/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.emdev.ui.tasks;

import android.os.Handler;
import android.os.Message;
import android.os.Process;

import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * <p>
 * AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and
 * publish results on the UI thread without having to manipulate threads and/or handlers.
 * </p>
 *
 * <p>
 * An asynchronous task is defined by a computation that runs on a background thread and whose result is published on
 * the UI thread. An asynchronous task is defined by 3 generic types, called <code>Params</code>, <code>Progress</code>
 * and <code>Result</code>, and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>,
 * <code>onProgressUpdate</code> and <code>onPostExecute</code>.
 * </p>
 *
 * <div class="special reference"> <h3>Developer Guides</h3>
 * <p>
 * For more information about using tasks and threads, read the <a href="{@docRoot}
 * guide/topics/fundamentals/processes-and-threads.html">Processes and Threads</a> developer guide.
 * </p>
 * </div>
 *
 * <h2>Usage</h2>
 * <p>
 * AsyncTask must be subclassed to be used. The subclass will override at least one method ({@link #doInBackground}),
 * and most often will override a second one ({@link #onPostExecute}.)
 * </p>
 *
 * <p>
 * Here is an example of subclassing:
 * </p>
 *
 * <pre class="prettyprint">
 *
 *
 * private class DownloadFilesTask extends AsyncTask&lt;URL, Integer, Long&gt; {
 *
 *     protected Long doInBackground(URL... urls) {
 *         int count = urls.length;
 *         long totalSize = 0;
 *         for (int i = 0; i &lt; count; i++) {
 *             totalSize += Downloader.downloadFile(urls[i]);
 *             publishProgress((int) ((i / (float) count) * 100));
 *         }
 *         return totalSize;
 *     }
 *
 *     protected void onProgressUpdate(Integer... progress) {
 *         setProgressPercent(progress[0]);
 *     }
 *
 *     protected void onPostExecute(Long result) {
 *         showDialog(&quot;Downloaded &quot; + result + &quot; bytes&quot;);
 *     }
 * }
 * </pre>
 *
 * <p>
 * Once created, a task is executed very simply:
 * </p>
 *
 * <pre class="prettyprint">
 * new DownloadFilesTask().execute(url1, url2, url3);
 * </pre>
 *
 * <h2>AsyncTask's generic types</h2>
 * <p>
 * The three types used by an asynchronous task are the following:
 * </p>
 * <ol>
 * <li><code>Params</code>, the type of the parameters sent to the task upon execution.</li>
 * <li><code>Progress</code>, the type of the progress units published during the background computation.</li>
 * <li><code>Result</code>, the type of the result of the background computation.</li>
 * </ol>
 * <p>
 * Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type {@link Void}:
 * </p>
 *
 * <pre>
 * private class MyTask extends AsyncTask&lt;Void, Void, Void&gt; { ... }
 * </pre>
 *
 * <h2>The 4 steps</h2>
 * <p>
 * When an asynchronous task is executed, the task goes through 4 steps:
 * </p>
 * <ol>
 * <li>{@link #onPreExecute()}, invoked on the UI thread immediately after the task is executed. This step is normally
 * used to setup the task, for instance by showing a progress bar in the user interface.</li>
 * <li>{@link #doInBackground}, invoked on the background thread immediately after {@link #onPreExecute()} finishes
 * executing. This step is used to perform background computation that can take a long time. The parameters of the
 * asynchronous task are passed to this step. The result of the computation must be returned by this step and will be
 * passed back to the last step. This step can also use {@link #publishProgress} to publish one or more units of
 * progress. These values are published on the UI thread, in the {@link #onProgressUpdate} step.</li>
 * <li>{@link #onProgressUpdate}, invoked on the UI thread after a call to {@link #publishProgress}. The timing of the
 * execution is undefined. This method is used to display any form of progress in the user interface while the
 * background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a
 * text field.</li>
 * <li>{@link #onPostExecute}, invoked on the UI thread after the background computation finishes. The result of the
 * background computation is passed to this step as a parameter.</li>
 * </ol>
 *
 * <h2>Cancelling a task</h2>
 * <p>
 * A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking this method will cause subsequent
 * calls to {@link #isCancelled()} to return true. After invoking this method, {@link #onCancelled(Object)}, instead of
 * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])} returns. To ensure that a task
 * is cancelled as quickly as possible, you should always check the return value of {@link #isCancelled()} periodically
 * from {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)
 * </p>
 *
 * <h2>Threading rules</h2>
 * <p>
 * There are a few threading rules that must be followed for this class to work properly:
 * </p>
 * <ul>
 * <li>The task instance must be created on the UI thread.</li>
 * <li>{@link #execute} must be invoked on the UI thread.</li>
 * <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute}, {@link #doInBackground}, {@link #onProgressUpdate}
 * manually.</li>
 * <li>The task can be executed only once (an exception will be thrown if a second execution is attempted.)</li>
 * </ul>
 *
 * <h2>Memory observability</h2>
 * <p>
 * AsyncTask guarantees that all callback calls are synchronized in such a way that the following operations are safe
 * without explicit synchronizations.
 * </p>
 * <ul>
 * <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them in {@link #doInBackground}.
 * <li>Set member fields in {@link #doInBackground}, and refer to them in {@link #onProgressUpdate} and
 * {@link #onPostExecute}.
 * </ul>
 */
public abstract class AsyncTask<Params, Progress, Result> {

