/*
* 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 android.os;
import java.util.ArrayDeque;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
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>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
* and does not constitute a generic threading framework. AsyncTasks should ideally be
* used for short operations (a few seconds at the most.) If you need to keep threads
* running for long periods of time, it is highly recommended you use the various APIs
* provided by the <code>java.util.concurrent</code> package such as {@link Executor},
* {@link ThreadPoolExecutor} and {@link FutureTask}.</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));
* // Escape early if cancel() is called
* if (isCancelled()) break;
* }
* 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 before 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 AsyncTask class must be loaded on the UI thread. This is done
* automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}.</li>
* <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>
*
* <h2>Order of execution</h2>
* <p>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. Starting with
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single
* thread to avoid common application errors caused by parallel execution.</p>
* <p>If you truly want parallel execution, you can invoke
* {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with
* {@link #THREAD_POOL_EXECUTOR}.</p>
*/
public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = "AsyncTask";
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE = 1;
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
/**
* An {@link Executor} that can be used to execute tasks in parallel.
*/
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
/**
* An {@link Executor} that executes tasks one at a time in serial
* order. This serialization is global to a particular process.
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static InternalHandler sHandler;
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private volatile Status mStatus = Status.PENDING;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
/**
* 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,
}
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
/** @hide */
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
return postResult(doInBackground(mParams));
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occured while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().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)
*/
@SuppressWarnings({"UnusedDeclaration"})
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
*/
@SuppressWarnings({"UnusedDeclaration"})
protected void onProgressUpdate(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()
*/
@SuppressWarnings({"UnusedParameters"})
protected void onCancelled(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 mCancelled.get();
}
/**
* <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(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
/**
* 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(long timeout, 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. Starting
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
* executed on 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}.
*
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
* @see #execute(Runnable)
*/
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, 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}.
*
* @see #execute(Object[])
*/
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
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)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
/**
* Convenience version of {@link #execute(Object...)} for use with
* a simple Runnable object. See {@link #execute(Object[])} for more
* information on the order of execution.
*
* @see #execute(Object[])
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
*/
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
/**
* 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 not 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(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
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;
}
@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
}
