Returning value while dispatching to the main thread

Question

I have a function func getValue() -> Bool that's called from a background thread. This is intentional and also required. Now, the getValue() needs to execute something on the main thread, in this case it needs to access UIApplication.shared.canOpenURL, which must be run on the main queue.

This is my current function:

func getValue() -> Bool {
    guard let url = URL(string: "someurl") else { return false }
    return UIApplication.shared.canOpenURL(url)
}

How can I convert that function to a thread safe one, namely to make sure it always runs on the main thread, without

• calling the function from the main thread to begin with
• refactoring the function to return the value in a closure

I've tried this:

// This causes a deadlock, see https://stackoverflow.com/a/42484670/1531270
func getValue() -> Bool {
    var flag = false

    let group = DispatchGroup()
    group.enter()
    DispatchQueue.main.async {
        if let url = URL(string: "someurl"), UIApplication.shared.canOpenURL(url) {
            flag = true
        }
        group.leave()
    }
    group.wait()

    return flag
}

and this:

// This crashes with EXC_BREAKPOINT (SIGTRAP) dispatch_sync called on queue already owned by current thread
func getValue() -> Bool {
    return DispatchQueue.main.sync {
        guard let url = URL(string: "someurl") else { return false }
        return UIApplication.shared.canOpenURL(url)
    }
}

but neither of them works. Any ideas?

Answer 1

You're looking for a semaphore - try this:

DispatchQueue.global(qos: .background).async {
    var value: Bool? = nil
    let semaphore = DispatchSemaphore(value: 0)
    DispatchQueue.main.async {
        let alert = UIAlertController(title: "Choose one", message: "Take your time, I'll wait", preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "true", style: .default, handler: { _ in
            value = true
            semaphore.signal()
        }))
        alert.addAction(UIAlertAction(title: "false", style: .default, handler: { _ in
            value = false
            semaphore.signal()
        }))
        self.present(alert, animated: true, completion: nil)
    }
    semaphore.wait()
    print("Choice: \(value!)")
}

Or to use your example from above:

func getValue() -> Bool {
    var flag = false
    let semaphore = DispatchSemaphore(value: 0)

    DispatchQueue.main.async {
        if let url = URL(string: "someurl"), UIApplication.shared.canOpenURL(url) {
            flag = true
            semaphore.signal()
        }
    }
    semaphore.wait()
    return flag
}