Android編程之圖片加載類

關于圖片加載類。其實，網上有很多這樣的類，而且比較推崇的是來自google中開源中的一篇。他寫的比較好了，而且注意了內存優化，下面貼出它的圖片下載類：

/*


Copyright (C) 2010 The Android Open Source Project
*
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
*
http://www.apache.org/licenses/LICENSE-2.0
*
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package com.example.android.imagedownloader;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.net.http.AndroidHttpClient;
import android.os.AsyncTask;
import android.os.Handler;
import android.util.Log;
import android.widget.ImageView;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.concurrent.ConcurrentHashMap;
/**

This helper class download images from the Internet and binds those with the provided ImageView.
*
<p>It requires the INTERNET permission, which should be added to your application's manifest
file.</p>
*
A local cache of downloaded images is maintained internally to improve performance.
*/
public class ImageDownloader {
 private static final String LOG_TAG = "ImageDownloader";
public enum Mode { NO_ASYNC_TASK, NO_DOWNLOADED_DRAWABLE, CORRECT }
 private Mode mode = Mode.NO_ASYNC_TASK;
/**

Download the specified image from the Internet and binds it to the provided ImageView. The
binding is immediate if the image is found in the cache and will be done asynchronously
otherwise. A null bitmap will be associated to the ImageView if an error occurs.
*
@param url The URL of the image to download.
@param imageView The ImageView to bind the downloaded image to.
*/
public void download(String url, ImageView imageView) {
 resetPurgeTimer();
 Bitmap bitmap = getBitmapFromCache(url);
if (bitmap == null) {

 forceDownload(url, imageView);

} else {

 cancelPotentialDownload(url, imageView);
 imageView.setImageBitmap(bitmap);

}
}
/*

Same as download but the image is always downloaded and the cache is not used.
Kept private at the moment as its interest is not clear.
private void forceDownload(String url, ImageView view) {
   forceDownload(url, view, null);
}
*/
/**

Same as download but the image is always downloaded and the cache is not used.
Kept private at the moment as its interest is not clear.
*/
private void forceDownload(String url, ImageView imageView) {
 // State sanity: url is guaranteed to never be null in DownloadedDrawable and cache keys.
 if (url == null) {

 imageView.setImageDrawable(null);
 return;

}
if (cancelPotentialDownload(url, imageView)) {

 switch (mode) {
     case NO_ASYNC_TASK:
         Bitmap bitmap = downloadBitmap(url);
         addBitmapToCache(url, bitmap);
         imageView.setImageBitmap(bitmap);
         break;

     case NO_DOWNLOADED_DRAWABLE:
         imageView.setMinimumHeight(156);
         BitmapDownloaderTask task = new BitmapDownloaderTask(imageView);
         task.execute(url);
         break;

     case CORRECT:
         task = new BitmapDownloaderTask(imageView);
         DownloadedDrawable downloadedDrawable = new DownloadedDrawable(task);
         imageView.setImageDrawable(downloadedDrawable);
         imageView.setMinimumHeight(156);
         task.execute(url);
         break;
 }

}
}
/**

Returns true if the current download has been canceled or if there was no download in
progress on this image view.
Returns false if the download in progress deals with the same url. The download is not
stopped in that case.
*/
private static boolean cancelPotentialDownload(String url, ImageView imageView) {
 BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
if (bitmapDownloaderTask != null) {

 String bitmapUrl = bitmapDownloaderTask.url;
 if ((bitmapUrl == null) || (!bitmapUrl.equals(url))) {
     bitmapDownloaderTask.cancel(true);
 } else {
     // The same URL is already being downloaded.
     return false;
 }

}
 return true;
}
/**

@param imageView Any imageView
@return Retrieve the currently active download task (if any) associated with this imageView.
null if there is no such task.
*/
private static BitmapDownloaderTask getBitmapDownloaderTask(ImageView imageView) {
 if (imageView != null) {

 Drawable drawable = imageView.getDrawable();
 if (drawable instanceof DownloadedDrawable) {
     DownloadedDrawable downloadedDrawable = (DownloadedDrawable)drawable;
     return downloadedDrawable.getBitmapDownloaderTask();
 }

