AsyncTaskLoader

public abstract class AsyncTaskLoader
extends Loader<D>

Known Direct Subclasses CursorLoader CursorLoader A loader that queries the ContentResolver and returns a Cursor.

Abstract Loader that provides an AsyncTask to do the work. See Loader and LoaderManager for more details.

Here is an example implementation of an AsyncTaskLoader subclass that loads the currently installed applications from the package manager. This implementation takes care of retrieving the application labels and sorting its result set from them, monitoring for changes to the installed applications, and rebuilding the list when a change in configuration requires this (such as a locale change).

/**
 * This class holds the per-item data in our Loader.
 */
public static class AppEntry {
    public AppEntry(AppListLoader loader, ApplicationInfo info) {
        mLoader = loader;
        mInfo = info;
        mApkFile = new File(info.sourceDir);
    }

    public ApplicationInfo getApplicationInfo() {
        return mInfo;
    }

    public String getLabel() {
        return mLabel;
    }

    public Drawable getIcon() {
        if (mIcon == null) {
            if (mApkFile.exists()) {
                mIcon = mInfo.loadIcon(mLoader.mPm);
                return mIcon;
            } else {
                mMounted = false;
            }
        } else if (!mMounted) {
            // If the app wasn't mounted but is now mounted, reload
            // its icon.
            if (mApkFile.exists()) {
                mMounted = true;
                mIcon = mInfo.loadIcon(mLoader.mPm);
                return mIcon;
            }
        } else {
            return mIcon;
        }

        return mLoader.getContext().getResources().getDrawable(
                android.R.drawable.sym_def_app_icon);
    }

    @Override public String toString() {
        return mLabel;
    }

    void loadLabel(Context context) {
        if (mLabel == null || !mMounted) {
            if (!mApkFile.exists()) {
                mMounted = false;
                mLabel = mInfo.packageName;
            } else {
                mMounted = true;
                CharSequence label = mInfo.loadLabel(context.getPackageManager());
                mLabel = label != null ? label.toString() : mInfo.packageName;
            }
        }
    }

    private final AppListLoader mLoader;
    private final ApplicationInfo mInfo;
    private final File mApkFile;
    private String mLabel;
    private Drawable mIcon;
    private boolean mMounted;
}

/**
 * Perform alphabetical comparison of application entry objects.
 */
public static final Comparator<AppEntry> ALPHA_COMPARATOR = new Comparator<AppEntry>() {
    private final Collator sCollator = Collator.getInstance();
    @Override
    public int compare(AppEntry object1, AppEntry object2) {
        return sCollator.compare(object1.getLabel(), object2.getLabel());
    }
};

/**
 * Helper for determining if the configuration has changed in an interesting
 * way so we need to rebuild the app list.
 */
public static class InterestingConfigChanges {
    final Configuration mLastConfiguration = new Configuration();
    int mLastDensity;

    boolean applyNewConfig(Resources res) {
        int configChanges = mLastConfiguration.updateFrom(res.getConfiguration());
        boolean densityChanged = mLastDensity != res.getDisplayMetrics().densityDpi;
        if (densityChanged || (configChanges&(ActivityInfo.CONFIG_LOCALE
                |ActivityInfo.CONFIG_UI_MODE|ActivityInfo.CONFIG_SCREEN_LAYOUT)) != 0) {
            mLastDensity = res.getDisplayMetrics().densityDpi;
            return true;
        }
        return false;
    }
}

/**
 * Helper class to look for interesting changes to the installed apps
 * so that the loader can be updated.
 */
public static class PackageIntentReceiver extends BroadcastReceiver {
    final AppListLoader mLoader;

    public PackageIntentReceiver(AppListLoader loader) {
        mLoader = loader;
        IntentFilter filter = new IntentFilter(Intent.ACTION_PACKAGE_ADDED);
        filter.addAction(Intent.ACTION_PACKAGE_REMOVED);
        filter.addAction(Intent.ACTION_PACKAGE_CHANGED);
        filter.addDataScheme("package");
        mLoader.getContext().registerReceiver(this, filter);
        // Register for events related to sdcard installation.
        IntentFilter sdFilter = new IntentFilter();
        sdFilter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE);
        sdFilter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE);
        mLoader.getContext().registerReceiver(this, sdFilter);
    }

