Propagates notification that operations should be canceled.

Developers writing methods that return a Task should take a CancellationToken as a parameter if they wish to make the Task cancelable (see below code snippet). A CancellationToken can only be created by creating a new instance of CancellationTokenSource. CancellationToken is immutable and must be canceled by calling cancel() on the CancellationTokenSource that creates it. It can only be canceled once. If canceled, it should not be passed to future operations.

When cancel() is called, all the Tasks with the CancellationToken from that CancellationTokenSource will be canceled. This operation only flags those Tasks as canceled, and the API author is responsible for stopping whatever the Task is actually doing to free up the resources.

Cancellable Task example:

 public Task<Integer> doSomething(CancellationToken token) {

   // Attach a listener that will be called once cancellation is requested.
   token.onCanceledRequested(new OnTokenCanceledListener() {
     @Override
     public void onCanceled() {
       // Some other operations to cancel this Task, such as free resources...
     }
   });

   final TaskCompletionSource<Integer> tcs = new TaskCompletionSource<>(token);

   // do something...

 }

 CancellationTokenSource cts = new CancellationTokenSource();
 Task<Integer> task = doSomething(cts.getToken());
 cts.cancel();
 

 public class MyActivity extends Activity {
   // ...

   @Override
   public void onStart() {
     super.onStart();

     // Typically use one cancellation source per lifecycle.
     cancellationSource = new TaskCancellationSource();

     // That source's token can be passed to multiple calls.
     doSomethingCancellable(cancellationSource.getToken())
       .onSuccessTask(result -> doSomethingElse(result, cancellationSource.getToken()));
   }

   @Override
   public void onStop() {
     super.onStop();
     cancellationSource.cancel();
   }
 }