}
 return null;
}
Bitmap downloadBitmap(String url) {
 final int IO_BUFFER_SIZE = 4 * 1024;
// AndroidHttpClient is not allowed to be used from the main thread
 final HttpClient client = (mode == Mode.NO_ASYNC_TASK) ? new DefaultHttpClient() :

 AndroidHttpClient.newInstance("Android");

final HttpGet getRequest = new HttpGet(url);
try {

 HttpResponse response = client.execute(getRequest);
 final int statusCode = response.getStatusLine().getStatusCode();
 if (statusCode != HttpStatus.SC_OK) {
     Log.w("ImageDownloader", "Error " + statusCode +
             " while retrieving bitmap from " + url);
     return null;
 }

 final HttpEntity entity = response.getEntity();
 if (entity != null) {
     InputStream inputStream = null;
     try {
         inputStream = entity.getContent();
         // return BitmapFactory.decodeStream(inputStream);
         // Bug on slow connections, fixed in future release.
         return BitmapFactory.decodeStream(new FlushedInputStream(inputStream));
     } finally {
         if (inputStream != null) {
             inputStream.close();
         }
         entity.consumeContent();
     }
 }

} catch (IOException e) {

 getRequest.abort();
 Log.w(LOG_TAG, "I/O error while retrieving bitmap from " + url, e);

} catch (IllegalStateException e) {

 getRequest.abort();
 Log.w(LOG_TAG, "Incorrect URL: " + url);

} catch (Exception e) {

 getRequest.abort();
 Log.w(LOG_TAG, "Error while retrieving bitmap from " + url, e);

} finally {

 if ((client instanceof AndroidHttpClient)) {
     ((AndroidHttpClient) client).close();
 }

}
 return null;
}
/*

An InputStream that skips the exact number of bytes provided, unless it reaches EOF.
*/
static class FlushedInputStream extends FilterInputStream {
 public FlushedInputStream(InputStream inputStream) {

 super(inputStream);

}
@Override
 public long skip(long n) throws IOException {

 long totalBytesSkipped = 0L;
 while (totalBytesSkipped < n) {
     long bytesSkipped = in.skip(n - totalBytesSkipped);
     if (bytesSkipped == 0L) {
         int b = read();
         if (b < 0) {
             break;  // we reached EOF
         } else {
             bytesSkipped = 1; // we read one byte
         }
     }
     totalBytesSkipped += bytesSkipped;
 }
 return totalBytesSkipped;

}
}
/**

The actual AsyncTask that will asynchronously download the image.
*/
class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
 private String url;
 private final WeakReference<ImageView> imageViewReference;
public BitmapDownloaderTask(ImageView imageView) {

 imageViewReference = new WeakReference<ImageView>(imageView);

}
/**

Actual download method.
*/
@Override
protected Bitmap doInBackground(String... params) {
 url = params[0];
 return downloadBitmap(url);
}
/**

Once the image is downloaded, associates it to the imageView
*/
@Override
protected void onPostExecute(Bitmap bitmap) {
 if (isCancelled()) {

 bitmap = null;

}
addBitmapToCache(url, bitmap);
if (imageViewReference != null) {

 ImageView imageView = imageViewReference.get();
 BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
 // Change bitmap only if this process is still associated with it
 // Or if we don't use any bitmap to task association (NO_DOWNLOADED_DRAWABLE mode)
 if ((this == bitmapDownloaderTask) || (mode != Mode.CORRECT)) {
     imageView.setImageBitmap(bitmap);
 }

