Implement native web auth

ios/Runner/AppDelegate.swift

@@ -7,7 +7,20 @@ import Flutter
     _ application: UIApplication,
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
+
+    self?.registerTbWebAuth()
+
     GeneratedPluginRegistrant.register(with: self)
     return super.application(application, didFinishLaunchingWithOptions: launchOptions)
   }
+
+  private func registerTbWebAuth() {
+    let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
+    let channel = FlutterMethodChannel(name: "tb_web_auth", binaryMessenger: controller.binaryMessenger)
+    let instance = TbWebAuthHandler()
+    channel.setMethodCallHandler({
+          (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
+          instance.handle(call: call, result: result)
+    })
+  }
 }

ios/Runner/TbWebAuthHandler.swift

@@ -0,0 +1,74 @@
+import AuthenticationServices
+import SafariServices
+import Flutter
+import UIKit
+
+public class TbWebAuthHandler: NSObject {
+
+    public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
+        if call.method == "authenticate" {
+            let url = URL(string: (call.arguments as! Dictionary<String, AnyObject>)["url"] as! String)!
+            let callbackURLScheme = (call.arguments as! Dictionary<String, AnyObject>)["callbackUrlScheme"] as! String
+
+            var sessionToKeepAlive: Any? = nil // if we do not keep the session alive, it will get closed immediately while showing the dialog
+            let completionHandler = { (url: URL?, err: Error?) in
+                sessionToKeepAlive = nil
+
+                if let err = err {
+                    if #available(iOS 12, *) {
+                        if case ASWebAuthenticationSessionError.canceledLogin = err {
+                            result(FlutterError(code: "CANCELED", message: "User canceled login", details: nil))
+                            return
+                        }
+                    }
+
+                    if #available(iOS 11, *) {
+                        if case SFAuthenticationError.canceledLogin = err {
+                            result(FlutterError(code: "CANCELED", message: "User canceled login", details: nil))
+                            return
+                        }
+                    }
+
+                    result(FlutterError(code: "EUNKNOWN", message: err.localizedDescription, details: nil))
+                    return
+                }
+
+                result(url!.absoluteString)
+            }
+
+            if #available(iOS 12, *) {
+                let session = ASWebAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme, completionHandler: completionHandler)
+
+                if #available(iOS 13, *) {
+                    guard let provider = UIApplication.shared.delegate?.window??.rootViewController as? FlutterViewController else {
+                        result(FlutterError(code: "FAILED", message: "Failed to aquire root FlutterViewController" , details: nil))
+                        return
+                    }
+
+                    session.presentationContextProvider = provider
+                }
+
+                session.start()
+                sessionToKeepAlive = session
+            } else if #available(iOS 11, *) {
+                let session = SFAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme, completionHandler: completionHandler)
+                session.start()
+                sessionToKeepAlive = session
+            } else {
+                result(FlutterError(code: "FAILED", message: "This plugin does currently not support iOS lower than iOS 11" , details: nil))
+            }
+        } else if (call.method == "cleanUpDanglingCalls") {
+            // we do not keep track of old callbacks on iOS, so nothing to do here
+            result(nil)
+        } else {
+            result(FlutterMethodNotImplemented)
+        }
+    }
+}
+
+@available(iOS 13, *)
+extension FlutterViewController: ASWebAuthenticationPresentationContextProviding {
+    public func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
+        return self.view.window!
+    }
+}