我使用 ImageLoader 来获取图像并显示进度条，直到图像被下载。ArrayAdapter 使用此类将图像加载到 GridView。
我想知道当加载此网格视图的片段已与活动分离时如何释放图像内存。此外，如果加载此视图后发生多个片段转换，将其存储在内存（即回栈）中是否昂贵。
这是我的代码
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.HttpURLConnection;
import java.net.URL;
import java.util.Collections;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.util.Log;
import android.view.View;
import android.widget.ImageView;
import android.widget.ProgressBar;
public class ImageLoader {
MemoryCache memoryCache = new MemoryCache();
FileCache fileCache;
private Map<ImageView, String> imageViews = Collections
.synchronizedMap(new WeakHashMap<ImageView, String>());
ExecutorService executorService;
Activity activity = null;
public ImageLoader(Context context) {
fileCache = new FileCache(context);
executorService = Executors.newFixedThreadPool(5);
}
public void DisplayImage(String url, Activity activity,
ImageView imageView, ProgressBar p) {
imageViews.put(imageView, url);
this.activity = activity;
Bitmap bitmap = memoryCache.get(url);
if (bitmap != null) {
imageView.setImageBitmap(bitmap);
imageView.setTag("loaded");
imageView.setVisibility(View.VISIBLE);
p.setVisibility(View.GONE);
} else {
queuePhoto(url, imageView, p);
imageView.setImageResource(Utils.stub_id);
}
}
private void queuePhoto(String url, ImageView imageView, ProgressBar pb) {
PhotoToLoad p = new PhotoToLoad(url, imageView, activity, pb);
executorService.submit(new PhotosLoader(p));
}
private Bitmap getBitmap(String url) {
File f = fileCache.getFile(url);
// from SD cache
Bitmap b = decodeFile(f);
if (b != null)
return b;
// from web
try {
Bitmap bitmap = null;
URL imageUrl = new URL(url);
HttpURLConnection conn = (HttpURLConnection) imageUrl
.openConnection();
conn.setConnectTimeout(Utils.timeout);
conn.setReadTimeout(Utils.timeout);
conn.setInstanceFollowRedirects(true);
InputStream is = conn.getInputStream();
OutputStream os = new FileOutputStream(f);
Utils.CopyStream(is, os);
os.close();
bitmap = decodeFile(f);
return bitmap;
} catch (Exception ex) {
return null;
}
}
// decodes image and scales it to reduce memory consumption
private Bitmap decodeFile(File f) {
try {
// decode image size
BitmapFactory.Options o = new BitmapFactory.Options();
o.inJustDecodeBounds = true;
BitmapFactory.decodeStream(new FileInputStream(f), null, o);
// Find the correct scale value. It should be the power of 2.
final int REQUIRED_SIZE = 70;
int width_tmp = o.outWidth, height_tmp = o.outHeight;
int scale = 1;
while (true) {
if (width_tmp / 2 < REQUIRED_SIZE
|| height_tmp / 2 < REQUIRED_SIZE)
break;
width_tmp /= 2;
height_tmp /= 2;
scale *= 2;
}
// decode with inSampleSize
BitmapFactory.Options o2 = new BitmapFactory.Options();
o2.inSampleSize = scale;
return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);
} catch (FileNotFoundException e) {
}
return null;
}
// Task for the queue
private class PhotoToLoad {
public String url;
public ImageView imageView;
Activity activity;
public ProgressBar progressBar;
public PhotoToLoad(String u, ImageView i, Activity a, ProgressBar p) {
url = u;
imageView = i;
activity = a;
progressBar = p;
}
}
class PhotosLoader implements Runnable {
PhotoToLoad photoToLoad;
PhotosLoader(PhotoToLoad photoToLoad) {
this.photoToLoad = photoToLoad;
}
@Override
public void run() {
if (imageViewReused(photoToLoad))
return;
try {
Bitmap bmp = getBitmap(photoToLoad.url);
memoryCache.put(photoToLoad.url, bmp);
if (imageViewReused(photoToLoad))
return;
BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad);
// Activity a = (Activity) photoToLoad.imageView.getContext();
photoToLoad.activity.runOnUiThread(bd);
} catch (Exception e) {
String s = e.getMessage();
}
}
}
boolean imageViewReused(PhotoToLoad photoToLoad) {
String tag = imageViews.get(photoToLoad.imageView);
if (tag == null || !tag.equals(photoToLoad.url))
return true;
return false;
}
// Used to display bitmap in the UI thread
class BitmapDisplayer implements Runnable {
Bitmap bitmap;
PhotoToLoad photoToLoad;
public BitmapDisplayer(Bitmap b, PhotoToLoad p) {
bitmap = b;
photoToLoad = p;
}
public void run() {
if (imageViewReused(photoToLoad))
return;
if (bitmap != null)
{
photoToLoad.imageView.setImageBitmap(bitmap);
photoToLoad.imageView.setTag("loaded");
}
else
photoToLoad.imageView.setImageResource(Utils.stub_id);
photoToLoad.imageView.setVisibility(View.VISIBLE);
photoToLoad.progressBar.setVisibility(View.GONE);
}
}
public void clearCache() {
memoryCache.clear();
fileCache.clear();
}
}
public class MemoryCache {
private Map<String, SoftReference<Bitmap>> cache=Collections.synchronizedMap(new HashMap<String, SoftReference<Bitmap>>());
public Bitmap get(String id){
if(!cache.containsKey(id))
return null;
SoftReference<Bitmap> ref=cache.get(id);
return ref.get();
}
public void put(String id, Bitmap bitmap){
cache.put(id, new SoftReference<Bitmap>(bitmap));
}
public void clear() {
cache.clear();
}
}