    private static final String LOG_TAG = "AsyncTask";

    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;

    private static final InternalHandler sHandler = new InternalHandler();

    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;

    private volatile Status mStatus = Status.PENDING;

    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();

    /**
     * Indicates the current status of the task. Each status will be set only once
     * during the lifetime of a task.
     */
    public enum Status {
        /**
         * Indicates that the task has not been executed yet.
         */
        PENDING,
        /**
         * Indicates that the task is running.
         */
        RUNNING,
        /**
         * Indicates that {@link AsyncTask#onPostExecute} has finished.
         */
        FINISHED,
    }

    /** @hide Used to force static handler to be created. */
    public static void init() {
        sHandler.getLooper();
    }

    /**
     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
     */
    public AsyncTask() {
        mWorker = new WorkerRunnable<Params, Result>() {

            @Override
            public Result call() throws Exception {
                mTaskInvoked.set(true);

                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                return postResult(doInBackground(mParams));
            }
        };

        mFuture = new FutureTask<Result>(mWorker) {

            @Override
            protected void done() {
                try {
                    final Result result = get();

                    postResultIfNotInvoked(result);
                } catch (final InterruptedException e) {
                    android.util.Log.w(LOG_TAG, e);
                } catch (final ExecutionException e) {
                    throw new RuntimeException("An error occured while executing doInBackground()", e.getCause());
                } catch (final CancellationException e) {
                    postResultIfNotInvoked(null);
                } catch (final Throwable t) {
                    throw new RuntimeException("An error occured while executing " + "doInBackground()", t);
                }
            }
        };
    }

    private void postResultIfNotInvoked(final Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

    @SuppressWarnings("unchecked")
    private Result postResult(final Result result) {
        final Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

    /**
     * Returns the current status of this task.
     *
     * @return The current status.
     */
    public final Status getStatus() {
        return mStatus;
    }

    /**
     * Override this method to perform a computation on a background thread. The
     * specified parameters are the parameters passed to {@link #execute} by the caller of this task.
     *
     * This method can call {@link #publishProgress} to publish updates
     * on the UI thread.
     *
     * @param params
     *            The parameters of the task.
     *
     * @return A result, defined by the subclass of this task.
     *
     * @see #onPreExecute()
     * @see #onPostExecute
     * @see #publishProgress
     */
    protected abstract Result doInBackground(Params... params);

    /**
     * Runs on the UI thread before {@link #doInBackground}.
     *
     * @see #onPostExecute
     * @see #doInBackground
     */
    protected void onPreExecute() {
    }

    /**
     * <p>
     * Runs on the UI thread after {@link #doInBackground}. The specified result is the value returned by
     * {@link #doInBackground}.
     * </p>
     *
     * <p>
     * This method won't be invoked if the task was cancelled.
     * </p>
     *
     * @param result
     *            The result of the operation computed by {@link #doInBackground}.
     *
     * @see #onPreExecute
     * @see #doInBackground
     * @see #onCancelled(Object)
     */
    protected void onPostExecute(final Result result) {
    }

