Swift - Dispatch Queues and the U/I running serially

Question

I have a secondary LaunchScreenViewController for an App that has some animation whilst it gathers three types of background data.

Everything works but the order in which the DispatchQueues.async are run is random. However if I change them to DispatchQueues.sync everything happens in the right order but runs so fast (even with sleeps) you don't see the animations.

This needs to be .sync but how do I control the U/I so that I can see the animation? (Shown here as, e.g. self.subLogo1View.isHidden = true)

Here's the code:

// Queuing Variables    
var semaphore    = DispatchSemaphore(value: 1)    
var semaphoreSub = DispatchSemaphore(value: 1)

override func viewDidLoad() {
    super.viewDidLoad()
    DispatchQueue.global().async {
        self.semaphore.wait()
        self.gatherData()
        self.semaphore.signal()
    }

    DispatchQueue.global().async {
        self.semaphore.wait()
        self.checkNetworkAvailability()
        self.semaphore.signal()
    }

    DispatchQueue.global().async {
        self.semaphore.wait()
        self.checkSomething()
        self.semaphore.signal()
    }   
}


func gatherData() {
    DispatchQueue.main.async {
        self.semaphoreSub.wait()
        print ("1")
        self.subLogo1View.isHidden = true
        self.subLogo1View.setNeedsDisplay()
        self.semaphoreSub.signal()
    }
}

func checkNetworkAvailability() {
    DispatchQueue.main.async {
        self.semaphoreSub.wait()
        print ("2")
        self.subLogo2View.isHidden = true
        self.subLogo2View.setNeedsDisplay()    
        self.semaphoreSub.signal()
    }
}

func checkSomething() {
    DispatchQueue.main.async {
        self.semaphoreSub.wait()
        print ("3")
        self.subLogo3View.isHidden = true
        self.subLogo3View.setNeedsDisplay()
        self.semaphoreSub.signal()
    }
}

Where is the code for animation? You better try do things with completions/callbacks. — Starsky, Jun 06 '19 at 08:44
Psuedo - referred to in third para - "This needs to be .sync but how do I control the U/I so that I can see the animation? (Shown here as, e.g. self.subLogo1View.isHidden = true)" Checking out callbacks - thanks. — Edward Hasted, Jun 06 '19 at 10:20

score 1 · Answer 1 · answered Jun 06 '19 at 10:35

Instead of manually serializing your closures with a bunch of semaphores, you maybe better use a custom serial queue. For animation, user UIView.animate

Something like this:

func gatherData() {
    DispatchQueue.main.async {  // or sync, depending on your animation needs
        print ("1: gather Data")
        UIView.animate(withDuration: 0.5) { 
            self.subLogo1View.alpha = 0  // instead of isHidden
        }
    }
}

func viewDidLoad() {
    var mySerialQueue = DispatchQueue (label:"my.serial")
    mySerialQueue.async {
        self.gatherData()
    }
    mySerialQueue.async {
        self.checkNetworkAvailability()
    }
    // ...
}

Many thanks - when I'm doing proper Animations I'll follow your advice - again, many thanks. — Edward Hasted, Jun 06 '19 at 11:31

Swift - Dispatch Queues and the U/I running serially

1 Answers1