1// Copyright 2014 The Flutter Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5import 'framework.dart';
6
7/// Base class for stateful widgets that have exactly one inflated instance in
8/// the tree.
9///
10/// Such widgets must be given a [GlobalKey]. This key can be generated by the
11/// subclass from its [Type] object, e.g. by calling `super(key: new
12/// GlobalObjectKey(MyWidget))` where `MyWidget` is the name of the subclass.
13///
14/// Since only one instance can be inflated at a time, there is only ever one
15/// corresponding [State] object. That object is exposed, for convenience, via
16/// the [currentState] property.
17///
18/// When subclassing [UniqueWidget], provide the corresponding [State] subclass
19/// as the type argument.
20abstract class UniqueWidget<T extends State<StatefulWidget>> extends StatefulWidget {
21 /// Creates a widget that has exactly one inflated instance in the tree.
22 ///
23 /// The [key] argument is required because it identifies the unique inflated
24 /// instance of this widget.
25 const UniqueWidget({required GlobalKey<T> super.key});
26
27 @override
28 T createState();
29
30 /// The state for the unique inflated instance of this widget.
31 ///
32 /// Might be null if the widget is not currently in the tree.
33 T? get currentState {
34 final GlobalKey<T> globalKey = key! as GlobalKey<T>;
35 return globalKey.currentState;
36 }
37}
38