gistfile1.java

package com.example.test_loader;

import java.util.Calendar;

import android.app.Activity;
import android.app.LoaderManager;
import android.app.LoaderManager.LoaderCallbacks;
import android.content.AsyncTaskLoader;
import android.content.Context;
import android.content.Loader;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

/**
 * <p><strong>App</strong> Shows a {@link Button} which is clickable. Clicking replaces the
 * {@link Loader} with a new one. When {@link LoaderCallbacks#onLoadFinished(Loader, Object)}
 * gets called, the UI gets updated to show the update's time stamp.</p>
 *
 * <p><strong>Problem</strong> The {@link Loader} does some "heavy lifting" and requires at least
 * 4000ms. Changing the orientation while waiting for an successful update lets the app forget the
 * old time stamp and shows the initial message.</p>
 *
 * <p>(The real app has a {@link Loader} which queries data from a database and then processes it.
 * Serializing and throwing the data into a {@link Bundle} is no option.)</p>
 */
public class MainActivity extends Activity implements LoaderCallbacks<String> {
	private static final String LOG_TAG = MainActivity.class.getSimpleName();

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		LoaderManager.enableDebugLogging(true);

		final LoaderManager lm = getLoaderManager();
		lm.initLoader(0, null, this);

		/**
		 * The button which restarts loaders.
		 */
		findViewById(R.id.button1).setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				lm.restartLoader(0, null, MainActivity.this);
			}
		});
	}

	/**
	 * FIXME Fix missing time when loading and changing orientation.
	 *       Workaround: Pass old data to new {@link Loader} and let it deliver the old data first?
	 */
	@Override
	public Loader<String> onCreateLoader(int id, Bundle args) {
		return new MyLoader(getApplicationContext());
	}

	@Override
	public void onLoadFinished(Loader<String> loader, String result) {
		TextView t = (TextView) findViewById(R.id.text1);
		t.setText(result);
	}

	/**
	 * FIXME Rescue old content? Force pull cached data from the loader?
	 */
	@Override
	public void onLoaderReset(Loader<String> loaderWithStringDebugInformation) {
		Log.d(LOG_TAG, "onLoaderReset(" + loaderWithStringDebugInformation.toString() + ")");
	}
}


/**
 * The loader which loads complex data (a {@link String}!).
 */
class MyLoader extends AsyncTaskLoader<String> {
	String mCache = null;

	public MyLoader(Context context) {
		super(context.getApplicationContext());
	}

	@Override
	protected void onStartLoading() {
		super.onStartLoading();

		if (mCache == null) {
			forceLoad();
		}

		else {
			deliverResult(mCache);
		}
	}

	@Override
	public void deliverResult(String data) {
		mCache = data;
		super.deliverResult(data);
	}

	@Override
	public String loadInBackground() {
		try {
			Thread.sleep(4000); // Heavy lifting.
		}

		catch (InterruptedException e) {
			/* NOOP */
		}

		Calendar c = Calendar.getInstance();
		return c.get(Calendar.HOUR_OF_DAY) + ":" + c.get(Calendar.MINUTE) + ":" + c.get(Calendar.SECOND);
	}

	@Override
	public String toString() {
		return "MyLoader [mCache=" + mCache + "]";
	}
}