self/app/ios/NotificationService/NotificationService.m

#import "NotificationService.h"
#import <Firebase.h>

@interface NotificationService ()

@property (nonatomic, strong) void (^contentHandler)(UNNotificationContent *contentToDeliver);
@property (nonatomic, strong) UNMutableNotificationContent *bestAttemptContent;

@end

@implementation NotificationService

- (void)didReceiveNotificationRequest:(UNNotificationRequest *)request withContentHandler:(void (^)(UNNotificationContent * _Nonnull))contentHandler {
    self.contentHandler = contentHandler;
    self.bestAttemptContent = [request.content mutableCopy];

    // Configure Firebase if needed
    if (![FIRApp defaultApp]) {
        [FIRApp configure];
    }

    // Get the message ID
    NSDictionary *userInfo = request.content.userInfo;
    NSString *messageID = userInfo[@"gcm.message_id"];
    if (messageID) {
        [[FIRMessaging messaging] appDidReceiveMessage:userInfo];
    }

    // Modify the notification content here...
    self.bestAttemptContent.title = [NSString stringWithFormat:@"%@ [modified]", self.bestAttemptContent.title];

    // Always call the content handler with the modified content
    self.contentHandler(self.bestAttemptContent);
}

- (void)serviceExtensionTimeWillExpire {
    // Called just before the extension will be terminated by the system.
    // Use this as an opportunity to deliver your "best attempt" at modified content, otherwise the original push payload will be used.
    self.contentHandler(self.bestAttemptContent);
}

@end