add disposable_build_context.cs

Samples/UIWidgetsSamples_2019_4/Packages/manifest.json

     "com.unity.collab-proxy": "1.2.16",
     "com.unity.ide.rider": "1.1.4",
     "com.unity.ide.vscode": "1.2.1",
-    "com.unity.uiwidgets": "file:../../../com.unity.uiwidgets",
-    "com.unity.test-framework": "1.1.14",
+    "com.unity.test-framework": "1.1.16",
+    "com.unity.uiwidgets": "file:../../../com.unity.uiwidgets",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.androidjni": "1.0.0",
     "com.unity.modules.animation": "1.0.0",

Samples/UIWidgetsSamples_2019_4/ProjectSettings/ProjectVersion.txt

-m_EditorVersion: 2019.4.2f1
-m_EditorVersionWithRevision: 2019.4.2f1 (f997fc5c673b)
+m_EditorVersion: 2019.4.10f1
+m_EditorVersionWithRevision: 2019.4.10f1 (a819905b7a01)

com.unity.uiwidgets/Runtime/widgets/disposable_build_context.cs

+using Unity.UIWidgets.foundation;
+
+namespace Unity.UIWidgets.widgets {
+    
+    public class DisposableBuildContext<T> where T : 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.
+        public DisposableBuildContext(T state) {
+            D.assert(_state != null);
+            D.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.
+        public BuildContext context {
+            get {
+                D.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.
+        public bool _debugValidate() {
+            D.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;
+        }
+    }
+}

Samples/UIWidgetsSamples_2019_4/Logs/Packages-Update.log

+
+=== Mon Jan 11 10:42:38 2021
+
+Packages were changed.
+Update Mode: updateDependencies
+
+The following packages were updated:
+  com.unity.test-framework from version 1.1.14 to 1.1.16