CancelableBlock.swift

// Modified from: https://github.com/katleta3000/CancelBlocks/blob/master/CancelBlocks.swift

typealias dispatch_cancelable_block_t = (cancel: Bool) -> (Void)

private func dispatch_after_delay(delay: Double, queue: dispatch_queue_t, block: dispatch_block_t?) -> dispatch_cancelable_block_t? {
    guard let block = block else { return nil }
    var originalBlock: dispatch_block_t? = block
    var cancelableBlock: dispatch_cancelable_block_t? = nil
    let delayBlock: dispatch_cancelable_block_t = {(cancel: Bool) -> Void in
        if let originalBlock = originalBlock where !cancel {
            dispatch_async(queue, originalBlock)
        }
        cancelableBlock = nil
        originalBlock = nil
    }
    cancelableBlock = delayBlock
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC))), queue) { () -> Void in
        if let cancelableBlock = cancelableBlock {
            cancelableBlock(cancel: false)
        }
    }
    return cancelableBlock
}

func dispatch_after_delay(delay: Double, block: dispatch_block_t?) -> dispatch_cancelable_block_t? {
    let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    return dispatch_after_delay(delay, queue: queue, block: block)
}

func dispatch_on_main_queue_after_delay(delay: Double, block: dispatch_block_t?) -> dispatch_cancelable_block_t? {
    return dispatch_after_delay(delay, queue: dispatch_get_main_queue(), block: block)
}

func cancel_block(block: dispatch_cancelable_block_t?) {
    if let block = block {
        block(cancel: true)
    }
}