How to synchronize result of UIAlertController? - ios

Question

I'm trying to implement a UINavigationBarDelegate, and I want to show a alert to let user determine whether leave this view.
This is my code:
extension CDFFormController: UINavigationBarDelegate {
public func navigationBar(_ navigationBar: UINavigationBar, shouldPop item: UINavigationItem) -> Bool {
if let entityViewController = self.topViewController as? MyEntityViewController {
if entityViewController.isEditing {
let semaphore = DispatchSemaphore(value: 0)
var result = false
let alert = UIAlertController(title: "Leave the view？", message: nil, preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "leave", style: .default, handler: { _ in
result = true
semaphore.signal()
}))
alert.addAction(UIAlertAction(title: "cancel", style: .cancel, handler: { _ in
semaphore.signal()
}))
entityViewController.present(alert, animated: true, completion: nil)
semaphore.wait()
return result
} else {
return true
}
} else {
return true
}
}
}
I need return the result, so I use DispatchSemaphore to block the method. But the question is: This method is called on main queue, and blocking it means blocking the ui thread and the method never returns.
Or any other solution?

Answer 1

I am not sure it's will be work. I does not tested this.
extension CDFFormController: UINavigationBarDelegate {
public func navigationBar(_ navigationBar: UINavigationBar, shouldPop item: UINavigationItem) -> Bool {
if let entityViewController = self.topViewController as? MyEntityViewController {
if entityViewController.isEditing {
let alert = UIAlertController(title: "Leave the view？", message: nil, preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "leave", style: .default, handler: { _ in
let saveDelegate = navigationBar.delegate;
navigationBar.delegate = nil;
navigationBar.popItem(animated:YES);
navigationBar.delegate = saveDelegate;
}))
alert.addAction(UIAlertAction(title: "cancel", style: .cancel, handler:nil))
entityViewController.present(alert, animated: true, completion: nil)
return false
} else {
return true
}
} else {
return true
}
}
}
Other way is use RunLoop. But I don't like this.
extension CDFFormController: UINavigationBarDelegate {
public func navigationBar(_ navigationBar: UINavigationBar, shouldPop item: UINavigationItem) -> Bool {
if let entityViewController = self.topViewController as? MyEntityViewController {
if entityViewController.isEditing {
let state = 0
var result = false
let alert = UIAlertController(title: "Leave the view？", message: nil, preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "leave", style: .default, handler: { _ in
state = 1
}))
alert.addAction(UIAlertAction(title: "cancel", style: .cancel, handler: { _ in
state = 2
}))
entityViewController.present(alert, animated: true, completion: nil)
while (state == 0)
{
RunLoop.current.run(until: Date(timeIntervalSinceNow: 0.5))
}
return state == 1
} else {
return true
}
} else {
return true
}
}
}