Understanding NSTimer as a Timeout Mechanism

Introduction

In this article, we will delve into the world of NSTimer and its role in implementing timeouts. We’ll explore how to use NSTimer effectively to create a timeout mechanism in your iOS applications.

What is NSTimer?

NSTimer is a class that allows you to schedule a block of code to run at a specified interval. It’s commonly used for tasks like updating UI elements, sending network requests, or performing long-running operations on background threads.

The Problem with Scheduling a Timer and Doing a Blocking Call

In the provided example, NSTimer is scheduled in viewDidLoad, but then [self doSomethingThatTakesALongTime] is called. This causes a problem because the timer is waiting for the blocking call to finish before firing.

- (void)viewDidLoad {
    timer = [NSTimer scheduledTimerWithTimeInterval:20
          target:self
        selector:@selector(initializationTimedOut:)
        userInfo:nil
         repeats:NO];

    [self doSomethingThatTakesALongTime];
}

As soon as doSomethingThatTakesALongTime is called, the timer’s run loop is blocked until it finishes. However, since we’re sleeping for 30 seconds inside this function, the timer never fires.

Solving the Problem with Detaching a Thread

To solve this problem, we need to detach a thread that performs the long operation. Once the long operation is finished, we can call back on the main thread to invalidate the timer.

- (void)viewDidLoad {
    timer = [NSTimer scheduledTimerWithTimeInterval:20
          target:self
        selector:@selector(initializationTimedOut:)
        userInfo:nil
         repeats:NO];

    NSThread *thread = [NSThread detachNewThreadSelector:@selector(doSomethingThatTakesALongTime:) toTarget:self withObject:nil];
}

- (void)doSomethingThatTakesALongTime:(id)arg {
    sleep(30); // for testing only
    // web service calls etc. go here

    // invalidate the timer on the main thread
    [self performSelectorOnMainThread:@selector(invalidate) withObject:nil waitUntilDone:NO];
}

- (void)invalidate {
    [timer invalidate];
}

In this revised version, we detach a new thread using NSThread detachNewThreadSelector:toTarget withObject:. The long-running operation is performed on this detached thread.

Once the long-running operation is finished, it calls back to the main thread using performSelectorOnMainThread:withObject:waitUntilDone: and invalidates the timer. This ensures that the timer’s run loop is not blocked by the long-running operation.

Understanding Key Concepts

• Blocking Calls: A blocking call is a function call that prevents the execution of other functions until it completes.
• NSTimer Run Loop: The NSTimer run loop is responsible for firing the timer at its scheduled interval. If a blocking call is made during this time, the run loop will be blocked until the call finishes.
• Detaching Threads: Detaching threads allows you to perform long-running operations in separate threads, which can improve performance and responsiveness.

Best Practices

When using NSTimer for timeouts, keep the following best practices in mind:

• Always invalidate the timer on the same thread it was scheduled.
• Use detached threads when performing long-running operations to avoid blocking the run loop.
• Consider using other timeout mechanisms like UIView.animate or dispatch_time for more complex scenarios.

Conclusion

By understanding how NSTimer works and using it effectively, you can implement timeouts in your iOS applications. Remember to detach threads when performing long-running operations to avoid blocking the run loop. By following these best practices and key concepts, you’ll be able to write efficient and responsive code for your iOS applications.

Additional Resources

For more information on NSTimer, consult the official Apple documentation:

https://developer.apple.com/documentation/nstimer

You can also explore other timeout mechanisms like UIView.animate or dispatch_time in the Apple Developer Library:

https://developer.apple.com/library/archive/documentation/General/Conceptual/CocoaFundamentals/Events/Events.html

By understanding these resources and implementing the best practices outlined in this article, you’ll be able to write high-performance iOS applications with efficient timeouts.

Last modified on 2023-12-21