/*
* 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<URL, Integer, Long> {
*
* protected Long doInBackground(URL... urls) {
* int count = urls.length;
* long totalSize = 0;
* for (int i = 0; i < 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("Downloaded " + result + " bytes");
* }
* }
* </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<Void, Void, Void> { ... }
* </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;
}
}
}