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

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.