    /**
     * Runs on the UI thread after {@link #publishProgress} is invoked.
     * The specified values are the values passed to {@link #publishProgress}.
     *
     * @param values
     *            The values indicating progress.
     *
     * @see #publishProgress
     * @see #doInBackground
     */
    protected void onProgressUpdate(final Progress... values) {
    }

    /**
     * <p>
     * Runs on the UI thread after {@link #cancel(boolean)} is invoked and {@link #doInBackground(Object[])} has
     * finished.
     * </p>
     *
     * <p>
     * The default implementation simply invokes {@link #onCancelled()} and ignores the result. If you write your own
     * implementation, do not call <code>super.onCancelled(result)</code>.
     * </p>
     *
     * @param result
     *            The result, if any, computed in {@link #doInBackground(Object[])}, can be null
     *
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    protected void onCancelled(final Result result) {
        onCancelled();
    }

    /**
     * <p>
     * Applications should preferably override {@link #onCancelled(Object)}. This method is invoked by the default
     * implementation of {@link #onCancelled(Object)}.
     * </p>
     *
     * <p>
     * Runs on the UI thread after {@link #cancel(boolean)} is invoked and {@link #doInBackground(Object[])} has
     * finished.
     * </p>
     *
     * @see #onCancelled(Object)
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    protected void onCancelled() {
    }

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally. If you are calling {@link #cancel(boolean)} on the task,
     * the value returned by this method should be checked periodically from {@link #doInBackground(Object[])} to end
     * the task as soon as possible.
     *
     * @return <tt>true</tt> if task was cancelled before it completed
     *
     * @see #cancel(boolean)
     */
    public final boolean isCancelled() {
        return mFuture.isCancelled();
    }

    /**
     * <p>
     * Attempts to cancel execution of this task. This attempt will fail if the task has already completed, already been
     * cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when
     * <tt>cancel</tt> is called, this task should never run. If the task has already started, then the
     * <tt>mayInterruptIfRunning</tt> parameter determines whether the thread executing this task should be interrupted
     * in an attempt to stop the task.
     * </p>
     *
     * <p>
     * Calling this method will result in {@link #onCancelled(Object)} being invoked on the UI thread after
     * {@link #doInBackground(Object[])} returns. Calling this method guarantees that {@link #onPostExecute(Object)} is
     * never invoked. After invoking this method, you should check the value returned by {@link #isCancelled()}
     * periodically from {@link #doInBackground(Object[])} to finish the task as early as possible.
     * </p>
     *
     * @param mayInterruptIfRunning
     *            <tt>true</tt> if the thread executing this
     *            task should be interrupted; otherwise, in-progress tasks are allowed
     *            to complete.
     *
     * @return <tt>false</tt> if the task could not be cancelled,
     *         typically because it has already completed normally; <tt>true</tt> otherwise
     *
     * @see #isCancelled()
     * @see #onCancelled(Object)
     */
    public final boolean cancel(final boolean mayInterruptIfRunning) {
        try {
            return mFuture.cancel(mayInterruptIfRunning);
        } catch (RuntimeException ex) {
            return false;
        }
    }

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return The computed result.
     *
     * @throws CancellationException
     *             If the computation was cancelled.
     * @throws ExecutionException
     *             If the computation threw an exception.
     * @throws InterruptedException
     *             If the current thread was interrupted
     *             while waiting.
     */
    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result.
     *
     * @param timeout
     *            Time to wait before cancelling the operation.
     * @param unit
     *            The time unit for the timeout.
     *
     * @return The computed result.
     *
     * @throws CancellationException
     *             If the computation was cancelled.
     * @throws ExecutionException
     *             If the computation threw an exception.
     * @throws InterruptedException
     *             If the current thread was interrupted
     *             while waiting.
     * @throws TimeoutException
     *             If the wait timed out.
     */
    public final Result get(final long timeout, final TimeUnit unit) throws InterruptedException, ExecutionException,
            TimeoutException {
        return mFuture.get(timeout, unit);
    }

