gistfile1.m

-(IBAction)startWorking:(id)sender {
    [self showIndeterminateProgressBar];

    self.future = [worker startAsyncWork];
    [future onCompletion:^(id result, NSError * error) {
        // we got a result!
        [self hideIndeterminateProgressBar];
        self.future = nil;
    }];

    [future onCancel:^{
        // clean up anything that might depend on what we attempt to do in the completion handler
        [self hideIndeterminateProgressBar];
        self.future = nil;
    }];
}