问题
I have a tableview which, when loaded, each cell could possibly return an NSError, which I have chosen to display in a UIAlertController. Problem is I get this error in the console if multiple errors are returned.
Warning: Attempt to present UIAlertController: 0x14e64cb00 on MessagesMasterVC: 0x14e53d800 which is already presenting (null)
Ideally, I would ideally like to handle this in my UIAlertController extension method.
class func simpleAlertWithMessage(message: String!) -> UIAlertController {
let alertController = UIAlertController(title: nil, message: message, preferredStyle: UIAlertControllerStyle.Alert)
let cancel = UIAlertAction(title: "Ok", style: .Cancel, handler: nil)
alertController.addAction(cancel)
return alertController
}
Based on matt's answer, I changed the extension to a UIViewController extension, its much cleaner and saves lots of presentViewController code.
func showSimpleAlertWithMessage(message: String!) {
let alertController = UIAlertController(title: nil, message: message, preferredStyle: UIAlertControllerStyle.Alert)
let cancel = UIAlertAction(title: "Ok", style: .Cancel, handler: nil)
alertController.addAction(cancel)
if self.presentedViewController == nil {
self.presentViewController(alertController, animated: true, completion: nil)
}
}
回答1:
It is not the UIAlertController that is "already presenting", it is MessagesMasterVC. A view controller can only present one other view controller at a time. Hence the error message.
In other words, if you have told a view controller to presentViewController:..., you cannot do that again until the presented view controller has been dismissed.
You can ask the MessagesMasterVC whether it is already presenting a view controller by examining its presentedViewController. If not nil, do not tell it to presentViewController:... - it is already presenting a view controller.
回答2:
if ([self.navigationController.visibleViewController isKindOfClass:[UIAlertController class]]) {
// UIAlertController is presenting.Here
}
回答3:
Well, the suggested solutions above has an essential problem from my point of view:
If you ask your ViewController, whether the attribute 'presentedViewController' is nil and the answer is false, you can't come to the conclusion, that your UIAlertController is already presented. It could be any presented ViewController, e.g. a popOver. So my suggestion to surely check, whether the Alert is already on the screen is the following (cast the presentedViewController as a UIAlertController):
if self.presentedViewController == nil {
// do your presentation of the UIAlertController
// ...
} else {
// either the Alert is already presented, or any other view controller
// is active (e.g. a PopOver)
// ...
let thePresentedVC : UIViewController? = self.presentedViewController as UIViewController?
if thePresentedVC != nil {
if let thePresentedVCAsAlertController : UIAlertController = thePresentedVC as? UIAlertController {
// nothing to do , AlertController already active
// ...
print("Alert not necessary, already on the screen !")
} else {
// there is another ViewController presented
// but it is not an UIAlertController, so do
// your UIAlertController-Presentation with
// this (presented) ViewController
// ...
thePresentedVC!.presentViewController(...)
print("Alert comes up via another presented VC, e.g. a PopOver")
}
}
}
回答4:
Here's a solution I use in Swift 3. It is a function that shows an alert to the user, and if you call it multiple times before the user has dismissed the alert, it will add the new alert text to the alert that's already being presented. If some other view is being presented, the alert will not appear. Not all will agree with that behavior, but it works well for simple situations.
extension UIViewController {
func showAlert(_ msg: String, title: String = "") {
if let currentAlert = self.presentedViewController as? UIAlertController {
currentAlert.message = (currentAlert.message ?? "") + "\n\nUpdate:\(title): \(msg)"
return
}
// create the alert
let alert = UIAlertController(title: title, message: msg, preferredStyle: UIAlertControllerStyle.alert)
alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.default, handler: nil))
// show the alert
self.present(alert, animated: true, completion: nil)
}
}
回答5:
We can simply check if any view controller is presented.
if presented then check if it is kind of UIAlertController .
id alert = self.presentedViewController;
if (alert && [alert isKindOfClass:[UIAlertController class]])
{
*// YES UIAlertController is already presented*
}
else
{
// UIAlertController is not presented OR visible.
}
回答6:
This category can auto-manage all the modal controllers include of UIAlertController.
UIViewController+JCPresentQueue.h
回答7:
I used that to detect and remove and alert.
First we create an alert with following function.
var yourAlert :UIAlertController!
func useYouAlert (header: String, info:String){
yourAlert = UIAlertController(title:header as String, message: info as String, preferredStyle: UIAlertControllerStyle.alert)
let okAction = UIAlertAction(title: self.langText[62]as String, style: UIAlertActionStyle.default) { (result : UIAlertAction) -> Void in
print("OK")
}
yourAlert.addAction(okAction)
self.present(yourAlert.addAction, animated: true, completion: nil)
}
And in some other part of your code
if yourAlert != nil {
yourAlert.dismiss(animated: true, completion: nil)
}
回答8:
For latest Swift language you can use following:
var alert = presentedViewController
if alert != nil && (alert is UIAlertController) {
// YES UIAlertController is already presented*
} else {
// UIAlertController is not presented OR visible.
}
回答9:
Swift 4.2+ Answer
if UIApplication.topViewController()!.isKind(of: UIAlertController.self) {
print("UIAlertController is presented")}
For those who don't know how to get top most Viewcontroller
extension UIApplication {
public class func topViewController(_ base: UIViewController? = UIApplication.shared.keyWindow?.rootViewController) -> UIViewController? {
if let nav = base as? UINavigationController {
return topViewController(nav.visibleViewController)
}
if let tab = base as? UITabBarController {
if let selected = tab.selectedViewController {
return topViewController(selected)
}
}
if let presented = base?.presentedViewController {
return topViewController(presented)
}
return base
}}
回答10:
Dismiss the current controller and present the alert controller like
func alert(_ message:String) {
let alert = UIAlertController(title: "Error!", message: message, preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "Dismiss", style: .default, handler: nil))
self.dismiss(animated: false, completion: nil)
self.present(alert, animated: true,completion: nil)
}
回答11:
you can test - in a single line - if an alert is already presented:
if self.presentedViewController as? UIAlertController != nil {
print ("alert already presented")
}
回答12:
Simply dismiss the current controller and present the one you want i.e.
self.dismiss(animated: false, completion: nil)
self.displayAlertController()
来源：https://stackoverflow.com/questions/28270282/what-is-the-best-way-to-check-if-a-uialertcontroller-is-already-presenting