AsyncTaskLoader basic example. (Android)
If you wish to use AsyncTaskLoader, here's a nice sample for you.
EDIT: I've decided to make a simpler solution (based on this repo):
public abstract class AsyncTaskLoaderEx<T> extends AsyncTaskLoader<T> {
private static final AtomicInteger sCurrentUniqueId = new AtomicInteger(0);
private T mData;
public boolean hasResult = false;
public static int getNewUniqueLoaderId() {
return sCurrentUniqueId.getAndIncrement();
}
public AsyncTaskLoaderEx(final Context context) {
super(context);
onContentChanged();
}
@Override
protected void onStartLoading() {
if (takeContentChanged())
forceLoad();
//this part should be removed from support library 27.1.0 :
//else if (hasResult)
// deliverResult(mData);
}
@Override
public void deliverResult(final T data) {
mData = data;
hasResult = true;
super.deliverResult(data);
}
@Override
protected void onReset() {
super.onReset();
onStopLoading();
if (hasResult) {
onReleaseResources(mData);
mData = null;
hasResult = false;
}
}
protected void onReleaseResources(T data) {
//nothing to do.
}
public T getResult() {
return mData;
}
}
Usage:
in your activity:
getSupportLoaderManager().initLoader(TASK_ID, TASK_BUNDLE, new LoaderManager.LoaderCallbacks<Bitmap>() {
@Override
public Loader<Bitmap> onCreateLoader(final int id, final Bundle args) {
return new ImageLoadingTask(MainActivity.this);
}
@Override
public void onLoadFinished(final Loader<Bitmap> loader, final Bitmap result) {
if (result == null)
return;
//TODO use result
}
@Override
public void onLoaderReset(final Loader<Bitmap> loader) {
}
});
inner static class , or a normal class:
private static class ImageLoadingTask extends AsyncTaskLoaderEx<Bitmap> {
public ImageLoadingTask (Context context) {
super(context);
}
@Override
public Bitmap loadInBackground() {
//TODO load and return bitmap
}
}
Update: starting from support library 27.1.0, things changed a bit (link here) :
In version 27.1.0, onStartLoading() is called every time the Activity is started. Since you call deliverResult() in onStartLoading(), you trigger onLoadFinished(). This is Working as Intended.
You should remove your call to deliverResult() from onStartLoading() as it is not needed (Loaders already deliver results computed in loadInBackground() without any additional work needed on your part).
I've updated the code above for this change.
EDIT: Updated, kotlin version can be found here.
Since Honeycomb and the v4 Compatibility Library it is possible to use AsyncTaskLoader
. From what I understand, the AsyncTaskLoader
can survive through config changes like screen flips. But using AsyncTask
you can mess up with configuration changes.
Key information: AsyncTaskLoader
is subclass of Loader
. This class performs the same function as the AsyncTask, but a bit better, it can also be useful in handling configuration changes (screen orientation).
A very good example and explanation is given here. http://www.javacodegeeks.com/2013/01/android-loaders-versus-asynctask.html
Google has a pretty good example directly in the API Docs. Android Design Patterns provides some more detail and the reasoning behind Loaders.
