Add Onesignal to Project

Open Your Flutter Ios Folder in Xcode.

• Create New Target.

• Select Notification Extension and Name It OneSignalNotificationServiceExtension

• 
• Press Cancel on the Activate Scheme prompt

• In your project, in the OneSignalNotificationServiceExtension/ folder, open NotificationService.m and replace the whole file contents with:

NotificationService.Swift

import UserNotifications

import OneSignal

class NotificationService: UNNotificationServiceExtension {
    
    var contentHandler: ((UNNotificationContent) -> Void)?
    var receivedRequest: UNNotificationRequest!
    var bestAttemptContent: UNMutableNotificationContent?
    
    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
        self.receivedRequest = request
        self.contentHandler = contentHandler
        self.bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)
        
        if let bestAttemptContent = bestAttemptContent {   
            OneSignal.didReceiveNotificationExtensionRequest(self.receivedRequest, with: bestAttemptContent, withContentHandler: self.contentHandler)
        }
    }
    
    override func serviceExtensionTimeWillExpire() {
        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
            OneSignal.serviceExtensionTimeWillExpireRequest(self.receivedRequest, with: self.bestAttemptContent)
            contentHandler(bestAttemptContent)
        }
    } 
}

• In the main app target, select Signing & Capabilities > All > + Capability, enable Push Notifications.

• 
• Next, enable Background Modes and check Remote Notifications.

•