    @Override public void onReceive(Context context, Intent intent) {
        // Tell the loader about the change.
        mLoader.onContentChanged();
    }
}

/**
 * A custom Loader that loads all of the installed applications.
 */
public static class AppListLoader extends AsyncTaskLoader<List<AppEntry>> {
    final InterestingConfigChanges mLastConfig = new InterestingConfigChanges();
    final PackageManager mPm;

    List<AppEntry> mApps;
    PackageIntentReceiver mPackageObserver;

    public AppListLoader(Context context) {
        super(context);

        // Retrieve the package manager for later use; note we don't
        // use 'context' directly but instead the save global application
        // context returned by getContext().
        mPm = getContext().getPackageManager();
    }

    /**
     * This is where the bulk of our work is done.  This function is
     * called in a background thread and should generate a new set of
     * data to be published by the loader.
     */
    @Override public List<AppEntry> loadInBackground() {
        // Retrieve all known applications.
        List<ApplicationInfo> apps = mPm.getInstalledApplications(
                PackageManager.GET_UNINSTALLED_PACKAGES |
                PackageManager.GET_DISABLED_COMPONENTS);
        if (apps == null) {
            apps = new ArrayList<ApplicationInfo>();
        }

        final Context context = getContext();

        // Create corresponding array of entries and load their labels.
        List<AppEntry> entries = new ArrayList<AppEntry>(apps.size());
        for (int i=0; i<apps.size(); i++) {
            AppEntry entry = new AppEntry(this, apps.get(i));
            entry.loadLabel(context);
            entries.add(entry);
        }

        // Sort the list.
        Collections.sort(entries, ALPHA_COMPARATOR);

        // Done!
        return entries;
    }

    /**
     * Called when there is new data to deliver to the client.  The
     * super class will take care of delivering it; the implementation
     * here just adds a little more logic.
     */
    @Override public void deliverResult(List<AppEntry> apps) {
        if (isReset()) {
            // An async query came in while the loader is stopped.  We
            // don't need the result.
            if (apps != null) {
                onReleaseResources(apps);
            }
        }
        List<AppEntry> oldApps = mApps;
        mApps = apps;

        if (isStarted()) {
            // If the Loader is currently started, we can immediately
            // deliver its results.
            super.deliverResult(apps);
        }

        // At this point we can release the resources associated with
        // 'oldApps' if needed; now that the new result is delivered we
        // know that it is no longer in use.
        if (oldApps != null) {
            onReleaseResources(oldApps);
        }
    }

    /**
     * Handles a request to start the Loader.
     */
    @Override protected void onStartLoading() {
        if (mApps != null) {
            // If we currently have a result available, deliver it
            // immediately.
            deliverResult(mApps);
        }

        // Start watching for changes in the app data.
        if (mPackageObserver == null) {
            mPackageObserver = new PackageIntentReceiver(this);
        }

        // Has something interesting in the configuration changed since we
        // last built the app list?
        boolean configChange = mLastConfig.applyNewConfig(getContext().getResources());

        if (takeContentChanged() || mApps == null || configChange) {
            // If the data has changed since the last time it was loaded
            // or is not currently available, start a load.
            forceLoad();
        }
    }

    /**
     * Handles a request to stop the Loader.
     */
    @Override protected void onStopLoading() {
        // Attempt to cancel the current load task if possible.
        cancelLoad();
    }

    /**
     * Handles a request to cancel a load.
     */
    @Override public void onCanceled(List<AppEntry> apps) {
        super.onCanceled(apps);

        // At this point we can release the resources associated with 'apps'
        // if needed.
        onReleaseResources(apps);
    }

    /**
     * Handles a request to completely reset the Loader.
     */
    @Override protected void onReset() {
        super.onReset();