This tutorial will definetly help You. http://www.javacodegeeks.com/2013/08/android-custom-loader-to-load-data-directly-from-sqlite-database.html
Here's step by step tutorial to implement
AsyncTaskLoader
. or check out this same article on Medium
-
Implement
LoaderManager.LoaderCallbacks<String>
on MainActivity and create astatic int
to uniquely identify your loader and create a String key to pass string url to your loaderpublic class MainActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<String>{ public static final int OPERATION_SEARCH_LOADER = 22; public static final String OPERATION_QUERY_URL_EXTRA = "query"; //...}
-
Override
onCreateLoader
,onLoadFinished
andonLoaderReset
functions inside MainActivity@Override public Loader<String> onCreateLoader(int id, final Bundle args) { //Here we will initiate AsyncTaskLoader return null; } @Override public void onLoadFinished(Loader<String> loader, String operationResult) { //Think of this as AsyncTask onPostExecute method, the result from onCreateLoader will be available in operationResult variable and here you can update UI with the data fetched. Log.d("MAINACTIVITY","result : "+ operationResult); } @Override public void onLoaderReset(Loader<String> loader) { //Don't bother about it, Android Studio will override it for you }
-
inside
onCreateLoader()
return a newAsyncTaskLoader<String>
as an anonymous inner class withthis
as the constructor's parameter and overrideloadInBackground
&onStartLoading
inside anonymous inner class@Override public Loader<String> onCreateLoader(int id, final Bundle args) { return new AsyncTaskLoader<String>(this) { @Override public String loadInBackground() { //Think of this as AsyncTask doInBackground() method, here you will actually initiate Network call return null; } @Override protected void onStartLoading() { //Think of this as AsyncTask onPreExecute() method,start your progress bar,and at the end call forceLoad(); forceLoad(); } }; }
-
Inside
loadInBackground
make a network call using HTTPUrlConnection or OKHttp or anything that you use.@Override public String loadInBackground() { String url = args.getString(OPERATION_QUERY_URL_EXTRA);//This is a url in string form if (url!=null&&"".equals(url)) { return null;//if url is null, return } String operationResult=""; try { operationResult = NetworkUtils.getResponseFromHttpUrl(url);//This just create a HTTPUrlConnection and return result in strings } catch (IOException e) { e.printStackTrace(); } return operationResult; }
-
Inside
onCreate
initialize the loader with OPERATION_SEARCH_LOADER as the ID, null for the bundle, and this for the contextgetSupportLoaderManager().initLoader(OPERATION_SEARCH_LOADER, null, this);
-
Now call this method, whenever and wherever you want to trigger the loader
private void makeOperationSearchQuery(String url) { // Create a bundle called queryBundle Bundle queryBundle = new Bundle(); // Use putString with OPERATION_QUERY_URL_EXTRA as the key and the String value of the URL as the value queryBundle.putString(OPERATION_QUERY_URL_EXTRA,url); // Call getSupportLoaderManager and store it in a LoaderManager variable LoaderManager loaderManager = getSupportLoaderManager(); // Get our Loader by calling getLoader and passing the ID we specified Loader<String> loader = loaderManager.getLoader(OPERATION_SEARCH_LOADER); // If the Loader was null, initialize it. Else, restart it. if(loader==null){ loaderManager.initLoader(OPERATION_SEARCH_LOADER, queryBundle, this); }else{ loaderManager.restartLoader(OPERATION_SEARCH_LOADER, queryBundle, this); } }
Walla, you are done, just to remind you NetworkUtils.getResponseFromHttpUrl(url);
is my custom function which take string convert it into URL
which in turn used to create HTTPUrlConnection
I like this brief example AsyncTask and AsyncTaskLoader.
class FooLoader extends AsyncTaskLoader {
public FooLoader(Context context, Bundle args) {
super(context);
// do some initializations here
}
public String loadInBackground() {
String result = "";
// ...
// do long running tasks here
// ...
return result;
}
}
class FooLoaderClient implements LoaderManager.LoaderCallbacks {
Activity context;
// to be used for support library:
// FragmentActivity context2;
public Loader onCreateLoader(int id, Bundle args) {
// init loader depending on id
return new FooLoader(context, args);
}
public void onLoadFinished(Loader loader, String data) {
// ...
// update UI here
//
}
public void onLoaderReset(Loader loader) {
// ...
}
public void useLoader() {
Bundle args = new Bundle();
// ...
// fill in args
// ...
Loader loader =
context.getLoaderManager().initLoader(0, args, this);
// with support library:
// Loader loader =
// context2.getSupportLoaderManager().initLoader(0, args, this);
// call forceLoad() to start processing
loader.forceLoad();
}
}