restructured the add to app samples (#698)

add_to_app/plugin/ios_using_plugin/IOSUsingPlugin/ViewController.swift

@@ -0,0 +1,51 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import UIKit
+import Flutter
+
+class ViewController: UIViewController {
+
+    @IBOutlet weak var counterLabel: UILabel!
+    
+    var methodChannel : FlutterMethodChannel?
+    var count = 0
+    
+    override func viewDidLoad() {
+        super.viewDidLoad()
+
+        if let flutterEngine = (UIApplication.shared.delegate as? AppDelegate)?.flutterEngine {
+            methodChannel = FlutterMethodChannel(name: "dev.flutter.example/counter",
+                                                 binaryMessenger: flutterEngine.binaryMessenger)
+            methodChannel?.setMethodCallHandler({ [weak self]
+                (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
+                if let strongSelf = self {
+                    switch(call.method) {
+                    case "incrementCounter":
+                        strongSelf.count += 1
+                        strongSelf.counterLabel.text = "Current counter: \(strongSelf.count)"
+                        strongSelf.reportCounter()
+                    case "requestCounter":
+                        strongSelf.reportCounter()
+                    default:
+                        // Unrecognized method name
+                        print("Unrecognized method name: \(call.method)")
+                    }
+                }
+            })
+        }
+    }
+
+    func reportCounter() {
+        methodChannel?.invokeMethod("reportCounter", arguments: count)
+    }
+    
+    @IBAction func buttonWasTapped(_ sender: Any) {
+        if let flutterEngine = (UIApplication.shared.delegate as? AppDelegate)?.flutterEngine {
+            let flutterViewController = FlutterViewController(engine: flutterEngine, nibName: nil, bundle: nil)
+            self.present(flutterViewController, animated: false, completion: nil)
+        }
+    }    
+}
+