/**
 * Copyright (C) 2006 The Android Open Source Project
 * <p/>
 * 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
 * <p/>
 * http://www.apache.org/licenses/LICENSE-2.0
 * <p/>
 * 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 com.yahala.ImageLoader.loader.util;

import android.os.Handler;
import android.os.Message;
import android.util.Log;

import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * <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/>
 * <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>begin</code>, <code>doInBackground</code>, <code>processProgress</code>
 * and <code>end</code>.
 * </p>
 * <p/>
 * <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/>
 * <p>
 * Here is an example of subclassing:
 * </p>
 * <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;
 * }
 * <p/>
 * protected void onProgressUpdate(Integer... progress) {
 * setProgressPercent(progress[0]);
 * }
 * <p/>
 * protected void onPostExecute(Long result) {
 * showDialog(&quot;Downloaded &quot; + result + &quot; bytes&quot;);
 * }
 * }
 * </pre>
 * <p/>
 * <p>
 * Once created, a task is executed very simply:
 * </p>
 * <p/>
 * <pre class="prettyprint">
 * new DownloadFilesTask().execute(url1, url2, url3);
 * </pre>
 * <p/>
 * <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>
 * <p/>
 * <pre>
 * private class MyTask extends AsyncTask&lt;Void, Void, Void&gt; { ... }
 * </pre>
 * <p/>
 * <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>
 * <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>
 */
public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";

    private static final int CORE_POOL_SIZE = 4;
    private static final int MAXIMUM_POOL_SIZE = 4;
    private static final int KEEP_ALIVE = 4;

    private static final BlockingQueue<Runnable> sWorkQueue = new LinkedBlockingQueue<Runnable>();

    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger count = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "ImageLoader #" + count.getAndIncrement());
        }
    };

    private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE,
            KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);

    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;
    private static final int MESSAGE_POST_CANCEL = 0x3;

    private static final InternalHandler sHandler = new InternalHandler();

    private final WorkerRunnable<Params, Result> worker;
    private final FutureTask<Result> futureTask;

    private volatile Status status = Status.PENDING;

    /**
     * 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 com.yahala.ImageLoader.loader.util.AsyncTask#onPostExecute} has finished.
         */
        FINISHED,
    }

    /**
     * Creates a new asynchronous task. This constructor must be invoked on the
     * UI thread.
     */
    public AsyncTask() {
        worker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_BACKGROUND);
                return doInBackground(params);
            }
        };

        futureTask = new FutureTask<Result>(worker) {
            @SuppressWarnings("unchecked")
            @Override
            protected void done() {
                Message message;
                Result result = null;

                try {
                    result = get();
                } catch (InterruptedException e) {
                    Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occured while executing doInBackground()", e.getCause());
                } catch (CancellationException e) {
                    message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, new AsyncTaskResult<Result>(AsyncTask.this,
                            (Result[]) null));
                    message.sendToTarget();
                    return;
                } catch (Throwable t) {
                    throw new RuntimeException("An error occured while executing " + "doInBackground()", t);
                }

                message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(AsyncTask.this,
                        result));
                message.sendToTarget();
            }
        };
    }

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

    /**
     * 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.
     * <p/>
     * 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() {
    }

    /**
     * Runs on the UI thread after {@link #doInBackground}. The specified result
     * is the value returned by {@link #doInBackground} or null if the task was
     * cancelled or an exception occured.
     *
     * @param result The result of the operation computed by
     *               {@link #doInBackground}.
     * @see #onPreExecute
     * @see #doInBackground
     */
    protected void onPostExecute(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(Progress... values) {
    }

    /**
     * Runs on the UI thread after {@link #cancel(boolean)} is invoked.
     *
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    protected void onCancelled() {
    }

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally.
     *
     * @return <tt>true</tt> if task was cancelled before it completed
     * @see #cancel(boolean)
     */
    public final boolean isCancelled() {
        return futureTask.isCancelled();
    }

    /**
     * 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.
     *
     * @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()
     */
    public final boolean cancel(boolean mayInterruptIfRunning) {
        return futureTask.cancel(mayInterruptIfRunning);
    }

    /**
     * Waits if necessary for the computation to complete, and then retrieves
     * its result.
     *
     * @return The computed result.
     * @throws java.util.concurrent.CancellationException If the computation was cancelled.
     * @throws java.util.concurrent.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 futureTask.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 java.util.concurrent.CancellationException If the computation was cancelled.
     * @throws java.util.concurrent.ExecutionException    If the computation threw an exception.
     * @throws InterruptedException  If the current thread was interrupted while waiting.
     * @throws java.util.concurrent.TimeoutException      If the wait timed out.
     */
    public final Result get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException,
            TimeoutException {
        return futureTask.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/>
     * 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 com.yahala.ImageLoader.loader.util.AsyncTask.Status#RUNNING} or
     *                               {@link com.yahala.ImageLoader.loader.util.AsyncTask.Status#FINISHED}.
     */
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        if (status != Status.PENDING) {
            switch (status) {
                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:
                    Log.w(AsyncTask.class.getSimpleName(), "Switch did not deal with following status " + status);
                    break;
            }
        }

        status = Status.RUNNING;

        onPreExecute();

        worker.params = params;
        sExecutor.execute(futureTask);

        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.
     *
     * @param values The progress values to update the UI with.
     * @see #onProgressUpdate
     * @see #doInBackground
     */
    protected final void publishProgress(Progress... values) {
        sHandler.obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult<Progress>(this, values)).sendToTarget();
    }

    private void finish(Result result) {
        if (isCancelled()) {
            result = null;
        }
        onPostExecute(result);
        status = Status.FINISHED;
    }

    private static class InternalHandler extends Handler {
        @SuppressWarnings({"rawtypes", "unchecked"})
        @Override
        public void handleMessage(Message msg) {
            AsyncTaskResult result = (AsyncTaskResult) msg.obj;
            switch (msg.what) {
                case MESSAGE_POST_RESULT:
                    // There is only one result
                    result.asyncTask.finish(result.data[0]);
                    break;
                case MESSAGE_POST_PROGRESS:
                    result.asyncTask.onProgressUpdate(result.data);
                    break;
                case MESSAGE_POST_CANCEL:
                    result.asyncTask.onCancelled();
                    break;
                default:
                    break;
            }
        }
    }

    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        Params[] params;
    }

    private static class AsyncTaskResult<Data> {
        @SuppressWarnings("rawtypes")
        final AsyncTask asyncTask;
        final Data[] data;

        @SuppressWarnings("rawtypes")
        AsyncTaskResult(AsyncTask task, Data... data) {
            asyncTask = task;
            this.data = data;
        }
    }
}
