问题
I have used the async task to download the data and then showing those images in the list view I used the cache to store the images as they are repeating but not in an order. But the images gets jumbled up and sometimes they don't download. I tried searching this, but couldn't find that.
This was one of mine dream company question and i didn't clear because of this. Please help me around.
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private static final String BASE_URL = "https://ajax.googleapis.com/ajax/services/search/images?v=1.0&q=android&start=";
private static final String IMAGE_JSON_KEY = "unescapedUrl";
private static final String RESULTS_JSON_KEY = "results";
private static final String RESPONSE_DATA_JSON_KEY = "responseData";
private int mCurrentPage;
private ListView mListView;
private Context mContext;
ArrayList<String> mImageUrls;
private LruCache<String, Bitmap> mCache;
private CustomListAdapter mAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Get memory class of this device, exceeding this amount will throw an
// OutOfMemory exception.
final int memClass = ((ActivityManager) getSystemService(Context.ACTIVITY_SERVICE)).getMemoryClass();
// Use 1/8th of the available memory for this memory cache.
final int cacheSize = 1024 * 1024 * memClass / 8;
// Initialize variables
mContext = this;
mCache = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap value) {
// The cache size will be measured in bytes rather than number of items.
return value.getByteCount();
}
};
mListView = (ListView) findViewById(R.id.list_view);
mAdapter = new CustomListAdapter();
mImageUrls = new ArrayList<>();
// If the urls are wrong then
if (mImageUrls.isEmpty() || checkDiff()) {
fetchNewImageUrls();
}
// Set the adapter to the list view
mListView.setAdapter(mAdapter);
}
class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {
private final WeakReference<ImageView> imageViewReference;
private String mUrl;
public BitmapWorkerTask(ImageView imageView) {
// Use a WeakReference to ensure the ImageView can be garbage collected
imageViewReference = new WeakReference<ImageView>(imageView);
}
@Override
protected Bitmap doInBackground(String... params) {
String imageUrl = params[0];
mUrl = imageUrl;
Bitmap bitmap = getBitmapFromMemCache(imageUrl);
if (bitmap == null) {
try {
URL url = new URL(imageUrl);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setDoInput(true);
connection.connect();
// Closing the stream after getting the sample size
InputStream inputStream = connection.getInputStream();
byte[] imageByteArray = convertToByteArray(inputStream);
bitmap = decodeSampledBitmap(imageByteArray, 200, 200);
inputStream.close();
if (bitmap != null) {
Log.d(TAG, "Image downloaded: " + imageUrl);
addBitmapToMemoryCache(imageUrl, bitmap);
} else {
Log.d(TAG, "Null Bitmap downloaded for: " + imageUrl);
}
connection.disconnect();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
} else {
Log.d(TAG, "Already present in the Cache");
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
if (imageViewReference != null && bitmap != null) {
final ImageView imageView = (ImageView) imageViewReference.get();
if (imageView != null && imageView.getTag().equals(mUrl)) {
imageView.setImageBitmap(bitmap);
}
}
}
}
public class CustomListAdapter extends BaseAdapter {
@Override
public int getCount() {
return mImageUrls.size();
}
@Override
public Object getItem(int position) {
return mImageUrls.get(position);
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
Log.d(TAG, "Get View is called for position: " + position);
View view = convertView;
Holder holder = null;
// Holder represents the elements of the view to use
// Here are initialized
if (view == null) {
view = LayoutInflater.from(mContext).inflate(R.layout.row_item, parent, false);
holder = new Holder();
holder.mRowImage = (ImageView) view.findViewById(R.id.image_view);
holder.mRowText = (TextView) view.findViewById(R.id.row_text);
view.setTag(holder);
} else {
holder = (Holder) view.getTag();
}
// Set default image background
holder.mRowImage.setImageResource(R.drawable.ic_launcher);
// here do operations in holder variable example
holder.mRowText.setText("Image Number: " + position);
// Set the tag for the imageview
holder.mRowImage.setTag(mImageUrls.get(position));
new BitmapWorkerTask(holder.mRowImage).execute(mImageUrls.get(position));
return view;
}
}
public static class Holder {
TextView mRowText;
ImageView mRowImage;
}
public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
if (getBitmapFromMemCache(key) == null) {
mCache.put(key, bitmap);
}
}
public Bitmap getBitmapFromMemCache(String key) {
return (Bitmap) mCache.get(key);
}
public Bitmap decodeSampledBitmap(byte[] imageByteArray, int reqWidth, int reqHeight) {
Bitmap bm = null;
