More robust Android LoaderTestCase implementation (works with support library Loaders, cancellation support, extends instrumentation so the Context is available)

SupportLoaderTestCase.java

package com.nfhsnetwork.stream;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.annotation.Nullable;
import android.support.v4.content.Loader;
import android.test.ActivityInstrumentationTestCase2;

import java.util.concurrent.ArrayBlockingQueue;

/**
 * A convenience class for testing {@link Loader}s. This test case
 * provides a simple way to synchronously get the result from a Loader making
 * it easy to assert that the Loader returns the expected result.
 * <p/>
 * This is a slightly modified version of {@link android.test.LoaderTestCase} that supports
 * returning null from {@link #getLoaderResultSynchronously(Loader)} on cancellation (instead of
 * blocking forever as the default LoaderTestCase does), supports the Android support library
 * loaders, and extends {@link ActivityInstrumentationTestCase2} so that an application context is
 * available for integration testing of the loader.
 */
public class SupportLoaderTestCase<T extends Activity> extends ActivityInstrumentationTestCase2<T> {
    private final static Cancelled cancelled = new Cancelled();

    static {
        // Force class loading of AsyncTask on the main thread so that it's handlers are tied to
        // the main thread and responses from the worker thread get delivered on the main thread.
        // The tests are run on another thread, allowing them to block waiting on a response from
        // the code running on the main thread. The main thread can't block since the AysncTask
        // results come in via the event loop.
        new AsyncTask<Void, Void, Void>() {
            @Override
            protected Void doInBackground(Void... args) {
                return null;
            }

            @Override
            protected void onPostExecute(Void result) {
            }
        };
    }

    public SupportLoaderTestCase(Class<T> activityClass) {
        super(activityClass);
    }

    /**
     * Runs a Loader synchronously and returns the result of the load. The loader will
     * be started, stopped, and destroyed by this method so it cannot be reused.
     *
     * @param loader the loader to run synchronously
     * @return the result from the loader; may be null only if the loader was canceled
     */
    @Nullable
    public <T> T getLoaderResultSynchronously(final Loader<T> loader) {
        // The test thread blocks on this queue until the loader puts it's result in
        final ArrayBlockingQueue<Object> queue = new ArrayBlockingQueue<Object>(1);
        // This callback runs on the "main" thread and unblocks the test thread
        // when it puts the result into the blocking queue
        final Loader.OnLoadCompleteListener<T> listener = new Loader.OnLoadCompleteListener<T>() {
            @Override
            public void onLoadComplete(Loader<T> completedLoader, T data) {
                // Shut the loader down
                completedLoader.unregisterListener(this);
                completedLoader.stopLoading();
                completedLoader.reset();
                // Store the result, unblocking the test thread
                queue.add(data);
            }
        };

        final Loader.OnLoadCanceledListener<T> cancelListener =
                new Loader.OnLoadCanceledListener<T>() {
                    @Override
                    public void onLoadCanceled(Loader<T> canceledLoader) {
                        // Shut the loader down
                        canceledLoader.unregisterOnLoadCanceledListener(this);
                        canceledLoader.stopLoading();
                        canceledLoader.reset();
                        // Remove the expectation of a result, unblocking the test thread
                        queue.add(cancelled);
                    }
                };
        // This handler runs on the "main" thread of the process since AsyncTask
        // is documented as needing to run on the main thread and many Loaders use
        // AsyncTask
        final Handler mainThreadHandler = new Handler(Looper.getMainLooper()) {
            @Override
            public void handleMessage(Message msg) {
                loader.registerListener(0, listener);
                loader.registerOnLoadCanceledListener(cancelListener);
                loader.startLoading();
            }
        };
        // Ask the main thread to start the loading process
        mainThreadHandler.sendEmptyMessage(0);
        // Block on the queue waiting for the result of the load to be inserted
        T result;
        while (true) {
            try {
                Object queueObj = queue.take();
                if (cancelled.equals(queueObj)) {
                    return null;
                }

                //noinspection unchecked because the above private code only fills with instance of cancelled or with the type-checked delivery result
                result = (T) queueObj;

                break;
            } catch (InterruptedException e) {
                throw new RuntimeException("waiting thread interrupted", e);
            }
        }

        return result;
    }

    /**
     * Used to uniquely check if the response was actually a loader cancellation, not a valid result
     */
    private static class Cancelled {
    }
}