Optimistically remove listeners using Finalizer (#5)

Author: lukehutch

CHANGELOG.md

@@ -1,3 +1,7 @@
+### 2.0.1
+
+Optimistically remove listeners when the `Element` they are listening to is garbage collected, reducing memory pressure. (#5)
+
 ### 2.0.0
 
 Fix memory leak (listeners were being added with every build). This required converting `ReactiveValueNotifier` from an extension to a subclass, because it needed an extra private field. (#5)

lib/src/reactive_value_notifier.dart

@@ -13,9 +13,17 @@ class ReactiveValueNotifier<T> extends ValueNotifier<T> {
   ReactiveValueNotifier(super.value);
 
   /// An Expando mapping from `Element` objects to `true`, if the `Element`
-  ///  is subscribed to this `ValueNotifier`.
+  /// is subscribed to this `ValueNotifier`. Used to ensure that an `Element`
+  /// is only subscribed once.
   final _subscribedElements = Expando<bool>();
 
+  /// A [Finalizer] that will be called when an [Element] is garbage collected.
+  /// Used to optimistically remove the listener corresponding to that
+  /// [Element]. (The listener will be removed anyway when the [ValueNotifier]
+  /// is disposed, or when the [ValueNotifier]'s value changes after the
+  /// [Element] has been garbage collected, but this reduces memory pressure.)
+  final Finalizer<void Function()> _finalizer = Finalizer((fn) => fn());
+
   /// Fetch the [value] of this [ValueNotifier], and subscribe the element
   /// that is currently being built (the [context]) to any changes in the
   /// value.
@@ -55,13 +63,27 @@ class ReactiveValueNotifier<T> extends ValueNotifier<T> {
         }
         // Remove the element from the Expando of subscribed elements
         _subscribedElements[elementRefTarget] = null;
+
+        // Tell the finalizer it no longer needs to watch the element, since
+        // we're about to manually remove the listener
+        _finalizer.detach(elementRefTarget);
       }
+
       // Remove the listener -- only listen to one change per build
       // (each subsequent build will resubscribe)
       removeListener(listenerWrapper.listener!);
     };
+
     // Listen to changes to the ReactiveValue
     addListener(listenerWrapper.listener!);
+
+    // Tell the finalizer to remove the listener when the element is garbage
+    // collected (this just reduces memory pressure, since the listener is
+    // no longer needed, but even without this, when the listener is next
+    // called, it would remove itself)
+    _finalizer.attach(element, () => removeListener(listenerWrapper.listener!),
+        detach: element);
+
     // Return the current value
     return value;
   }

pubspec.yaml

@@ -1,6 +1,6 @@
 name: flutter_reactive_value
 description: Simple reactive state management for Flutter.
-version: 2.0.0
+version: 2.0.1
 homepage: https://github.com/lukehutch/flutter_reactive_value
 
 environment: