/* * 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 android.annotation.MainThread; import android.annotation.WorkerThread; 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; /** *

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.

* *

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 java.util.concurrent package such as {@link Executor}, * {@link ThreadPoolExecutor} and {@link FutureTask}.

* *

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 Params, Progress and Result, * and 4 steps, called onPreExecute, doInBackground, * onProgressUpdate and onPostExecute.

* *
*

Developer Guides

*

For more information about using tasks and threads, read the * Processes and * Threads developer guide.

*
* *

Usage

*

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}.)

* *

Here is an example of subclassing:

*
 * 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");
 *     }
 * }
 * 
* *

Once created, a task is executed very simply:

*
 * new DownloadFilesTask().execute(url1, url2, url3);
 * 
* *

AsyncTask's generic types

*

The three types used by an asynchronous task are the following:

*
    *
  1. Params, the type of the parameters sent to the task upon * execution.
  2. *
  3. Progress, the type of the progress units published during * the background computation.
  4. *
  5. Result, the type of the result of the background * computation.
  6. *
*

Not all types are always used by an asynchronous task. To mark a type as unused, * simply use the type {@link Void}:

*
 * private class MyTask extends AsyncTask<Void, Void, Void> { ... }
 * 
* *

The 4 steps

*

When an asynchronous task is executed, the task goes through 4 steps:

*
    *
  1. {@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.
  2. *
  3. {@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.
  4. *
  5. {@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.
  6. *
  7. {@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.
  8. *
* *

Cancelling a task

*

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.)

* *

Threading rules

*

There are a few threading rules that must be followed for this class to * work properly:

* * *

Memory observability

*

AsyncTask guarantees that all callback calls are synchronized in such a way that the following * operations are safe without explicit synchronizations.

* * *

Order of execution

*

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.

*

If you truly want parallel execution, you can invoke * {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with * {@link #THREAD_POOL_EXECUTOR}.

*/ public abstract class AsyncTask { 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 sPoolWorkQueue = new LinkedBlockingQueue(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 mWorker; private final FutureTask 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 mTasks = new ArrayDeque(); 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() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked Result result = doInBackground(mParams); Binder.flushPendingCommands(); return postResult(result); } }; mFuture = new FutureTask(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 occurred 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(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 */ @WorkerThread protected abstract Result doInBackground(Params... params); /** * Runs on the UI thread before {@link #doInBackground}. * * @see #onPostExecute * @see #doInBackground */ @MainThread protected void onPreExecute() { } /** *

Runs on the UI thread after {@link #doInBackground}. The * specified result is the value returned by {@link #doInBackground}.

* *

This method won't be invoked if the task was cancelled.

* * @param result The result of the operation computed by {@link #doInBackground}. * * @see #onPreExecute * @see #doInBackground * @see #onCancelled(Object) */ @SuppressWarnings({"UnusedDeclaration"}) @MainThread 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"}) @MainThread protected void onProgressUpdate(Progress... values) { } /** *

Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.

* *

The default implementation simply invokes {@link #onCancelled()} and * ignores the result. If you write your own implementation, do not call * super.onCancelled(result).

* * @param result The result, if any, computed in * {@link #doInBackground(Object[])}, can be null * * @see #cancel(boolean) * @see #isCancelled() */ @SuppressWarnings({"UnusedParameters"}) @MainThread protected void onCancelled(Result result) { onCancelled(); } /** *

Applications should preferably override {@link #onCancelled(Object)}. * This method is invoked by the default implementation of * {@link #onCancelled(Object)}.

* *

Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.

* * @see #onCancelled(Object) * @see #cancel(boolean) * @see #isCancelled() */ @MainThread protected void onCancelled() { } /** * Returns true 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 true if task was cancelled before it completed * * @see #cancel(boolean) */ public final boolean isCancelled() { return mCancelled.get(); } /** *

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 cancel is called, * this task should never run. If the task has already started, * then the mayInterruptIfRunning parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task.

* *

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.

* * @param mayInterruptIfRunning true if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete. * * @return false if the task could not be cancelled, * typically because it has already completed normally; * true 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. * *

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. * *

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) */ @MainThread public final AsyncTask 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. * *

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. * *

Warning: Allowing multiple tasks to run in parallel from * a thread pool is generally not 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}. * *

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[]) */ @MainThread public final AsyncTask 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[]) */ @MainThread 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 */ @WorkerThread protected final void publishProgress(Progress... values) { if (!isCancelled()) { getHandler().obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult(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 implements Callable { Params[] mParams; } @SuppressWarnings({"RawUseOfParameterizedType"}) private static class AsyncTaskResult { final AsyncTask mTask; final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) { mTask = task; mData = data; } } }