我正在使用ListView来显示与这些图像相关的一些图像和标题。我正在从互联网上获取图像。是否有一种方法可以延迟加载图像,以便在文本显示时,UI不会被阻止,图像在下载时显示?
图像总数不是固定的。
我正在使用ListView来显示与这些图像相关的一些图像和标题。我正在从互联网上获取图像。是否有一种方法可以延迟加载图像,以便在文本显示时,UI不会被阻止,图像在下载时显示?
图像总数不是固定的。
当前回答
更新:请注意,这个答案现在非常无效。垃圾回收器对SoftReference和WeakReference进行攻击,因此此代码不适用于新应用。(相反,请尝试其他答案中建议的通用图像加载器之类的库。)
感谢James提供的代码,感谢鲍龙提供的使用SoftReference的建议。我在James的代码上实现了SoftReference更改。不幸的是,SoftReferences导致我的图像被垃圾收集得太快。在我的情况下,没有SoftReference的东西很好,因为我的列表大小有限,图像也很小。
一年前有一场关于谷歌群组上SoftReferences的讨论:链接到线程。作为过早垃圾收集的解决方案,他们建议使用dalvik.system.VMRuntime.setMinimumHeapSize()手动设置VM堆大小,这对我来说不是很有吸引力。
public DrawableManager() {
drawableMap = new HashMap<String, SoftReference<Drawable>>();
}
public Drawable fetchDrawable(String urlString) {
SoftReference<Drawable> drawableRef = drawableMap.get(urlString);
if (drawableRef != null) {
Drawable drawable = drawableRef.get();
if (drawable != null)
return drawable;
// Reference has expired so remove the key from drawableMap
drawableMap.remove(urlString);
}
if (Constants.LOGGING) Log.d(this.getClass().getSimpleName(), "image url:" + urlString);
try {
InputStream is = fetch(urlString);
Drawable drawable = Drawable.createFromStream(is, "src");
drawableRef = new SoftReference<Drawable>(drawable);
drawableMap.put(urlString, drawableRef);
if (Constants.LOGGING) Log.d(this.getClass().getSimpleName(), "got a thumbnail drawable: " + drawable.getBounds() + ", "
+ drawable.getIntrinsicHeight() + "," + drawable.getIntrinsicWidth() + ", "
+ drawable.getMinimumHeight() + "," + drawable.getMinimumWidth());
return drawableRef.get();
} catch (MalformedURLException e) {
if (Constants.LOGGING) Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
return null;
} catch (IOException e) {
if (Constants.LOGGING) Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
return null;
}
}
public void fetchDrawableOnThread(final String urlString, final ImageView imageView) {
SoftReference<Drawable> drawableRef = drawableMap.get(urlString);
if (drawableRef != null) {
Drawable drawable = drawableRef.get();
if (drawable != null) {
imageView.setImageDrawable(drawableRef.get());
return;
}
// Reference has expired so remove the key from drawableMap
drawableMap.remove(urlString);
}
final Handler handler = new Handler() {
@Override
public void handleMessage(Message message) {
imageView.setImageDrawable((Drawable) message.obj);
}
};
Thread thread = new Thread() {
@Override
public void run() {
//TODO : set imageView to a "pending" image
Drawable drawable = fetchDrawable(urlString);
Message message = handler.obtainMessage(1, drawable);
handler.sendMessage(message);
}
};
thread.start();
}
其他回答
URLImageViewHelper是一个非常棒的库,可以帮助您做到这一点。
我用图像制作了一个懒惰列表(位于GitHub)的简单演示。
基本用途ImageLoader ImageLoader=新的ImageLoader(上下文)。。。imageLoader.DisplayImage(url,imageView);不要忘记添加对AndroidManifest.xml的以下权限:<uses permission android:name=“android.ppermission.INTERNET”/><uses permission android:name=“android.permission.WRITE_EXTERNAL_STORAGE”/>请只创建ImageLoader的一个实例,并在您的应用这样,图像缓存将更加高效。
这可能对某人有帮助。它在后台线程中下载图像。图像缓存在SD卡和内存中。缓存的实现非常简单,对于演示来说就足够了。我使用inSampleSize解码图像以减少内存消耗。我还尝试正确处理回收的视图。
我写了一篇教程,解释如何在列表视图中延迟加载图像。我详细讨论了回收和并发的问题。我还使用固定线程池来防止产生大量线程。
在Listview教程中延迟加载图像
使用滑动库。它适用于我,也适用于你的代码。它也适用于图像和gif。
ImageView imageView = (ImageView) findViewById(R.id.test_image);
GlideDrawableImageViewTarget imagePreview = new GlideDrawableImageViewTarget(imageView);
Glide
.with(this)
.load(url)
.listener(new RequestListener<String, GlideDrawable>() {
@Override
public boolean onException(Exception e, String model, Target<GlideDrawable> target, boolean isFirstResource) {
return false;
}
@Override
public boolean onResourceReady(GlideDrawable resource, String model, Target<GlideDrawable> target, boolean isFromMemoryCache, boolean isFirstResource) {
return false;
}
})
.into(imagePreview);
}
public class ImageDownloader {
Map<String, Bitmap> imageCache;
public ImageDownloader() {
imageCache = new HashMap<String, Bitmap>();
}
// download function
public void download(String url, ImageView imageView) {
if (cancelPotentialDownload(url, imageView)) {
// Caching code right here
String filename = String.valueOf(url.hashCode());
File f = new File(getCacheDirectory(imageView.getContext()),
filename);
// Is the bitmap in our memory cache?
