ShowFlutterViewController.swift

import UIKit
import Flutter
import FlutterPluginRegistrant

class ShowFlutterViewControler: UIViewController{
  
  let userToken = "dummy123"
  
  func showFlutter(){
        let flutterEnggine = (UIApplication.shared.delegate as! AppDelegate).flutter_engine
        let flutterController = FlutterViewController(engine:flutterEnggine,nibName:nil,bundle:nil)
        let newsChannel = FlutterMethodChannel(name:"com.example.myflutterapp/data", binaryMessenger: flutterController.binaryMessenger)
        newsChannel.setMethodCallHandler({
            (call:FlutterMethodCall,result:@escaping FlutterResult) -> Void in
            guard call.method == "getUserToken" else {
                result("error")
                return
            }
            
            if userToken.isEmpty {
                result(FlutterError(code: "UNAVAILABLE",message: "Data not available",details: nil))
            }else{
                result(newsToken)
            }
        })
        
        //start
        //if your have scrollview in your flutter app
        if #available(iOS 13.0, *) {
            flutterController.isModalInPresentation = false
        } else {
            // Fallback on earlier versions
        }
        flutterController.modalPresentationStyle = .fullScreen
        //end
        self.present(flutterController, animated: true, completion: nil)
    }
}