android - 这是在 android ListView 中从网络加载图像的流畅快速的方法!

Question

我需要列出来自网络的数据，因为我有 1 个图像和 2 个文本。我解析所有数据并显示它，但图像在列表中显示非常慢。所以我正在寻找最好的方法来做到这一点。

请帮帮我。

提前致谢

Answer 1

请复制下面的类。该类是从网上下载图像并存储到存储卡或应用程序的内部存储器。

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.util.HashMap;
import java.util.Stack;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
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 = null;
    private Bitmap  useThisBitmap = null;
    @SuppressWarnings("unused")
    private Context ctx = null;

    public Imageloader(Context context) 
    {
        // Make the background thead low priority. This way it will not affect
        // the UI performance

        ctx = 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(),"DownloadImages/AlbumArt/");
        else
            cacheDir = context.getCacheDir();

        if (!cacheDir.exists())
            cacheDir.mkdirs();
    }

    public void DisplayImage(String url, Activity activity, ImageView imageView) 
    {
        if(!url.equals(""))
         {
             if (cache.containsKey(url))
             {
                 imageView.setImageBitmap(cache.get(url));
             }
             else
             {
                queuePhoto(url, activity, imageView);
            }
         }
    }

    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();
    }

    public Bitmap getBitmap(String url) 
    {
        try
        {
            // 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;       
                if(!url.equals("")){
                InputStream is = new URL(url).openStream();
                OutputStream os = new FileOutputStream(f);
                Utils.CopyStream(is, os);
                os.close();
                bitmap = decodeFile(f);
                }
                return bitmap;
            } 
            catch (Exception ex) 
            {
                ex.printStackTrace();
            return null;
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();

            return null;    
        }
    }

    /*decodes image and scales it to reduce memory consumption
     * @param file path
     * @throws FileNotFoundException
     * @return bitmap
     * */
    private Bitmap decodeFile(File f){
        Bitmap b = null;
        try {

            useThisBitmap = null;
            //Decode image size
            BitmapFactory.Options o = new BitmapFactory.Options();
            o.inJustDecodeBounds = true;
            final int IMAGE_MAX_SIZE = 70;
            BitmapFactory.decodeStream(new FileInputStream(f), null, o);
            int scale = 2;
            if (o.outHeight > IMAGE_MAX_SIZE || o.outWidth > IMAGE_MAX_SIZE) {
                scale = 2 ^ (int) Math.ceil(Math.log(IMAGE_MAX_SIZE / (double) Math.max(o.outHeight, o.outWidth)) / Math.log(0.5));
            }

            //Decode with inSampleSize
            BitmapFactory.Options o2 = new BitmapFactory.Options();

            o2.inSampleSize = scale;
            b = BitmapFactory.decodeStream(new FileInputStream(f), null, o2);
            useThisBitmap = b;

        } 
        catch (FileNotFoundException e) {
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
        finally
        {
            System.gc();
        }
        return useThisBitmap;
    }


    // Task for the queue
    private class PhotoToLoad 
    {
        public String url;
        public ImageView imageView;

        public PhotoToLoad(String u, ImageView i) {
            url = u;
            imageView = i;
        }
    }

    private PhotosQueue photosQueue = new PhotosQueue();



    // stores list of photos to download
    private class PhotosQueue 
    {
        private Stack<PhotoToLoad> photosToLoad = new Stack<PhotoToLoad>();
        // removes all instances of this ImageView
        private void Clean(ImageView image) 
        {
            for (int j = 0; j < photosToLoad.size();) 
            {
                if (photosToLoad.get(j).imageView == image)
                    photosToLoad.remove(j);
                else
                    ++j;
            }
        }
    }

    private 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
            }
        }
    }

    private PhotosLoader photoLoaderThread = new PhotosLoader();

    // Used to display bitmap in the UI thread
    private class BitmapDisplayer implements Runnable 
    {
        private Bitmap bitmap;
        private ImageView imageView;

        private BitmapDisplayer(Bitmap b, ImageView i) 
        {
            bitmap = b;
            imageView = i;
        }

        public void run() 
        {
            if (bitmap != null)
                imageView.setImageBitmap(bitmap);
        }
    }

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

    public void clearCache() 
    {
        cache.clear();
        File[] files = cacheDir.listFiles();
        for (File f : files)
            f.delete();
    }
}

例子

首先创建您的图像 Loader 类对象。

Imageloader imageLoader = new Imageloader(getApplicationContext());

然后将图片 url 设置到 imageview 的 setTag 属性中。

imgImageView.setTag(Your Image Url);

然后调用您的 ImageLoader 类显示图像功能。需要3个参数。

1) 图片地址

2) 你当前的类(class)名称

3) 图片 View

imageLoader.DisplayImage(Your Image Url,ClassName.this,imgImageView);

该函数从网络上下载图片并存储到内存中，并从内存中显示。