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This library is deprecated and shouldn't be used anymore. Please use something better like RxJava. No more development will be taking place.


An utility library for Android to run actions in background. It handles orientation changes and delegates results to the visible Activity.


Download the latest version or grab via Gradle:

dependencies {

  compile '' 


The class TaskExecutor serves as entry point. Your background actions need to extend the class Task. Your callback method needs be annotated with TaskResult and has to accept exactly one result object, which the Task returns. You may want to take a look at the demo.

public class MyActivity extends Activity {

 @Override  protected void onCreate(Bundle savedInstanceState) {


int taskId = new MyTask().start(this);

@TaskResult  public void onResult(Integer result) {

// handle result, this method gets called on the UI thread and only if the activity is visible  
  public class MyTask extends Task<Integer> {

 @Override  protected Integer execute() {

return 5;  


You can create your own TaskExecutor instance, if you want to change the behavior.

new Builder()  .setExecutorService(Executors.newSingleThreadExecutor()) // default CachedThreadPool  .setPostResult(PostResult.ON_ANY_THREAD) // default PostResult.UI_THREAD  .build()  .asSingleton();

The TaskExecutor returns an ID. You can restore this ID in onCreate(Bundle savedInstanceState to find your Task.

@Override protected void onCreate(Bundle savedInstanceState) {


if (savedInstanceState != null) {

int taskId = savedInstanceState.getInt(TASK_ID_KEY, -1);

MyTask task = TaskExecutor.getInstance().getTask(taskId);


A callback method can have a second parameter to get the specific Task instance.

@TaskResult public void onResult(Integer result, MyTask task) {
  // handle result, this method gets called on the UI thread and only if the activity is visible 

You can annotate the callback method with a concrete ID, if you want to reuse a Task but provide different callbacks. Attention: If your callback method has an ID, then a Task submitted without an ID won't find this callback method.

public void startTask() {
  Task myTask = new MyTask();
  TaskExecutor.getInstance().execute(myTask, this, "my_id");
  @TaskResult(id = "my_id") public void onResult(Integer result, MyTask task) {
  // handle result, this method gets called on the UI thread and only if the activity is visible 

If you want to submit a Task, which should not return any result and shouldn't invoke any callback method, then extend TaskNoCallback. Compared to a normal Thread the purpose of this class is that you still have access to the Context and can find the Task in the TaskExecutor.

public class SimpleNoCallbackTask extends TaskNoCallback {

  protected void executeTask() {

// do anything

It's possible to replace the callback, if another Activity or Fragment should handle the result.

public class ReplaceCallbackActivity extends Activity {

  protected void onCreate(Bundle savedInstanceState) {


 List<IntegerTask> tasks = TaskExecutor.getInstance().getAllTasks(IntegerTask.class);

for (IntegerTask task : tasks) {




  public void onResult(Integer integer) {

Toast.makeText(this, "Replaced " + integer, Toast.LENGTH_SHORT).show();


How it works

The TaskExecutor monitors the state of the Activity. When a Task finished, the TaskExecutor searches the callback method and invokes it on the visible Activity or Fragment.

UI components are only referenced with a WeakReference to avoid memory leaks. If a callback method can't be found or the Activity was already cleared, then the result from the Task is dropped.


Your class, which can receive callbacks, must be an instance of Activity, FragmentActivity or Fragment from the support library.

The library uses reflection to find your callback method at runtime. This costs time, but classes are scanned in the background.


Copyright 2015 Ralf Wondratschek  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  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. 


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