AsyncTask

AsyncTask

extends  Object

java.lang.Object
↳	android.os.AsyncTask<Params, Progress, Result>

Class Overview

---

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 pacakge 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 andonPostExecute.

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

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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> { ... }

   

    
Summary
    

    
Nested
 Classes

        
         enum
        
AsyncTask.Status
Indicates
 the current status of the task. 
    
Fields

        
         public static final 
         Executor
        
SERIAL_EXECUTOR
An Executor that
 executes tasks one at a time in serial order.

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

        

        
         AsyncTask()
        
        

         Creates a new asynchronous task.
        
    
Public
 Methods

        
         final boolean
        

        
         cancel(boolean mayInterruptIfRunning)
        
        

         
Attempts to cancel execution of this task.
        

        
         static void
        

        
         execute(
         Runnable runnable)
        
        

         Convenience version of 
         execute(Object) for use with a simple Runnable object.
        

        
         final 
         AsyncTask<Params, Progress, Result>
        

        
         execute(Params... params)
        
        

         Executes the task with the specified parameters.
        

        
         final 
         AsyncTask<Params, Progress, Result>
        

        
         executeOnExecutor(
         Executor exec, Params... params)
        
        

         Executes the task with the specified parameters.
        

        
         final Result
        

        
         get(long timeout, 
         TimeUnit unit)
        
        

         Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.
        

        
         final Result
        

        
         get()
        
        

         Waits if necessary for the computation to complete, and then retrieves its result.
        

        
         final 
         AsyncTask.Status
        

        
         getStatus()
        
        

         Returns the current status of this task.
        

        
         final boolean
        

        
         isCancelled()
        
        

         Returns 
         true if this task was cancelled before it completed normally.
        
    
Protected
 Methods

        
         abstract Result
        

        
         doInBackground(Params... params)
        
        

         Override this method to perform a computation on a background thread.
        

        
         void
        

        
         onCancelled(Result result)
        
        

         
Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished.
        

        
         void
        

        
         onCancelled()
        
        

         
Applications should preferably override onCancelled(Object).
        

        
         void
        

        
         onPostExecute(Result result)
        
        

         
Runs on the UI thread after doInBackground(Params...).
        

        
         void
        

        
         onPreExecute()
        
        

         Runs on the UI thread before 
         doInBackground(Params...).
        

        
         void
        

        
         onProgressUpdate(Progress... values)
        
        

         Runs on the UI thread after 
         publishProgress(Progress...) is invoked.
        

        
         final void
        

        
         publishProgress(Progress... values)
        
        

         This method can be invoked from 
         doInBackground(Params...) to publish updates on the UI thread while the background computation is still running.
        
    
[Expand]
        

         Inherited Methods
        
 From
 class java.lang.Object
        

         

        
   
Fields
   

   

    
public static final Executor SERIAL_EXECUTOR
    

     Added in 
     API level 11
    
    

     

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

   

    
public static final Executor THREAD_POOL_EXECUTOR
    

     Added in 
     API level 11
    
    

     

      
An Executor that can be used to execute tasks in parallel.