[web_dashboard] add logout (#447)

* logout wip

* Use AnimatedSwitcher, change lingo from "login" to signIn"

* add automatic sign-in

* fix flashing sign in button

* sign out of FirebaseAuth and GoogleSignIn

* formatting

* change isSignedIn() to getter

* Add error handling for sign in

* improve error handling at login screen

experimental/web_dashboard/lib/src/pages/sign_in.dart

@@ -6,7 +6,7 @@ import 'package:flutter/material.dart';
 
 import '../auth/auth.dart';
 
-class SignInPage extends StatefulWidget {
+class SignInPage extends StatelessWidget {
   final Auth auth;
   final ValueChanged<User> onSuccess;
 
@@ -15,31 +15,89 @@ class SignInPage extends StatefulWidget {
     @required this.onSuccess,
   });
 
-  @override
-  _SignInPageState createState() => _SignInPageState();
-}
-
-class _SignInPageState extends State<SignInPage> {
-  @override
-  void initState() {
-    super.initState();
-  }
-
   @override
   Widget build(BuildContext context) {
     return Scaffold(
       body: Center(
-        child: RaisedButton(
-          child: Text('Sign In'),
-          onPressed: () async {
-            var user = await widget.auth.signIn();
-            if (user != null) {
-              widget.onSuccess(user);
-            } else {
-              throw ('Unable to sign in');
-            }
-          },
-        ),
+        child: SignInButton(auth: auth, onSuccess: onSuccess),
+      ),
+    );
+  }
+}
+
+class SignInButton extends StatefulWidget {
+  final Auth auth;
+  final ValueChanged<User> onSuccess;
+
+  SignInButton({
+    @required this.auth,
+    @required this.onSuccess,
+  });
+
+  @override
+  _SignInButtonState createState() => _SignInButtonState();
+}
+
+class _SignInButtonState extends State<SignInButton> {
+  Future<bool> _checkSignInFuture;
+
+  @override
+  void initState() {
+    super.initState();
+    _checkSignInFuture = _checkIfSignedIn();
+  }
+
+  // Check if the user is signed in. If the user is already signed in (for
+  // example, if they signed in and refreshed the page), invoke the `onSuccess`
+  // callback right away.
+  Future<bool> _checkIfSignedIn() async {
+    var alreadySignedIn = await widget.auth.isSignedIn;
+    if (alreadySignedIn) {
+      var user = await widget.auth.signIn();
+      widget.onSuccess(user);
+    }
+    return alreadySignedIn;
+  }
+
+  Future<void> _signIn() async {
+    try {
+      var user = await widget.auth.signIn();
+      widget.onSuccess(user);
+    } on SignInException {
+      _showError();
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return FutureBuilder<bool>(
+      future: _checkSignInFuture,
+      builder: (context, snapshot) {
+        // If signed in, or the future is incomplete, show a circular
+        // progress indicator.
+        var alreadySignedIn = snapshot.data;
+        if (snapshot.connectionState != ConnectionState.done ||
+            alreadySignedIn == true) {
+          return CircularProgressIndicator();
+        }
+
+        // If sign in failed, show toast and the login button
+        if (snapshot.hasError) {
+          _showError();
+        }
+
+        return RaisedButton(
+          child: Text('Sign In with Google'),
+          onPressed: () => _signIn(),
+        );
+      },
+    );
+  }
+
+  void _showError() {
+    Scaffold.of(context).showSnackBar(
+      SnackBar(
+        content: Text('Unable to sign in.'),
       ),
     );
   }