Added validation for the new task screen

lib/src/blocs/new_task_bloc.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 
 import 'package:rxdart/rxdart.dart';
 
+import '../utils.dart' show Validators;
 import '../models/task_model.dart';
 import '../models/user_model.dart';
 import '../resources/authService.dart';
@@ -9,37 +10,38 @@ import '../resources/firestore_provider.dart';
 
 // TODO: Add validation.
 
-class NewTaskBloc {
+class NewTaskBloc extends Object with Validators {
   final AuthService _auth = authService;
   final FirestoreProvider _firestore = firestoreProvider;
   final _user = BehaviorSubject<UserModel>();
   final _eventName = BehaviorSubject<String>();
+  final _taskText = BehaviorSubject<String>();
 
-  String text = '';
   TaskPriority priority = TaskPriority.high;
 
   //Stream getters.
   Observable<UserModel> get userModelStream => _user.stream;
   Observable<String> get eventName => _eventName.stream;
+  Observable<String> get taskText =>
+      _taskText.stream.transform(validateStringNotEmpty);
+  Observable<bool> get submitEnabled =>
+      Observable.combineLatest2(eventName, taskText, (a, b) => true);
 
   //Sinks getters.
   Function(String) get changeEventName => _eventName.sink.add;
+  Function(String) get changeTaskText => _taskText.sink.add;
 
   NewTaskBloc() {
     setCurrentUser();
   }
 
-  void setText(String newText) {
-    text = newText;
-  }
-
   void setPriority(TaskPriority newPriority) {
     priority = newPriority;
   }
 
   Future<void> submit() {
     final newTask = TaskModel(
-      text: text,
+      text: _taskText.value,
       priority: priority,
       event: _eventName.value,
       ownerUsername: _user.value.username,
@@ -55,6 +57,7 @@ class NewTaskBloc {
   }
 
   void dispose() {
+    _taskText.close();
     _user.close();
     _eventName.close();
   }

lib/src/screens/new_task_screen.dart

@@ -41,7 +41,7 @@ class _NewTaskScreenState extends State<NewTaskScreen> {
                   children: <Widget>[
                     BigTextInput(
                       height: 95,
-                      onChanged: bloc.setText,
+                      onChanged: bloc.changeTaskText,
                     ),
                     SizedBox(
                       height: 15,
@@ -54,15 +54,21 @@ class _NewTaskScreenState extends State<NewTaskScreen> {
                     SizedBox(
                       height: 20,
                     ),
-                    GradientTouchableContainer(
-                      height: 40,
-                      radius: 8,
-                      isExpanded: true,
-                      onTap: () => onSubmit(context, bloc),
-                      child: Text(
-                        'Submit',
-                        style: kSmallTextStyle,
-                      ),
+                    StreamBuilder<Object>(
+                      stream: bloc.submitEnabled,
+                      builder: (context, submitSnap) {
+                        return GradientTouchableContainer(
+                          height: 40,
+                          radius: 8,
+                          isExpanded: true,
+                          enabled: submitSnap.hasData,
+                          onTap: () => onSubmit(context),
+                          child: Text(
+                            'Submit',
+                            style: kSmallTextStyle,
+                          ),
+                        );
+                      },
                     ),
                   ],
                 ),
@@ -127,7 +133,7 @@ class _NewTaskScreenState extends State<NewTaskScreen> {
     );
   }
 
-  void onSubmit(BuildContext context, NewTaskBloc bloc) async {
+  void onSubmit(BuildContext context) async {
     await bloc.submit();
     Navigator.of(context).pop();
   }

lib/src/utils.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import './models/task_model.dart';
 
@@ -37,3 +39,15 @@ Color getColorFromPriority(TaskPriority priority) {
       return Colors.white;
   }
 }
+
+class Validators {
+  final validateStringNotEmpty = StreamTransformer<String, String>.fromHandlers(
+    handleData: (String string, EventSink<String> sink) {
+      if (string.isEmpty) {
+        sink.addError('Text cannot be empty');
+      } else {
+        sink.add(string);
+      }
+    },
+  );
+}