Flutter with platform

AppDelegate.swift

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate, MyViewDelegate {
    var flutterViewController: FlutterViewController?
    var flutterResult: FlutterResult?

    override func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        let fvc : FlutterViewController = window?.rootViewController as! FlutterViewController
        let channel = FlutterMethodChannel(name: "my channel", binaryMessenger: fvc.binaryMessenger)
        channel.setMethodCallHandler(self.handle)
        
        flutterViewController = fvc
        let nav = UINavigationController.init(rootViewController: fvc)
        nav.isNavigationBarHidden = true
        window.rootViewController = nav
        
        GeneratedPluginRegistrant.register(with: self)
        return super.application(application, didFinishLaunchingWithOptions: launchOptions)
    }

    func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) -> Void {
        switch call.method {
        case "hello":
            result("iOS!")
        case "text":
            flutterResult = result
            let mvc = MyViewController()
            mvc.delegate = self
            let nav = window?.rootViewController as! UINavigationController
            nav.present(mvc, animated: true, completion: nil)
        default:
            result(FlutterMethodNotImplemented)
        }
    }

    func textDidEdit(_ text: String) {
        flutterResult?(text)
        flutterResult = nil
    }
    
    override func registrar(forPlugin: String) -> FlutterPluginRegistrar {
        return (self.flutterViewController?.registrar(forPlugin: forPlugin))!
    }
    
    override func hasPlugin(_ pluginKey: String) -> Bool {
        return (self.flutterViewController?.hasPlugin(pluginKey))!
    }
    
    override func valuePublished(byPlugin pluginKey: String) -> NSObject {
        return (self.flutterViewController?.valuePublished(byPlugin: pluginKey))!
    }
}

protocol MyViewDelegate {
    func textDidEdit(_ text: String)
}

class MyViewController: UIViewController, UITextViewDelegate {
    var delegate: MyViewDelegate?
    let editText = UITextView.init(frame: .zero)
    let submitButton = UIButton.init(frame: .zero)

    override func viewDidLoad() {
        self.view.backgroundColor = .white

        editText.font = UIFont.systemFont(ofSize: 16.0)
        editText.frame = CGRect(x: 8.0, y: 48.0, width: self.view.frame.width - 16.0, height: 200.0)
        editText.delegate = self
        editText.keyboardType = .default
        editText.returnKeyType = .default
        editText.backgroundColor = .white
        
        submitButton.setTitle("OK", for: .normal)
        submitButton.frame = CGRect(x: 8.0, y: 208.0, width: self.view.frame.width - 16.0, height: 40.0)
        submitButton.addTarget(self, action: #selector(self.submitButtonDidTouch(sender:)), for: .touchUpInside)
        submitButton.backgroundColor = .systemBlue
        
        self.view.addSubview(editText)
        self.view.addSubview(submitButton)
    }
    
    @objc func submitButtonDidTouch(sender: UIButton) {
        editText.resignFirstResponder()
        delegate?.textDidEdit(editText.text)
        if let nav = self.navigationController {
            nav.dismiss(animated: true)
        } else {
            self.dismiss(animated: true, completion: nil)
        }
    }
}

main.dart

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

const channel = MethodChannel('my channel');

void main() {
  //runApp(MyApp());
  runApp(MyApp2());
  //runApp(MyApp3());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(),
        body: Center(
          child: FutureBuilder<String>(
            future: channel.invokeMethod<String>('hello'),
            builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
              return Text(snapshot.data ?? '...');
            },
          ),
        ),
      ),
    );
  }
}

class MyApp2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(),
        body: FutureBuilder<String>(
          future: channel.invokeMethod<String>('text'),
          builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
            return Text(snapshot.data ?? '...');
          },
        ),
      ),
    );
  }
}

MainActivity.java

package com.example.myapp;

import android.content.Intent;

import androidx.annotation.NonNull;

import io.flutter.embedding.android.FlutterActivity;
import io.flutter.embedding.engine.FlutterEngine;
import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;

public class MainActivity extends FlutterActivity implements MethodChannel.MethodCallHandler {
    static private final int REQUEST_CODE = 0xf1677e4;

    private MethodChannel.Result mResult;

    @Override
    public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
        super.configureFlutterEngine(flutterEngine);
        MethodChannel channel = new MethodChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), "my channel");
        channel.setMethodCallHandler(this);
    }

    @Override
    public void onMethodCall(@NonNull MethodCall call, @NonNull MethodChannel.Result result) {
        switch (call.method) {
            case "hello":
                result.success("Android!");
                break;
            case "text":
                mResult = result;
                Intent intent = new Intent(this, MyActivity.class);
                startActivityForResult(intent, REQUEST_CODE);
                break;
            default:
                result.notImplemented();
                break;
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == REQUEST_CODE) {
            if (resultCode == RESULT_OK) {
                mResult.success(data.getStringExtra("data"));
            } else {
                mResult.success("cancel");
            }
            mResult = null;
        } else {
            super.onActivityResult(requestCode, resultCode, data);
        }
    }
}

MyActivity.java

package com.example.myapp;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.LinearLayout;

public class MyActivity extends Activity implements View.OnClickListener {
    private EditText editText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        LinearLayout layout = new LinearLayout(this);
        layout.setOrientation(LinearLayout.VERTICAL);

        editText = new EditText(this);
        layout.addView(editText, new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT,
                500
        ));

        Button btn = new Button(this);
        btn.setText("OK");
        btn.setOnClickListener(this);
        layout.addView(btn, new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT,
                LinearLayout.LayoutParams.WRAP_CONTENT
        ));

        setContentView(layout);
    }

    @Override
    public void onClick(View view) {
        final Intent intent = new Intent();
        intent.putExtra("data", editText.getText().toString());
        setResult(RESULT_OK, intent);
        finish();
    }
}