// First decode with inJustDecodeBounds=true to check dimensions
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeByteArray(imageByteArray, 0, imageByteArray.length, options);
// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
options.inJustDecodeBounds = false;
bm = BitmapFactory.decodeByteArray(imageByteArray, 0, imageByteArray.length, options);
return bm;
}
public int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {
// Raw height and width of image
final int height = options.outHeight;
final int width = options.outWidth;
int inSampleSize = 1;
if (height > reqHeight || width > reqWidth) {
if (width > height) {
inSampleSize = Math.round((float) height / (float) reqHeight);
} else {
inSampleSize = Math.round((float) width / (float) reqWidth);
}
}
Log.d(TAG, "Sample size is: " + inSampleSize);
return inSampleSize;
}
private void fetchNewImageUrls() {
new AsyncTask<String, Void, Boolean>() {
@Override
protected Boolean doInBackground(String... params) {
try {
StringBuilder response = new StringBuilder();
URL url = new URL(params[0]);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.connect();
BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null)
response.append(inputLine);
in.close();
connection.disconnect();
String resp = response.toString();
Log.d(TAG, "Response is: " + response);
// Parsing the response
JSONObject jsonObject = new JSONObject(resp);
JSONObject jsonObject1 = jsonObject.getJSONObject(RESPONSE_DATA_JSON_KEY);
JSONArray jsonArray = jsonObject1.getJSONArray(RESULTS_JSON_KEY);
for (int i = 0; i < 4; i++) {
JSONObject dataObject = jsonArray.getJSONObject(i);
mImageUrls.add(dataObject.getString(IMAGE_JSON_KEY));
}
mCurrentPage++;
Log.d(TAG, "Number of image urls are: " + mImageUrls.size());
return true;
} catch (JSONException e) {
e.printStackTrace();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return false;
}
@Override
protected void onPostExecute(Boolean value) {
super.onPostExecute(value);
if (checkDiff() && value) {
Log.d(TAG, "Again fetching the Images");
fetchNewImageUrls();
}
if (!value) {
Log.d(TAG, "Error while getting the response");
}
mAdapter.notifyDataSetChanged();
}
}.execute(BASE_URL + mCurrentPage);
}
private boolean checkDiff() {
int diff = mImageUrls.size() - mCurrentPage * 4;
Log.d(TAG, "Diff is: " + diff);
return diff < 8;
}
public static byte[] convertToByteArray(InputStream input) {
byte[] buffer = new byte[8192];
int bytesRead;
ByteArrayOutputStream output = new ByteArrayOutputStream();
try {
while ((bytesRead = input.read(buffer)) != -1) {
output.write(buffer, 0, bytesRead);
}
} catch (IOException e) {
e.printStackTrace();
}
return output.toByteArray();
}
}
回答1:
Since you do not cancel the unfinished "out of view" downloads these may interfere with your gui.
Example
- listview line1 shows item#1 with has an async task to download image "a" not completed yet
- scroll down, line1 is now invisible;
- listview line8 becomes visible recycling item#1 new async task to download image "x"
- async task to download image "a" finishes displaying wrong image. Line8 shows image "a" instead of "x"
to solve this you have to cancel the pending unnecessary unfinished async task-s
public static class Holder {
ImageView mRowImage;
String mImageUrl;
// neccessary to cancel unfinished download
BitmapWorkerTask mDownloader;
}
static class BitmapWorkerTask extends AsyncTask<Holder, Void, Bitmap> {
Holder mHolder;
protected Bitmap doInBackground(Holder... holders) {
mHolder = holders[0];
...
}
protected void onPostExecute(...) {
mHolder.mDownloader = null;
if (!isCancelled()) {
this.mHolder.mRowImage.setImageBitmap(image);
}
this.mHolder = null;
}
}
public class CustomListAdapter extends BaseAdapter {
@Override
public View getView(int position, ...) {
...
if (view == null) {
holder = ...
...
} else {
holder = (Holder) view.getTag();
}
...
// cancel unfinished mDownloader
if (holder.mDownloader != null) {
holder.mDownloader.cancel(false);
holder.mDownloader = null;
}
holder.mImageUrl = mImageUrls.get(position);
holder.mDownloader = new BitmapWorkerTask()
holder.mDownloader.execute(holder);
}
}
Here is a working example for this combination of Adapter + Holder + AsyncTask
[Update] Potential problems with this solution.
- Most modern android versions execute only one async task at a time. If you are fetching the images via the web you cannot download multible images at the same time. See running-parallel-asynctask@stackoverflow for details.
- There may be promlems with configuration change(like orientation). See @Selvin-s comment below. I have posted this question "what-happens-with-view-references-in-unfinished-asynctask-after-orientation-chan@stackoverflow" to find out more about it.
来源：https://stackoverflow.com/questions/33366547/list-view-showing-images-jumbled-after-downloading-images-using-asynctask