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 'package:flutter/foundation.dart';
6
7import 'framework.dart';
8
9/// Provides non-leaking access to a [BuildContext].
10///
11/// A [BuildContext] is only valid if it is pointing to an active [Element].
12/// Once the [Element] is unmounted, the [BuildContext] should not be accessed
13/// further. This class makes it possible for a [StatefulWidget] to share its
14/// build context safely with other objects.
15///
16/// Creators of this object must guarantee the following:
17///
18/// 1. They create this object at or after [State.initState] but before
19/// [State.dispose]. In particular, do not attempt to create this from the
20/// constructor of a state.
21/// 2. They call [dispose] from [State.dispose].
22///
23/// This object will not hold on to the [State] after disposal.
24@optionalTypeArgs
25class DisposableBuildContext<T extends State> {
26 /// Creates an object that provides access to a [BuildContext] without leaking
27 /// a [State].
28 ///
29 /// Creators must call [dispose] when the [State] is disposed.
30 ///
31 /// [State.mounted] must be true.
32 DisposableBuildContext(T this._state)
33 : assert(
34 _state.mounted,
35 'A DisposableBuildContext was given a BuildContext for an Element that is not mounted.',
36 ) {
37 assert(debugMaybeDispatchCreated('widgets', 'DisposableBuildContext', this));
38 }
39
40 T? _state;
41
42 /// Provides safe access to the build context.
43 ///
44 /// If [dispose] has been called, will return null.
45 ///
46 /// Otherwise, asserts the [_state] is still mounted and returns its context.
47 BuildContext? get context {
48 assert(_debugValidate());
49 return _state?.context;
50 }
51
52 /// Called from asserts or tests to determine whether this object is in a
53 /// valid state.
54 ///
55 /// Always returns true, but will assert if [dispose] has not been called
56 /// but the state this is tracking is unmounted.
57 bool _debugValidate() {
58 assert(
59 _state == null || _state!.mounted,
60 'A DisposableBuildContext tried to access the BuildContext of a disposed '
61 'State object. This can happen when the creator of this '
62 'DisposableBuildContext fails to call dispose when it is disposed.',
63 );
64 return true;
65 }
66
67 /// Marks the [BuildContext] as disposed.
68 ///
69 /// Creators of this object must call [dispose] when their [Element] is
70 /// unmounted, i.e. when [State.dispose] is called.
71 void dispose() {
72 assert(debugMaybeDispatchDisposed(this));
73 _state = null;
74 }
75}
76