TIL You Can Cancel Enqueued GCD Blocks

Today I learned that you can cancel a delayed dispatch_block_t with the new dispatch_block_cancel (available since OS X 10.10/iOS 8.0). Thanks Matt for the post – here's a Swift example:

#!swift
let work = dispatch_block_create(0) { print("Hello!") }

# Execute after 10s
let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(10 * Double(NSEC_PER_SEC)))
dispatch_after(delayTime, dispatch_get_main_queue(), work)

dispatch_block_cancel(work)
# Will never print "Hello!"

Note: canceling doesn't work if the block is being executed.

If I knew that this API existed, I might not have used the very cumbersome approach from below in Move!.

Super-Weird Legacy Version of a Cancelable Delayed Block

For historic purposes, here's an adaptation of the cancelable dispatch block you may find on the internet that I once have adapted for Swift:

#!swift
typealias CancelableDispatchBlock = (cancel: Bool) -> Void

func dispatch(cancelableBlock block: dispatch_block_t, atDate date: NSDate) -> CancelableDispatchBlock? {

    // Use two pointers for the same block handle to make
    // the block reference itself.
    var cancelableBlock: CancelableDispatchBlock? = nil

    let delayBlock: CancelableDispatchBlock = { cancel in

        if !cancel {
            dispatch_async(dispatch_get_main_queue(), block)
        }

        cancelableBlock = nil
    }

    cancelableBlock = delayBlock

    let interval = Int64(date.timeIntervalSinceNow)
    let delay = interval * Int64(NSEC_PER_SEC)

    dispatch_after(dispatch_walltime(nil, delay), dispatch_get_main_queue()) {

        guard let cancelableBlock = cancelableBlock else { return }

        cancelableBlock(cancel: false)
    }

    return cancelableBlock
}

func cancelBlock(block: CancelableDispatchBlock?) {

    guard let block = block else { return }

    block(cancel: true)
}

The trick is this: the delayed block delayBlock: CancelableDispatchBlock captures its context where a reference to cancelableBlock is included – but not set yet. Then you make the reference point to the delayBlock itself.

The actual canceling is a fake, though. The block is still called. It aborts early if the cancel parameter is true, though.

Jiggle GCD Queues to Find Problems

To debug my threading issues and help bring forth future problems, I have created a simmple object that slows the current queue down:

#!swift
let IsRunningTests = NSClassFromString("XCTestCase") != nil

struct QueueJigglePoint {

    /// Randomly interfere with the thread.
    static func jiggle() {

        guard !IsRunningTests else { return }

        #if DEBUG
            usleep(2*1000000) // 2 seconds
        #endif
    }

}

I got this idea from Brett Schuchert on pages 188–90 of Uncle Bob's Clean Code. A Handbook of Agile Software Craftsmanship. There, interference with traditional threading should randomly sleep, yield, or fall through. Enqueued blocks are a lot less volatile, so I only came up with sleeping.

Randomizing the sleep interval is up next. But a fixed number of 2–10 seconds helps find UI-blocking code already.

Just throw in a QueueJigglePoint.jiggle() in NSManagedObjectContext.performBlock executions, when dispatching async to the background, or when reading files, for example.