        // Ensure the loader is stopped
        onStopLoading();

        // At this point we can release the resources associated with 'apps'
        // if needed.
        if (mApps != null) {
            onReleaseResources(mApps);
            mApps = null;
        }

        // Stop monitoring for changes.
        if (mPackageObserver != null) {
            getContext().unregisterReceiver(mPackageObserver);
            mPackageObserver = null;
        }
    }

    /**
     * Helper function to take care of releasing resources associated
     * with an actively loaded data set.
     */
    protected void onReleaseResources(List<AppEntry> apps) {
        // For a simple List<> there is nothing to do.  For something
        // like a Cursor, we would close it here.
    }
}

An example implementation of a fragment that uses the above loader to show the currently installed applications in a list is below.

public static class AppListAdapter extends ArrayAdapter<AppEntry> {
    private final LayoutInflater mInflater;

    public AppListAdapter(Context context) {
        super(context, android.R.layout.simple_list_item_2);
        mInflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    public void setData(List<AppEntry> data) {
        clear();
        if (data != null) {
            addAll(data);
        }
    }

    /**
     * Populate new items in the list.
     */
    @Override public View getView(int position, View convertView, ViewGroup parent) {
        View view;

        if (convertView == null) {
            view = mInflater.inflate(R.layout.list_item_icon_text, parent, false);
        } else {
            view = convertView;
        }

        AppEntry item = getItem(position);
        ((ImageView)view.findViewById(R.id.icon)).setImageDrawable(item.getIcon());
        ((TextView)view.findViewById(R.id.text)).setText(item.getLabel());

        return view;
    }
}

public static class AppListFragment extends ListFragment
        implements OnQueryTextListener, OnCloseListener,
        LoaderManager.LoaderCallbacks<List<AppEntry>> {

    // This is the Adapter being used to display the list's data.
    AppListAdapter mAdapter;

    // The SearchView for doing filtering.
    SearchView mSearchView;

    // If non-null, this is the current filter the user has provided.
    String mCurFilter;

    @Override public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        // Give some text to display if there is no data.  In a real
        // application this would come from a resource.
        setEmptyText("No applications");

        // We have a menu item to show in action bar.
        setHasOptionsMenu(true);

        // Create an empty adapter we will use to display the loaded data.
        mAdapter = new AppListAdapter(getActivity());
        setListAdapter(mAdapter);

        // Start out with a progress indicator.
        setListShown(false);

        // Prepare the loader.  Either re-connect with an existing one,
        // or start a new one.
        getLoaderManager().initLoader(0, null, this);
    }

    public static class MySearchView extends SearchView {
        public MySearchView(Context context) {
            super(context);
        }

        // The normal SearchView doesn't clear its search text when
        // collapsed, so we will do this for it.
        @Override
        public void onActionViewCollapsed() {
            setQuery("", false);
            super.onActionViewCollapsed();
        }
    }

    @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Place an action bar item for searching.
        MenuItem item = menu.add("Search");
        item.setIcon(android.R.drawable.ic_menu_search);
        item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM
                | MenuItem.SHOW_AS_ACTION_COLLAPSE_ACTION_VIEW);
        mSearchView = new MySearchView(getActivity());
        mSearchView.setOnQueryTextListener(this);
        mSearchView.setOnCloseListener(this);
        mSearchView.setIconifiedByDefault(true);
        item.setActionView(mSearchView);
    }

    @Override public boolean onQueryTextChange(String newText) {
        // Called when the action bar search text has changed.  Since this
        // is a simple array adapter, we can just have it do the filtering.
        mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;
        mAdapter.getFilter().filter(mCurFilter);
        return true;
    }

    @Override public boolean onQueryTextSubmit(String query) {
        // Don't care about this.
        return true;
    }

    @Override
    public boolean onClose() {
        if (!TextUtils.isEmpty(mSearchView.getQuery())) {
            mSearchView.setQuery(null, true);
        }
        return true;
    }

    @Override public void onListItemClick(ListView l, View v, int position, long id) {
        // Insert desired behavior here.
        Log.i("LoaderCustom", "Item clicked: " + id);
    }