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     *
     * <p>
     * Note: this function schedules the task on a queue for a single background thread or pool of threads depending on
     * the platform version. When first introduced, AsyncTasks were executed serially on a single background thread.
     * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed to a pool of threads allowing
     * multiple tasks to operate in parallel. After {@link android.os.Build.VERSION_CODES#HONEYCOMB}, it is planned to
     * change this back to a single thread to avoid common application errors caused by parallel execution. If you truly
     * want parallel execution, you can use the {@link #executeOnExecutor} version of this method with
     * {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings on its use.
     *
     * <p>
     * This method must be invoked on the UI thread.
     *
     * @param params
     *            The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException
     *             If {@link #getStatus()} returns either {@link AsyncTask.Status#RUNNING} or
     *             {@link AsyncTask.Status#FINISHED}.
     */
    @Deprecated
    public final AsyncTask<Params, Progress, Result> execute(final Params... params) {
        return AsyncTaskExecutor.DEFAULT.executeAsDefault(this, params);
    }

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     *
     * <p>
     * This method is typically used with {@link #THREAD_POOL_EXECUTOR} to allow multiple tasks to run in parallel on a
     * pool of threads managed by AsyncTask, however you can also use your own {@link Executor} for custom behavior.
     *
     * <p>
     * <em>Warning:</em> Allowing multiple tasks to run in parallel from a thread pool is generally <em>not</em> what
     * one wants, because the order of their operation is not defined. For example, if these tasks are used to modify
     * any state in common (such as writing a file due to a button click), there are no guarantees on the order of the
     * modifications. Without careful work it is possible in rare cases for the newer version of the data to be
     * over-written by an older one, leading to obscure data loss and stability issues. Such changes are best executed
     * in serial; to guarantee such work is serialized regardless of platform version you can use this function with
     * {@link #SERIAL_EXECUTOR}.
     *
     * <p>
     * This method must be invoked on the UI thread.
     *
     * @param exec
     *            The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a
     *            convenient process-wide thread pool for tasks that are loosely coupled.
     * @param params
     *            The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException
     *             If {@link #getStatus()} returns either {@link AsyncTask.Status#RUNNING} or
     *             {@link AsyncTask.Status#FINISHED}.
     */
    final AsyncTask<Params, Progress, Result> executeOnExecutor(final Executor exec, final Params... params) {
        if (mStatus != Status.PENDING) {
            switch (mStatus) {
                case RUNNING:
                    throw new IllegalStateException("Cannot execute task:" + " the task is already running.");
                case FINISHED:
                    throw new IllegalStateException("Cannot execute task:" + " the task has already been executed "
                            + "(a task can be executed only once)");
                default:
                    break;
            }
        }

        mStatus = Status.RUNNING;

        onPreExecute();

        mWorker.mParams = params;
        exec.execute(mFuture);

        return this;
    }

    /**
     * This method can be invoked from {@link #doInBackground} to
     * publish updates on the UI thread while the background computation is
     * still running. Each call to this method will trigger the execution of {@link #onProgressUpdate} on the UI thread.
     *
     * {@link #onProgressUpdate} will note be called if the task has been
     * canceled.
     *
     * @param values
     *            The progress values to update the UI with.
     *
     * @see #onProgressUpdate
     * @see #doInBackground
     */
    protected final void publishProgress(final Progress... values) {
        if (!isCancelled()) {
            sHandler.obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

    private void finish(final Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

    private static class InternalHandler extends Handler {

        @SuppressWarnings({ "rawtypes", "unchecked" })
        @Override
        public void handleMessage(final Message msg) {
            final AsyncTaskResult result = (AsyncTaskResult) msg.obj;
            switch (msg.what) {
                case MESSAGE_POST_RESULT:
                    // There is only one result
                    result.mTask.finish(result.mData[0]);
                    break;
                case MESSAGE_POST_PROGRESS:
                    result.mTask.onProgressUpdate(result.mData);
                    break;
            }
        }
    }

    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {

        Params[] mParams;
    }

    private static class AsyncTaskResult<Data> {

        @SuppressWarnings("rawtypes")
        final AsyncTask mTask;
        final Data[] mData;

        @SuppressWarnings("rawtypes")
        AsyncTaskResult(final AsyncTask task, final Data... data) {
            mTask = task;
            mData = data;
        }
    }
}