Bitmap bitmap = null;
bitmap = (Bitmap) imageCache.get(f.getPath());
if (bitmap == null) {
bitmap = BitmapFactory.decodeFile(f.getPath());
if (bitmap != null) {
imageCache.put(f.getPath(), bitmap);
}
}
// No? download it
if (bitmap == null) {
try {
BitmapDownloaderTask task = new BitmapDownloaderTask(
imageView);
DownloadedDrawable downloadedDrawable = new DownloadedDrawable(
task);
imageView.setImageDrawable(downloadedDrawable);
task.execute(url);
} catch (Exception e) {
Log.e("Error==>", e.toString());
}
} else {
// Yes? set the image
imageView.setImageBitmap(bitmap);
}
}
}
// cancel a download (internal only)
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;
}
// gets an existing download if one exists for the imageview
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;
}
// our caching functions
// Find the dir to save cached images
private static File getCacheDirectory(Context context) {
String sdState = android.os.Environment.getExternalStorageState();
File cacheDir;
if (sdState.equals(android.os.Environment.MEDIA_MOUNTED)) {
File sdDir = android.os.Environment.getExternalStorageDirectory();
// TODO : Change your diretcory here
cacheDir = new File(sdDir, "data/ToDo/images");
} else
cacheDir = context.getCacheDir();
if (!cacheDir.exists())
cacheDir.mkdirs();
return cacheDir;
}
private void writeFile(Bitmap bmp, File f) {
FileOutputStream out = null;
try {
out = new FileOutputStream(f);
bmp.compress(Bitmap.CompressFormat.PNG, 80, out);
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
if (out != null)
out.close();
} catch (Exception ex) {
}
}
}
// download asynctask
public class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
private String url;
private final WeakReference<ImageView> imageViewReference;
public BitmapDownloaderTask(ImageView imageView) {
imageViewReference = new WeakReference<ImageView>(imageView);
}
@Override
// Actual download method, run in the task thread
protected Bitmap doInBackground(String... params) {
// params comes from the execute() call: params[0] is the url.
url = (String) params[0];
return downloadBitmap(params[0]);
}
@Override
// Once the image is downloaded, associates it to the imageView
protected void onPostExecute(Bitmap bitmap) {
if (isCancelled()) {
bitmap = null;
}
if (imageViewReference != null) {
ImageView imageView = imageViewReference.get();
BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
// Change bitmap only if this process is still associated with
// it
if (this == bitmapDownloaderTask) {
imageView.setImageBitmap(bitmap);
// cache the image
String filename = String.valueOf(url.hashCode());
File f = new File(
getCacheDirectory(imageView.getContext()), filename);
imageCache.put(f.getPath(), bitmap);
writeFile(bitmap, f);
}
}
}
}
static class DownloadedDrawable extends ColorDrawable {
private final WeakReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;
public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) {
super(Color.WHITE);
bitmapDownloaderTaskReference = new WeakReference<BitmapDownloaderTask>(
bitmapDownloaderTask);
}
public BitmapDownloaderTask getBitmapDownloaderTask() {
return bitmapDownloaderTaskReference.get();
}
}
// the actual download code
static Bitmap downloadBitmap(String url) {
HttpParams params = new BasicHttpParams();
params.setParameter(CoreProtocolPNames.PROTOCOL_VERSION,
HttpVersion.HTTP_1_1);
HttpClient client = new DefaultHttpClient(params);
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();
final Bitmap bitmap = BitmapFactory
.decodeStream(inputStream);
return bitmap;
} finally {
if (inputStream != null) {
inputStream.close();
}
entity.consumeContent();
}
}
} catch (Exception e) {
// Could provide a more explicit error message for IOException or
// IllegalStateException
getRequest.abort();
Log.w("ImageDownloader", "Error while retrieving bitmap from "
+ url + e.toString());
} finally {
if (client != null) {
// client.close();
}
}
return null;
}
}