我编写了以下代码，该代码使用CWAC Endless Adapter实现了 ListView 的无限滚动。使用 AsyncTasks 从 Web 按需检索图像：
package com.myproject.ui.adapter;
import java.io.InputStream;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.util.LruCache;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import com.commonsware.cwac.endless.EndlessAdapter;
public class MyThumbsAdapter extends EndlessAdapter {
public static class FetchThumbTask extends AsyncTask<String, Void, Bitmap> {
private final Context mContext;
private final BaseAdapter mAdapter;
private final String mThumbId;
private final View mView;
public FetchThumbTask(Context context, BaseAdapter adapter, View view, String thumbId) {
mContext = context;
mAdapter = adapter;
mView = view;
mThumbId = thumbId;
}
@Override
protected Bitmap doInBackground(String... arg0) {
Bitmap bitmap = null;
if (cache.get(mThumbId) == null) {
// Fetch thumbnail
try {
URL url = new URL(...);
InputStream is = url.openStream();
bitmap = BitmapFactory.decodeStream(is);
cache.put(mThumbId, bitmap);
} catch (Exception e) {
...
}
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
// Set the loaded image on the ImageView
ImageView imageView = (ImageView) mView.findViewById(R.id.thumb_image);
if (imageView != null) {
imageView.setImageBitmap(bitmap);
}
}
}
public static class MyThumbsBaseAdapter extends BaseAdapter {
private final Context mContext;
private final List<String> mThumbIds = new ArrayList<String>();
public MyThumbsBaseAdapter(Context context) {
mContext = context;
}
public void addThumbIds(List<String> thumbIds) {
mThumbIds.addAll(thumbIds);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
String thumbId = mThumbIds.get(position);
View rootView = convertView;
if (rootView == null) {
rootView = LayoutInflater.from(parent.getContext()).inflate(
R.layout.thumbnails, null);
}
ImageView imageView =
(ImageView) rootView.findViewById(R.id.doodle_thumb_image);
Bitmap bitmap = cache.get(thumbId);
if (bitmap == null) {
loadThumbBitmap(rootView, thumbId);
} else if (imageView != null) {
imageView.setImageBitmap(bitmap);
}
return rootView;
}
@Override
public int getCount() {
return mThumbIds.size();
}
@Override
public Object getItem(int position) {
return null;
}
@Override
public long getItemId(int position) {
return 0;
}
private void loadThumbBitmap(View view, String thumbId) {
new FetchThumbTask(mContext, this, view, thumbId)
.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
}
}
static {
int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
int cacheSize = maxMemory / 8;
cache = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap bitmap) {
// The cache size will be measured in kilobytes rather than
// number of items.
return bitmap.getByteCount() / 1024;
}
};
}
private static final LruCache<String, Bitmap> cache;
private List<String> mThumbIdsCache;
public MyThumbsAdapter(Context context) {
super(new MyThumbsBaseAdapter(context));
}
@Override
protected View getPendingView(ViewGroup parent) {
return LayoutInflater.from(parent.getContext())
.inflate(R.layout.loading_thumb, null);
}
@Override
protected boolean cacheInBackground() throws Exception {
JsonReader reader = // Retrieve thumb ids list from server
mThumbIdsCache = // Returned thumb ids list
return true;
}
@Override
protected void appendCachedData() {
MyThumbsBaseAdapter adapter = (MyThumbsBaseAdapter) getWrappedAdapter();
adapter.addThumbIds(mThumbIdsCache);
}
}
我正在使用 LruCache 缓存从网络加载的位图。问题是在我的 Nexus 7 上进行测试时，我看到很多缓存未命中，它应该有足够的可用内存。当我向上/向下滚动 ListView 时，这会导致图像弹出到位。
更糟糕的是，我看到应用程序崩溃并出现 OutOfMemory 错误，但我无法始终如一地重现。
我在这里做错了什么？我不应该为每个图像触发单独的 AsyncTasks 吗？
编辑：我还应该提到我正在下载的图像是预先缩放的。