Question

I want to start a calculation task that will last for a couple of minutes. Since the solution is getting closer and closer to the optimal solution with time, I want to give the user a button to stop the calculation at any time and stick with the approximation.

But when I start my calculation in viewDidLoad the view with the stop button isn’t even presented to the user until the calculation ends. So I need some kind of background calculation process – but how can I do this on iOS 5?

I also want to be able to update a progress bar in my view (with the stop button).

Here’s my code in the UIViewController subclass:

- (void)viewDidLoad
{
    [super viewDidLoad];

    // doing some initialization

    [self startCalculation]; // do this in background for stopping capabilities?
}

- (void)startCalculation {
    // start calculation and update progress bar every 200 ms or so
}

Could somebody please help me?

Answer 1

General answer is to call startCalculation from within an NSThread

//thread is a property in your corrent class
NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(startCalculation) object:nil];

Then to stop it

//when you want to stop the thread
[thread cancel];

If you want to update a progress view from within this thread make sure to call the progress updates from the ui main thread like so

//in your calculation function, when you want to update the progressview
- (void)startGenerationProcess {
    //Calculations

    //update progress
    dispatch_async(dispatch_get_main_queue(), ^{
          //Update progress
    });

    //More Calculations
}