Fix #15 always terminate 'authenticate' callbacks on Android

android/src/main/kotlin/com/linusu/flutter_web_auth/CallbackActivity.kt

@@ -4,17 +4,17 @@ import android.app.Activity
 import android.net.Uri
 import android.os.Bundle
 
-public class CallbackActivity: Activity() {
+class CallbackActivity: Activity() {
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
 
-    val url = getIntent()?.getData() as? Uri
-    val scheme = url?.getScheme()
+    val url = intent?.data
+    val scheme = url?.scheme
 
     if (scheme != null) {
       FlutterWebAuthPlugin.callbacks.remove(scheme)?.success(url.toString())
     }
 
-    this.finish()
+    finish()
   }
 }

android/src/main/kotlin/com/linusu/flutter_web_auth/FlutterWebAuthPlugin.kt

@@ -1,7 +1,5 @@
 package com.linusu.flutter_web_auth
 
-import java.util.HashMap
-
 import android.content.Context
 import android.content.Intent
 import android.net.Uri
@@ -16,7 +14,7 @@ import io.flutter.plugin.common.PluginRegistry.Registrar
 
 class FlutterWebAuthPlugin(private val context: Context): MethodCallHandler {
   companion object {
-    public val callbacks = HashMap<String, Result>()
+    val callbacks = mutableMapOf<String, Result>()
 
     @JvmStatic
     fun registerWith(registrar: Registrar) {
@@ -25,22 +23,30 @@ class FlutterWebAuthPlugin(private val context: Context): MethodCallHandler {
     }
   }
 
-  override fun onMethodCall(call: MethodCall, result: Result) {
-    if (call.method == "authenticate") {
-      val url = Uri.parse(call.argument<String>("url"))
-      val callbackUrlScheme = call.argument<String>("callbackUrlScheme")!!
-
-      callbacks.put(callbackUrlScheme, result)
-
-      val intent = CustomTabsIntent.Builder().build()
-      val keepAliveIntent = Intent().setClassName(context.getPackageName(), KeepAliveService::class.java.canonicalName)
-
-      intent.intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP or Intent.FLAG_ACTIVITY_NO_HISTORY or Intent.FLAG_ACTIVITY_NEW_TASK)
-      intent.intent.putExtra("android.support.customtabs.extra.KEEP_ALIVE", keepAliveIntent)
-
-      intent.launchUrl(context, url)
-    } else {
-      result.notImplemented()
+  override fun onMethodCall(call: MethodCall, resultCallback: Result) {
+    when (call.method) {
+        "authenticate" -> {
+          val url = Uri.parse(call.argument("url"))
+          val callbackUrlScheme = call.argument<String>("callbackUrlScheme")!!
+
+          callbacks[callbackUrlScheme] = resultCallback
+
+          val intent = CustomTabsIntent.Builder().build()
+          val keepAliveIntent = Intent(context, KeepAliveService::class.java)
+
+          intent.intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP or Intent.FLAG_ACTIVITY_NO_HISTORY or Intent.FLAG_ACTIVITY_NEW_TASK)
+          intent.intent.putExtra("android.support.customtabs.extra.KEEP_ALIVE", keepAliveIntent)
+
+          intent.launchUrl(context, url)
+        }
+        "cleanUpDanglingCalls" -> {
+          callbacks.forEach{ (_, danglingResultCallback) ->
+              danglingResultCallback.error("CANCELED", "User canceled login", null)
+          }
+          callbacks.clear()
+          resultCallback.success(null)
+        }
+        else -> resultCallback.notImplemented()
     }
   }
 }

android/src/main/kotlin/com/linusu/flutter_web_auth/KeepAliveService.kt

@@ -5,12 +5,12 @@ import android.content.Intent
 import android.os.Binder
 import android.os.IBinder
 
