framework/lib/src/widgets/disposable_build_context.dart

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'framework.dart';

/// Provides non-leaking access to a [BuildContext].
///
/// A [BuildContext] is only valid if it is pointing to an active [Element].
/// Once the [Element] is unmounted, the [BuildContext] should not be accessed
/// further. This class makes it possible for a [StatefulWidget] to share its
/// build context safely with other objects.
///
/// Creators of this object must guarantee the following:
///
///   1. They create this object at or after [State.initState] but before
///      [State.dispose]. In particular, do not attempt to create this from the
///      constructor of a state.
///   2. They call [dispose] from [State.dispose].
///
/// This object will not hold on to the [State] after disposal.
@optionalTypeArgs
class DisposableBuildContext<T extends State> {
  /// Creates an object that provides access to a [BuildContext] without leaking
  /// a [State].
  ///
  /// Creators must call [dispose] when the [State] is disposed.
  ///
  /// The [State] must not be null, and [State.mounted] must be true.
  DisposableBuildContext(T this._state)
      : assert(_state != null),
        assert(_state.mounted, 'A DisposableBuildContext was given a BuildContext for an Element that is not mounted.');

  T? _state;

  /// Provides safe access to the build context.
  ///
  /// If [dispose] has been called, will return null.
  ///
  /// Otherwise, asserts the [_state] is still mounted and returns its context.
  BuildContext? get context {
    assert(_debugValidate());
    if (_state == null) {
      return null;
    }
    return _state!.context;
  }

  /// Called from asserts or tests to determine whether this object is in a
  /// valid state.
  ///
  /// Always returns true, but will assert if [dispose] has not been called
  /// but the state this is tracking is unmounted.
  bool _debugValidate() {
    assert(
      _state == null || _state!.mounted,
      'A DisposableBuildContext tried to access the BuildContext of a disposed '
      'State object. This can happen when the creator of this '
      'DisposableBuildContext fails to call dispose when it is disposed.',
    );
    return true;
  }


  /// Marks the [BuildContext] as disposed.
  ///
  /// Creators of this object must call [dispose] when their [Element] is
  /// unmounted, i.e. when [State.dispose] is called.
  void dispose() {
    _state = null;
  }
}