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 android.os.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 pacakge such as {@link java.util.concurrent.Executor}, * {@link java.util.concurrent.ThreadPoolExecutor} and {@link java.util.concurrent.FutureTask}.
java.util.concurrent
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.
Params
Progress
Result
onPreExecute
doInBackground
onProgressUpdate
onPostExecute
For more information about using tasks and threads, read the * Processes and * Threads developer guide.
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); *
The three types used by an asynchronous task are the following:
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> { ... } *
When an asynchronous task is executed, the task goes through 4 steps:
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.)
There are a few threading rules that must be followed for this class to * work properly:
AsyncTask guarantees that all callback calls are synchronized in such a way that the following * operations are safe without explicit synchronizations.
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}.
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.
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).
super.onCancelled(result)
Applications should preferably override {@link #onCancelled(Object)}. * This method is invoked by the default implementation of * {@link #onCancelled(Object)}.
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.
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) */ 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 java.util.concurrent.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[]) */ 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[]) */ 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 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(Progress... values) { if (!isCancelled()) { sHandler.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 { @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; } } }
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 java.util.concurrent.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[]) */ 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[]) */ 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 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(Progress... values) { if (!isCancelled()) { sHandler.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 { @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; } } }