-public class KeepAliveService: Service() {
+class KeepAliveService: Service() {
   companion object {
-    val sBinder = Binder()
+    val binder = Binder()
   }
 
   override fun onBind(intent: Intent): IBinder {
-    return sBinder
+    return binder
   }
 }

example/lib/main.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'dart:async';
 import 'dart:io' show HttpServer;
 
@@ -93,9 +94,12 @@ class _MyAppState extends State<MyApp> {
     final url = 'http://localtest.me:43823/';
     final callbackUrlScheme = 'foobar';
 
-    final result = await FlutterWebAuth.authenticate(url: url, callbackUrlScheme: callbackUrlScheme);
-
-    setState(() { _status = 'Got result: $result'; });
+    try {
+      final result = await FlutterWebAuth.authenticate(url: url, callbackUrlScheme: callbackUrlScheme);
+      setState(() { _status = 'Got result: $result'; });
+    } on PlatformException catch (e) {
+      setState(() { _status = 'Got error: $e'; });
+    }
   }
 
   @override

ios/Classes/SwiftFlutterWebAuthPlugin.swift

@@ -15,9 +15,9 @@ public class SwiftFlutterWebAuthPlugin: NSObject, FlutterPlugin {
             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 keepMe: Any? = nil
+            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
-                keepMe = nil
+                sessionToKeepAlive = nil
 
                 if let err = err {
                     if #available(iOS 12, *) {
@@ -52,12 +52,15 @@ public class SwiftFlutterWebAuthPlugin: NSObject, FlutterPlugin {
                 }
 
                 session.start()
-                keepMe = session
+                sessionToKeepAlive = session
             } else {
                 let session = SFAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme, completionHandler: completionHandler)
                 session.start()
-                keepMe = session
+                sessionToKeepAlive = session
             }
+        } 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)
         }

lib/flutter_web_auth.dart

@@ -1,17 +1,48 @@
 import 'dart:async';
 
+import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart' show required;
 import 'package:flutter/services.dart' show MethodChannel;
 
+class _OnAppLifecycleResumeObserver extends WidgetsBindingObserver {
+  final Function onResumed;
+
+  _OnAppLifecycleResumeObserver(this.onResumed);
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.resumed) {
+      onResumed();
+    }
+  }
+}
+
 class FlutterWebAuth {
   static const MethodChannel _channel = const MethodChannel('flutter_web_auth');
 
+  static final _OnAppLifecycleResumeObserver _resumedObserver = _OnAppLifecycleResumeObserver(() {
+    _cleanUpDanglingCalls(); // unawaited
+  });
+
   /// Ask the user to authenticate to the specified web service.
   ///
   /// The page pointed to by [url] will be loaded and displayed to the user. From the page, the user can authenticate herself and grant access to the app. On completion, the service will send a callback URL with an authentication token, and this URL will be result of the returned [Future].
   ///
   /// [callbackUrlScheme] should be a string specifying the scheme of the url that the page will redirect to upon successful authentication.
   static Future<String> authenticate({@required String url, @required String callbackUrlScheme}) async {
-    return await _channel.invokeMethod('authenticate', <String, dynamic>{'url': url, 'callbackUrlScheme': callbackUrlScheme}) as String;
+    WidgetsBinding.instance.removeObserver(_resumedObserver); // safety measure so we never add this observer twice
+    WidgetsBinding.instance.addObserver(_resumedObserver);
+    return await _channel.invokeMethod('authenticate', <String, dynamic>{
+      'url': url,
+      'callbackUrlScheme': callbackUrlScheme,
+    }) as String;
+  }
+
+  /// On Android, the plugin has to store the Result callbacks in order to pass the result back to the caller of
+  /// `authenticate`. But if that result never comes the callback will dangle around forever. This can be called to
+  /// terminate all `authenticate` calls with an error.
+  static Future<void> _cleanUpDanglingCalls() async {
+    await _channel.invokeMethod('cleanUpDanglingCalls');
+    WidgetsBinding.instance.removeObserver(_resumedObserver);
   }
 }