    @Override public Loader<List<AppEntry>> onCreateLoader(int id, Bundle args) {
        // This is called when a new Loader needs to be created.  This
        // sample only has one Loader with no arguments, so it is simple.
        return new AppListLoader(getActivity());
    }

    @Override public void onLoadFinished(Loader<List<AppEntry>> loader, List<AppEntry> data) {
        // Set the new data in the adapter.
        mAdapter.setData(data);

        // The list should now be shown.
        if (isResumed()) {
            setListShown(true);
        } else {
            setListShownNoAnimation(true);
        }
    }

    @Override public void onLoaderReset(Loader<List<AppEntry>> loader) {
        // Clear the data in the adapter.
        mAdapter.setData(null);
    }
}

Summary

Inherited methods
From class android.content.Loader void abandon() This function will normally be called for you automatically by LoaderManager when restarting a Loader. boolean cancelLoad() Attempt to cancel the current load task. void commitContentChanged() Commit that you have actually fully processed a content change that was returned by takeContentChanged(). String dataToString(D data) For debugging, converts an instance of the Loader's data class to a string that can be printed. void deliverCancellation() Informs the registered Loader.OnLoadCanceledListener that the load has been canceled. void deliverResult(D data) Sends the result of the load to the registered listener. void dump(String prefix, FileDescriptor fd, PrintWriter writer, String[] args) Print the Loader's state into the given stream. void forceLoad() Force an asynchronous load. Context getContext() int getId() boolean isAbandoned() Return whether this loader has been abandoned. boolean isReset() Return whether this load has been reset. boolean isStarted() Return whether this load has been started. void onAbandon() Subclasses implement this to take care of being abandoned. boolean onCancelLoad() Subclasses must implement this to take care of requests to cancelLoad(). void onContentChanged() Called when Loader.ForceLoadContentObserver detects a change. void onForceLoad() Subclasses must implement this to take care of requests to forceLoad(). void onReset() Subclasses must implement this to take care of resetting their loader, as per reset(). void onStartLoading() Subclasses must implement this to take care of loading their data, as per startLoading(). void onStopLoading() Subclasses must implement this to take care of stopping their loader, as per stopLoading(). void registerListener(int id, OnLoadCompleteListener<D> listener) Registers a class that will receive callbacks when a load is complete. void registerOnLoadCanceledListener(OnLoadCanceledListener<D> listener) Registers a listener that will receive callbacks when a load is canceled. void reset() This function will normally be called for you automatically by LoaderManager when destroying a Loader. void rollbackContentChanged() Report that you have abandoned the processing of a content change that was returned by takeContentChanged() and would like to rollback to the state where there is again a pending content change. final void startLoading() This function will normally be called for you automatically by LoaderManager when the associated fragment/activity is being started. void stopLoading() This function will normally be called for you automatically by LoaderManager when the associated fragment/activity is being stopped. boolean takeContentChanged() Take the current flag indicating whether the loader's content had changed while it was stopped. String toString() Returns a string representation of the object. void unregisterListener(OnLoadCompleteListener<D> listener) Remove a listener that was previously added with registerListener(int, Loader.OnLoadCompleteListener). void unregisterOnLoadCanceledListener(OnLoadCanceledListener<D> listener) Unregisters a listener that was previously added with registerOnLoadCanceledListener(Loader.OnLoadCanceledListener).
From class java.lang.Object Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long millis, int nanos) Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed. final void wait(long millis) Causes the current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed. final void wait() Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.

Public constructors

AsyncTaskLoader (Context context)

Public methods

void cancelLoadInBackground ()

void dump (String prefix, 
                FileDescriptor fd, 
                PrintWriter writer, 
                String[] args)

boolean isLoadInBackgroundCanceled ()

D loadInBackground ()

void onCanceled (D data)

void setUpdateThrottle (long delayMS)

Protected methods

boolean onCancelLoad ()

void onForceLoad ()

D onLoadInBackground ()