}
}
}







/**
 * A fake Drawable that will be attached to the imageView while the download is in progress.
 *
 * <p>Contains a reference to the actual download task, so that a download task can be stopped
 * if a new binding is required, and makes sure that only the last started download process can
 * bind its result, independently of the download finish order.</p>
 */
static class DownloadedDrawable extends ColorDrawable {
    private final WeakReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;

    public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) {
        super(Color.BLACK);
        bitmapDownloaderTaskReference =
            new WeakReference<BitmapDownloaderTask>(bitmapDownloaderTask);
    }

    public BitmapDownloaderTask getBitmapDownloaderTask() {
        return bitmapDownloaderTaskReference.get();
    }
}

public void setMode(Mode mode) {
    this.mode = mode;
    clearCache();
}


/*
 * Cache-related fields and methods.
 * 
 * We use a hard and a soft cache. A soft reference cache is too aggressively cleared by the
 * Garbage Collector.
 */

private static final int HARD_CACHE_CAPACITY = 10;
private static final int DELAY_BEFORE_PURGE = 10 * 1000; // in milliseconds

// Hard cache, with a fixed maximum capacity and a life duration
private final HashMap<String, Bitmap> sHardBitmapCache =
    new LinkedHashMap<String, Bitmap>(HARD_CACHE_CAPACITY / 2, 0.75f, true) {
    @Override
    protected boolean removeEldestEntry(LinkedHashMap.Entry<String, Bitmap> eldest) {
        if (size() > HARD_CACHE_CAPACITY) {
            // Entries push-out of hard reference cache are transferred to soft reference cache
            sSoftBitmapCache.put(eldest.getKey(), new SoftReference<Bitmap>(eldest.getValue()));
            return true;
        } else
            return false;
    }
};

// Soft cache for bitmaps kicked out of hard cache
private final static ConcurrentHashMap<String, SoftReference<Bitmap>> sSoftBitmapCache =
    new ConcurrentHashMap<String, SoftReference<Bitmap>>(HARD_CACHE_CAPACITY / 2);

private final Handler purgeHandler = new Handler();

private final Runnable purger = new Runnable() {
    public void run() {
        clearCache();
    }
};

/**
 * Adds this bitmap to the cache.
 * @param bitmap The newly downloaded bitmap.
 */
private void addBitmapToCache(String url, Bitmap bitmap) {
    if (bitmap != null) {
        synchronized (sHardBitmapCache) {
            sHardBitmapCache.put(url, bitmap);
        }
    }
}

/**
 * @param url The URL of the image that will be retrieved from the cache.
 * @return The cached bitmap or null if it was not found.
 */
private Bitmap getBitmapFromCache(String url) {
    // First try the hard reference cache
    synchronized (sHardBitmapCache) {
        final Bitmap bitmap = sHardBitmapCache.get(url);
        if (bitmap != null) {
            // Bitmap found in hard cache
            // Move element to first position, so that it is removed last
            sHardBitmapCache.remove(url);
            sHardBitmapCache.put(url, bitmap);
            return bitmap;
        }
    }

    // Then try the soft reference cache
    SoftReference<Bitmap> bitmapReference = sSoftBitmapCache.get(url);
    if (bitmapReference != null) {
        final Bitmap bitmap = bitmapReference.get();
        if (bitmap != null) {
            // Bitmap found in soft cache
            return bitmap;
        } else {
            // Soft reference has been Garbage Collected
            sSoftBitmapCache.remove(url);
        }
    }

    return null;
}

/**
 * Clears the image cache used internally to improve performance. Note that for memory
 * efficiency reasons, the cache will automatically be cleared after a certain inactivity delay.
 */
public void clearCache() {
    sHardBitmapCache.clear();
    sSoftBitmapCache.clear();
}

/**
 * Allow a new delay before the automatic cache clear is done.
 */
private void resetPurgeTimer() {
    purgeHandler.removeCallbacks(purger);
    purgeHandler.postDelayed(purger, DELAY_BEFORE_PURGE);
}

}</pre>



里面有一些圖片加載的方式方法，都很可取，不過對于ListView的優化而言，還應配合列表滾動監聽來寫。也就是說，當列表停止滾動，手離開屏幕的時候，再加載大數據，例如圖片。稍后，我也會整理一篇，給大家提供參考借鑒。





源代碼來自：http://code.google.com/p/android-imagedownloader/



滾動加載的例子：http://www.iteye.com/topic/1118828