🎉 Add support for Flutter "add to app" (LinusU#106)

* The current implementation assumes the plugin is used in a pure Flutter app. It casts the app's rootViewController to FlutterViewController.

This supports using this plugin in an "add to app" use case. The implementation does not assume the rootViewController is an instance of FlutterViewController, but rather starts with the presented view controller, and navigates up the stack to the root controller. It also handles in the case of a non-modal screen, in which case it would be a UINavigationController. We could probably just use the presented view controller, rather than navigating up the stack to the root to use as the session context provider.

* Use guarded cast instead of forced unwrap.

* fix(swift): make changes from code review

* fix(swift): eliminate !s and unused code

Co-authored-by: junying1 <jying@simplec.com>

Author: timshadel

ios/Classes/SwiftFlutterWebAuthPlugin.swift

@@ -11,10 +11,13 @@ public class SwiftFlutterWebAuthPlugin: NSObject, FlutterPlugin {
     }
 
     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
-            let preferEphemeral = (call.arguments as! Dictionary<String, AnyObject>)["preferEphemeral"] as! Bool
+        if call.method == "authenticate",
+           let arguments = call.arguments as? Dictionary<String, AnyObject>,
+           let urlString = arguments["url"] as? String,
+           let url = URL(string: urlString),
+           let callbackURLScheme = arguments["callbackUrlScheme"] as? String,
+           let preferEphemeral = arguments["preferEphemeral"] as? Bool
+        {
 
             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
@@ -39,20 +42,36 @@ public class SwiftFlutterWebAuthPlugin: NSObject, FlutterPlugin {
                     return
                 }
 
-                result(url!.absoluteString)
+                guard let url = url else {
+                    result(FlutterError(code: "EUNKNOWN", message: "URL was null, but no error provided.", 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))
+                    guard var topController = UIApplication.shared.keyWindow?.rootViewController else {
+                        result(FlutterError.aquireRootViewControllerFailed)
                         return
                     }
 
+                    while let presentedViewController = topController.presentedViewController {
+                        topController = presentedViewController
+                    }
+                    if let nav = topController as? UINavigationController {
+                        topController = nav.visibleViewController ?? topController
+                    }
+
+                    guard let contextProvider = topController as? ASWebAuthenticationPresentationContextProviding else {
+                        result(FlutterError.aquireRootViewControllerFailed)
+                        return
+                    }
+                    session.presentationContextProvider =  contextProvider
                     session.prefersEphemeralWebBrowserSession = preferEphemeral
-                    session.presentationContextProvider = provider
                 }
 
                 session.start()
@@ -79,3 +98,9 @@ extension FlutterViewController: ASWebAuthenticationPresentationContextProviding
         return self.view.window!
     }
 }
+
+fileprivate extension FlutterError {
+    static var aquireRootViewControllerFailed: FlutterError {
+        return FlutterError(code: "AQUIRE_ROOT_VIEW_CONTROLLER_FAILED", message: "Failed to aquire root view controller" , details: nil)
+    }
+}