NoteEditor: Show a confirmation dialog if discarding a note

lib/note_editor.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 import 'package:journal/note.dart';
@@ -15,7 +17,36 @@ class NoteEditor extends StatelessWidget {
     final container = StateContainer.of(context);
 
     var bodyWidget = new Container(
-      child: new TextFormField(
+      child: new Form(
+        // Show a dialog if discarding non-empty notes
+        onWillPop: () {
+          return Future(() {
+            var noteContent = noteTextKey.currentState.value.trim();
+            if (noteContent.isEmpty) {
+              return true;
+            }
+            return showDialog(
+              context: context,
+              builder: (BuildContext context) {
+                return new AlertDialog(
+                  title: new Text('Are you sure?'),
+                  content: new Text('Do you want to discard the entry'),
+                  actions: <Widget>[
+                    new FlatButton(
+                      onPressed: () => Navigator.of(context).pop(false),
+                      child: new Text('No'),
+                    ),
+                    new FlatButton(
+                      onPressed: () => Navigator.of(context).pop(true),
+                      child: new Text('Yes'),
+                    ),
+                  ],
+                );
+              },
+            );
+          });
+        },
+        child: TextFormField(
           key: noteTextKey,
           autofocus: true,
           keyboardType: TextInputType.multiline,
@@ -24,6 +55,7 @@ class NoteEditor extends StatelessWidget {
             hintText: 'Write here',
           ),
         ),
+      ),
       padding: const EdgeInsets.all(8.0),
     );
 
@@ -38,10 +70,10 @@ class NoteEditor extends StatelessWidget {
       floatingActionButton: FloatingActionButton(
           child: Icon(Icons.add),
           onPressed: () {
-            var body = noteTextKey.currentState.value;
+            var noteContent = noteTextKey.currentState.value;
             var note = new Note(
               createdAt: _createdAt,
-              body: body,
+              body: noteContent,
             );
             container.addNote(note);