytes());
                cipher.init(Cipher.DECRYPT_MODE, keySpec, ivSpec);

                InputStream input = mAssetManager.open(src);
                CipherInputStream cis = new CipherInputStream(input, cipher);

                myBitmap = BitmapFactory.decodeStream(cis);

                }
                catch(Exception e){
                    e.printStackTrace();
                    Log.v("ERROR","Error : "+e);
                }


                return myBitmap;


        }

    //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

        long size=0;
            cache.clear();

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




}

EDIT1:

我的 LazyAdapter 类中的 getView 方法：

   public View getView(int position, View convertView, ViewGroup parent) {
        View vi=convertView;
        ViewHolder holder;
        if(convertView==null){
            vi = inflater.inflate(R.layout.item, null);
            holder=new ViewHolder();
            holder.name=(TextView)vi.findViewById(R.id.name);
            holder.info=(TextView)vi.findViewById(R.id.info);
            holder.image=(ImageView)vi.findViewById(R.id.image);
            vi.setTag(holder);
        }
        else
            holder=(ViewHolder)vi.getTag();

        holder.name.setText("MotoGP Rider");
        holder.info.setText("Category");
        holder.image.setTag(data[position]);
        imageLoader.DisplayImage(data[position], activity, holder.image);
        return vi;
    }

有什么想法吗？

Answer 1

检查代码后：我确信只有可见的图像正在加载！

但是因为下面的一些图像具有相同的网址！您会认为它们是以不同的顺序加载的。但是，它们是根据可见图像加载的，但具有相同 url 的任何其他列表项都会显示图像。

---

我在应用程序中使用延迟加载列表，并且只需要加载 ListView 的可见元素。

ListView 上可见的每个位置都会调用 getView

需要加载的图片必须从 Adapter 的 getView 添加到队列中，

因此您应该重写

public View   getView  (int position, View convertView, ViewGroup parent){
    super(position,convertView,parent);
    //now add the picture at position to the queue
}

注意：小心

假设您将照片 1 添加到队列中：这并不意味着不会再次调用 getView位置 1。

换句话说，考虑以下事实：可以使用getView多次调用position。

Answer 2

要仅加载可见元素，您需要实现 AbsListView.OnScrollListener 接口。从这个接口中你可以从它的方法中找到“scroll event”和“scrolleventstate”的变化。

ListView list = (ListView) findViewById(R.id.listview);
MyAdapter adapter = new MyAdapter(context, items);
list.setAdapter(adapter);
list.setOnScrollListener(new OnScrollListener() {
@Override
public void onScrollStateChanged(AbsListView view, 
        int scrollState) {
     // Do nothing
}

@Override
public void onScroll(AbsListView view, int firstVisibleItem, 
        int visibleItemCount, int totalItemCount) {

    // threshold being indicator if bottom of list is hit
    if (firstVisibleItem = threshold) {
        pullMoreData();
    }
}
});

有关于此的更多解释。您可以按照本教程进行操作。 智能加载列表视图 , 动态数据listview，检测结束滚动，获取可见数据索引