Package android.os

Class AsyncTask<Params,​Progress,​Result>

  • public abstract class AsyncTask<Params,​Progress,​Result>
extends Object

    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 Thread and 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 Executor, ThreadPoolExecutor and 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 (doInBackground(Params...)), and most often will override a second one (onPostExecute(Result).)

    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. Progress, the type of the progress units published during the background computation.
    3. Result, the type of the result of the background computation.

    Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type 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. 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. doInBackground(Params...), invoked on the background thread immediately after 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 #publishProgress to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.
    3. onProgressUpdate(Progress...), invoked on the UI thread after a call to #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.
    4. onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

    Cancelling a task

    A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from 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 android.os.Build.VERSION_CODES#DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with 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 executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.

    • Field Detail

      • THREAD_POOL_EXECUTOR

        public static final Executor THREAD_POOL_EXECUTOR
        An Executor that can be used to execute tasks in parallel.

      • SERIAL_EXECUTOR

        public static final Executor SERIAL_EXECUTOR
        An Executor that executes tasks one at a time in serial order. This serialization is global to a particular process.

    • Constructor Detail

      • AsyncTask

        public AsyncTask()
        Creates a new asynchronous task. This constructor must be invoked on the UI thread.

    • Method Detail

      • setDefaultExecutor

        public static void setDefaultExecutor​(Executor exec)

      • getStatus

        public final AsyncTask.Status getStatus()
        Returns the current status of this task.
        Returns:
        The current status.

      • doInBackground

        protected abstract Result doInBackground​(Params... params)
        Override this method to perform a computation on a background thread. The specified parameters are the parameters passed to execute(Params...) by the caller of this task. This method can call #publishProgress to publish updates on the UI thread.
        Parameters:
        params - The parameters of the task.
        Returns:
        A result, defined by the subclass of this task.
        See Also:
        onPreExecute(), onPostExecute(Result), #publishProgress

      • onProgressUpdate

        protected void onProgressUpdate​(Progress... values)
        Runs on the UI thread after #publishProgress is invoked. The specified values are the values passed to #publishProgress.
        Parameters:
        values - The values indicating progress.
        See Also:
        #publishProgress, doInBackground(Params...)

      • isCancelled

        public final boolean isCancelled()
        Returns true if this task was cancelled before it completed normally. If you are calling cancel(boolean) on the task, the value returned by this method should be checked periodically from doInBackground(Object[]) to end the task as soon as possible.
        Returns:
        true if task was cancelled before it completed
        See Also:
        cancel(boolean)

      • cancel

        public final boolean cancel​(boolean mayInterruptIfRunning)

        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 onCancelled(Object) being invoked on the UI thread after doInBackground(Object[]) returns. Calling this method guarantees that onPostExecute(Object) is never invoked. After invoking this method, you should check the value returned by isCancelled() periodically from doInBackground(Object[]) to finish the task as early as possible.

        Parameters:
        mayInterruptIfRunning - true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete.
        Returns:
        false if the task could not be cancelled, typically because it has already completed normally; true otherwise
        See Also:
        isCancelled(), onCancelled(Object)

      • executeOnExecutor

        public final AsyncTask<Params,​Progress,​Result> executeOnExecutor​(Executor exec,
                                                                             Params... 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 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 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 SERIAL_EXECUTOR.

        This method must be invoked on the UI thread.

        Parameters:
        exec - The executor to use. THREAD_POOL_EXECUTOR is available as a convenient process-wide thread pool for tasks that are loosely coupled.
        params - The parameters of the task.
        Returns:
        This instance of AsyncTask.
        Throws:
        IllegalStateException - If getStatus() returns either AsyncTask.Status.RUNNING or AsyncTask.Status.FINISHED.
        See Also:
        execute(Object[])