AsyncTaskLoader with cache

suggest change

It’s a good practice to cache loaded result to avoid multiple loading of same data. To invalidate cache onContentChanged() should be called. If loader has been already started, forceLoad() will be called, otherwise (if loader in stopped state) loader will be able to understand content change with takeContentChanged() check.

Remark: onContentChanged() must be called from the process’s main thread.

Javadocs says about takeContentChanged():

Take the current flag indicating whether the loader’s content had changed while it was stopped. If it had, true is returned and the flag is cleared.

public abstract class BaseLoader<T> extends AsyncTaskLoader<T> {

    // Cached result saved here
    private final AtomicReference<T> cache = new AtomicReference<>();

    public BaseLoader(@NonNull final Context context) {
        super(context);
    }

    @Override
    public final void deliverResult(final T data) {
        if (!isReset()) {
            // Save loaded result
            cache.set(data);
            if (isStarted()) {
                super.deliverResult(data);
            }
        }
    }

    @Override
    protected final void onStartLoading() {            
        // Register observers
        registerObserver();

        final T cached = cache.get();    
        // Start new loading if content changed in background
        // or if we never loaded any data
        if (takeContentChanged() || cached == null) {
            forceLoad();
        } else {
            deliverResult(cached);
        }
    }

    @Override
    public final void onStopLoading() {
        cancelLoad();
    }

    @Override
    protected final void onReset() {
        super.onReset();
        onStopLoading();
        // Clear cache and remove observers
        cache.set(null);
        unregisterObserver();
    }

    /* virtual */
    protected void registerObserver() {
        // Register observers here, call onContentChanged() to invalidate cache
    }

    /* virtual */
    protected void unregisterObserver() {
        // Remove observers
    }
}

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