Cancellable dispatch block with delay

dispatch_after_cancellable

func dispatch_after_cancellable(when: dispatch_time_t, queue: dispatch_queue_t, block: dispatch_block_t) -> () -> Void {
    var isCancelled = false
    dispatch_after(when, queue) {
        if !isCancelled {
            block()
        }
    }
    
    return { isCancelled = true }
}

// Usage:

var cancelMessage: () -> Void = {}

cancelMessage()
cancelMessage = dispatch_after_cancellable(dispatch_time(DISPATCH_TIME_NOW, Int64(0.5 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()) {
    // do something
}