我有以下类解析JSON并显示名称和其他细节，我在listview中没有提到。
的 ShowViewActivity.java
public class ShowViewActivity extends ListActivity {
private static String url = "http://example.com/myfile.php";
private static final String TAG_ROOT = "root";
private static final String TAG_NAME = "name";
private static final String TAG_IMAGE = "image";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
ArrayList<HashMap<String, String>> myList = new ArrayList<HashMap<String, String>>();
JSONParser jParser = new JSONParser();
JSONObject json = jParser.getJSONFromUrl(url);
try {
root = json.getJSONArray(TAG_ROOT);
for (int i = 0; i < root.length(); i++) {
JSONObject c = root.getJSONObject(i);
String name = c.getString(TAG_NAME);
String image = c.getString(TAG_IMAGE);
HashMap<String, String> map = new HashMap<String, String>();
map.put(TAG_NAME, name);
map.put(TAG_IMAGE, image);
myList.add(map);
}
} catch (JSONException e) {
e.printStackTrace();
}
myAdapter adapter = new myAdapter(this, myList, R.layout.list_item,
new String[] {}, new int[] {});
setListAdapter(adapter);
final ListView lv = getListView();
// Launching new screen on Selecting Single ListItem
lv.setOnItemClickListener(new OnItemClickListener() {
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
}
});
}
private class myAdapter extends SimpleAdapter {
public myAdapter(Context context, List<? extends Map<String, ?>> data,
int resource, String[] from, int[] to) {
super(context, data, resource, from, to);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
convertView = getLayoutInflater().inflate(R.layout.list_item,
null);
}
HashMap<String, Object> data = (HashMap<String, Object>) getItem(position);
TextView nameTextView = (TextView) convertView
.findViewById(R.id.name);
ImageView iconImageView = (ImageView) convertView
.findViewById(R.id.listicon);
String nameString = (String) data.get(TAG_NAME);
nameTextView.setText(nameString);
String imageUrl = (String) data.get(TAG_IMAGE);
return convertView;
}
}
}
我想实现以下ImageDownloader类来显示来自网址的图片。
ImageDownloader.java
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;
public ImageLoader(Context context) {
// Make the background thead low priority. This way it will not affect
// the UI performance
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(),
"LazyList");
else
cacheDir = context.getCacheDir();
if (!cacheDir.exists())
cacheDir.mkdirs();
}
final int stub_id = R.drawable.icon;
public void DisplayImage(String url, Activity activity, ImageView imageView) {
if (cache.containsKey(url))
imageView.setImageBitmap(cache.get(url));
else {
queuePhoto(url, activity, imageView);
imageView.setImageResource(stub_id);
}
}
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();
}
private Bitmap getBitmap(String url) {
// 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;
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;
}
}
// 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++;
}
// 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;
public PhotoToLoad(String u, ImageView i) {
url = u;
imageView = i;
}
}
PhotosQueue photosQueue = new PhotosQueue();
public void stopThread() {
photoLoaderThread.interrupt();
}
// stores list of photos to download
class PhotosQueue {
private Stack<PhotoToLoad> photosToLoad = new Stack<PhotoToLoad>();
// removes all instances of this ImageView
public void Clean(ImageView image) {
for (int j = 0; j < photosToLoad.size();) {
if (photosToLoad.get(j).imageView == image)
photosToLoad.remove(j);
else
++j;
}
}
}
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
}
}
}
PhotosLoader photoLoaderThread = new PhotosLoader();
// Used to display bitmap in the UI thread
class BitmapDisplayer implements Runnable {
Bitmap bitmap;
ImageView imageView;
public BitmapDisplayer(Bitmap b, ImageView i) {
bitmap = b;
imageView = i;
}
public void run() {
if (bitmap != null)
imageView.setImageBitmap(bitmap);
else
imageView.setImageResource(stub_id);
}
}
public void clearCache() {
// clear memory cache
cache.clear();
// clear SD cache
File[] files = cacheDir.listFiles();
for (File f : files)
f.delete();
}
}
我看到扩展BaseAdapter的lazyload示例但在这里我使用的是我的自定义适配器名称myAdapter，它扩展了SimpleAdapter？
如何实现此ImageLoader类以显示来自web image url的imageview？
答案 0 :(得分：1)
第一步是获取ImageDownloader实例。我建议你为一个实例使用一些集中的位置，例如你自己的Application扩展名。这样你只需要创建一次，但在需要的地方使用它。此外，如果您（重新）每次调用都构造此对象，则提供的缓存将被限制为无用。或者，您可以将该类设置为单身，但我个人更喜欢前者。
获得ImageDownloader对象后，只需对自定义适配器进行更改，如下所述。由于您需要传入Activity引用，请保留传递给Context构造函数的myAdapter变量的句柄。在正常情况下，这实际上是运行时的Activity引用。
private class myAdapter extends SimpleAdapter {
private Context mContext;
public myAdapter(Context context, List<? extends Map<String, ?>> data, int resource, String[] from, int[] to) {
super(context, data, resource, from, to);
mContext = context;
}
...
@Override public View getView(int position, View convertView, ViewGroup parent) {
...
// assuming you have an instance of ImageDownloader here
//
mImageDownloader.DisplayImage(imageUrl,(Activity)mContext, iconImageView);
}
...
}