Implement some of the methods of the plugin.

packages/google_sign_in/google_sign_in_web/lib/google_sign_in_web.dart

@@ -3,19 +3,29 @@ import 'dart:html' as html;
 
 import 'package:flutter/services.dart';
 import 'package:flutter_web_plugins/flutter_web_plugins.dart';
+import 'package:google_sign_in_web/src/gapi.dart';
+import 'package:js/js.dart';
+
+const String _kClientIdMetaSelector = 'meta[name=google-signin-client_id]';
+const String _kClientIdAttributeName = 'content';
+const List<String> _kJsLibraries = <String>[
+  'https://apis.google.com/js/platform.js'
+];
 
 /// Implementation of the google_sign_in plugin for Web
 class GoogleSignInPlugin {
   GoogleSignInPlugin() {
-    _jsLibrariesLoading = _injectJSLibraries(<String>[
-      'https://apis.google.com/js/platform.js',
-    ])..then(_initGapi);    
+    _autoDetectedClientId = html
+        .querySelector(_kClientIdMetaSelector)
+        ?.getAttribute(_kClientIdAttributeName);
+
+    _isGapiInitialized =
+        Future.wait(_injectJSLibraries(_kJsLibraries)).then(_initGapi);
   }
 
-  Future<List<void>> _jsLibrariesLoading;
+  Future<void> _isGapiInitialized;
+  String _autoDetectedClientId;
 
-  Future<void> get isJsLoaded => _jsLibrariesLoading;
-
   static void registerWith(Registrar registrar) {
     final MethodChannel channel = MethodChannel(
         'plugins.flutter.io/google_sign_in',
@@ -28,14 +38,35 @@ class GoogleSignInPlugin {
 
   Future<dynamic> handleMethodCall(MethodCall call) async {
     // Await for initialization promises to complete, then do your thing...
-    await isJsLoaded;
+    await _isGapiInitialized;
+
+    html.window.console.debug(call);
 
-    html.window.console.log('Doing things! $call');
+    final GoogleUser currentUser =
+        gapi.auth2.getAuthInstance()?.currentUser?.get();
 
     switch (call.method) {
-      case 'init':
-        // Initialize the gapi
-        return _init(call.arguments);
+      case 'init': // void
+        return _init(call.arguments)
+            .toString(); // Anything serializable, really!
+        break;
+      case 'signInSilently':
+        await _silentSignIn(currentUser, call.arguments);
+        return _currentUserToPluginMap(currentUser);
+        break;
+      case 'signIn':
+        await _signIn(call.arguments);
+        return _currentUserToPluginMap(currentUser);
+        break;
+      case 'disconnect':
+        currentUser.disconnect();
+        return null;
+      case 'getTokens':
+        final Auth2AuthResponse response = currentUser.getAuthResponse();
+        return <String, String>{
+          'idToken': response.id_token,
+          'accessToken': response.access_token,
+        };
         break;
       default:
         throw PlatformException(
@@ -45,34 +76,67 @@ class GoogleSignInPlugin {
     }
   }
 
+  Map<String, dynamic> _currentUserToPluginMap(GoogleUser currentUser) {
+    assert(currentUser != null);
+    final Auth2BasicProfile profile = currentUser.getBasicProfile();
+    return <String, dynamic>{
+      'displayName': profile?.getName(),
+      'email': profile?.getEmail(),
+      'id': profile?.getId(),
+      'photoUrl': profile?.getImageUrl(),
+      'idToken': currentUser?.getAuthResponse()?.id_token,
+    };
+  }
+
   // Load the auth2 library
-  Future<void> _init(Map<String, dynamic> arguments) {
-    // 
+  GoogleAuth _init(dynamic arguments) => gapi.auth2.init(Auth2ClientConfig(
+        hostedDomain: arguments['hostedDomain'],
+        scope: arguments['scopes'].join(
+            ' '), // The backend wants a space-separated list of values, not an array
+        clientId: arguments['clientId'] ?? _autoDetectedClientId,
+      ));
+
+  Future<dynamic> _signIn(dynamic arguments) async {
+    return html.promiseToFuture<dynamic>(gapi.auth2.getAuthInstance().signIn());
   }
 
-  Future<void> _initGapi(List<void> _) {
-    // JS-interop with the global gapi method and call gapi.load('auth2'), and wait for the 
+  Future<dynamic> _silentSignIn(
+      GoogleUser currentUser, dynamic arguments) async {
+    return html.promiseToFuture<dynamic>(
+        gapi.auth2.getAuthInstance().signIn(Auth2SignInOptions(
+              prompt: 'none',
+            )));
+  }
+
+  Future<void> _initGapi(dynamic _) {
+    // JS-interop with the global gapi method and call gapi.load('auth2'), and wait for the
     // promise to resolve...
-
+    final Completer<void> gapiLoadCompleter = Completer<void>();
+    gapi.load('auth2', allowInterop(() {
+      gapiLoadCompleter.complete();
+    }));
+
+    return gapiLoadCompleter
+        .future; // After this is resolved, we can use gapi.auth2!
   }
 
-  /// Injects a bunch of libraries in the <head> and returns a 
+  /// Injects a bunch of libraries in the <head> and returns a
   /// Future that resolves when all load.
-  Future<List<void>> _injectJSLibraries(List<String> libraries, { Duration timeout }) {
-    final List<Future<void>> loading = <Future<void>>[
-      Future<bool>.delayed(const Duration(seconds: 10), () => true) // TODO: Remove this delay before submitting :)
-    ];
+  List<Future<void>> _injectJSLibraries(List<String> libraries,
+      {Duration timeout}) {
+    final List<Future<void>> loading = <Future<void>>[];
     final List<html.HtmlElement> tags = <html.HtmlElement>[];
 
     libraries.forEach((String library) {
       final html.ScriptElement script = html.ScriptElement()
         ..async = true
         ..defer = true
         ..src = library;
-      loading.add(script.onLoad.first); // TODO add a timeout race to fail this future
+      loading.add(
+          script.onLoad.first); // TODO add a timeout race to fail this future
       tags.add(script);
     });
     html.querySelector('head').children.addAll(tags);
-    return Future.wait(loading);
+    return loading;
   }
 }

packages/google_sign_in/google_sign_in_web/pubspec.yaml

@@ -18,6 +18,7 @@ dependencies:
   flutter_web_plugins:
     sdk: flutter
   meta: ^1.1.7
+  js: any
 
 dev_dependencies:
   flutter_test: