This guide helps you to launch an incoming call screen from the UI Kit library on receiving an incoming call notification.
CometChat SDK & UI Kit both need to be configured before launching the call screen.

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Step 1. Process push notification payload and grab Call object

To present an incoming call screen, firstly you will need a Call object. You can grab this from the push notification payload itself of incoming call notification. You need to call CometChat.processMessage() method to process push notification payload. 
 func userNotificationCenter(_ center: UNUserNotificationCenter,
                    didReceive response: UNNotificationResponse,
                    withCompletionHandler completionHandler: @escaping () -> Void) {
        
        if let userInfo = response.notification.request.content.userInfo as? [String : Any], let messageObject = userInfo["message"] as? [String:Any] {
           print("didReceive: \(userInfo)")
          if let baseMessage = CometChat.processMessage(messageObject).0 {
            switch baseMessage.messageCategory {
            case .message:
                print("Message Object Received: \(String(describing: (baseMessage as? TextMessage)?.stringValue()))")
                
            case .action: break
            case .call: break
            case .custom: break
            @unknown default: break
            }
          }
        }
        completionHandler()
      }

​
Step 2 . Launch call screen (Method 1)

You can directly launch the view controller from the app delegate once you receive Call Object. 
  if let call = baseMessage as? Call {
      DispatchQueue.main.async {
      let call = CometChatIncomingCall()
      call.modalPresentationStyle = .custom
      call.setCall(call: call)
     if let window = self.window, let rootViewController = window.rootViewController      {
     var currentController = rootViewController
     while let presentedController = currentController.presentedViewController {
     currentController = presentedController
     }
      if (!call.isViewLoaded && (call.view.window != nil)) {
     currentController.present(call, animated: true, completion: nil)
      }
     }
   }
 }
If you are facing any difficulties while launching the Call Screen from App Delegate, then you can use another method.

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Step 2 . Launch call screen (Method 2)

You can launch the call screen from your base view controller instead of launching it from the App Delegate. This method uses NotificationCenter to trigger and present Call Screen.
  1. In this method you need to fire notification after you receive Call Object.
  2. In Notification’s user info you can pass Call Object to that desired notification.

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Trigger notification from App Delegate

if let call = baseMessage as? Call {
  DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
 NotificationCenter.default.post(name: NSNotification.Name(rawValue: "didReceivedIncomingCall"), object: nil, userInfo: ["call":call])
   }
}
  1. On the other hand, you need to observe for the above notification in your base view controller
  1. Base view controller is a controller that launches immediately after the app delegate.
  2. Base view controller should be present to observe the notification when notification fires.
  3. If the view controller is not present in the memory when a new notification receives, then it won’t launch Call Screen.

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Observe notification in Base View Controller

class BaseViewController : UIViewController {
  
  override func viewDidLoad() {
    NotificationCenter.default.addObserver(self, selector:#selector(self.didReceivedIncomingCall(_:)), name: NSNotification.Name(rawValue: "didReceivedIncomingCall"), object: nil)
  }
}

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Add selector method & Launch call screen

 @objc func didReceivedIncomingCall(_ notification: NSNotification) {
        if let currentCall = notification.userInfo?["call"] as? Call {
            DispatchQueue.main.async {
              let call = CometChatIncomingCall()
              call.modalPresentationStyle = .custom
              call.setCall(call: currentcall)
              self.present(call, animated: true, completion: nil)
            }
        }
 }