| // Copyright 2013 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. |
| |
| /// Maintains instances used to communicate with the native objects they |
| /// represent. |
| /// |
| /// Added instances are stored as weak references and their copies are stored |
| /// as strong references to maintain access to their variables and callback |
| /// methods. Both are stored with the same identifier. |
| /// |
| /// When a weak referenced instance becomes inaccessible, |
| /// [onWeakReferenceRemoved] is called with its associated identifier. |
| /// |
| /// If an instance is retrieved and has the possibility to be used, |
| /// (e.g. calling [getInstanceWithWeakReference]) a copy of the strong reference |
| /// is added as a weak reference with the same identifier. This prevents a |
| /// scenario where the weak referenced instance was released and then later |
| /// returned by the host platform. |
| class InstanceManager { |
| /// Constructs an [InstanceManager]. |
| InstanceManager({required void Function(int) onWeakReferenceRemoved}) { |
| this.onWeakReferenceRemoved = (int identifier) { |
| _weakInstances.remove(identifier); |
| onWeakReferenceRemoved(identifier); |
| }; |
| _finalizer = Finalizer<int>(this.onWeakReferenceRemoved); |
| } |
| |
| // Identifiers are locked to a specific range to avoid collisions with objects |
| // created simultaneously by the host platform. |
| // Host uses identifiers >= 2^16 and Dart is expected to use values n where, |
| // 0 <= n < 2^16. |
| static const int _maxDartCreatedIdentifier = 65536; |
| |
| // Expando is used because it doesn't prevent its keys from becoming |
| // inaccessible. This allows the manager to efficiently retrieve an identifier |
| // of an instance without holding a strong reference to that instance. |
| // |
| // It also doesn't use `==` to search for identifiers, which would lead to an |
| // infinite loop when comparing an object to its copy. (i.e. which was caused |
| // by calling instanceManager.getIdentifier() inside of `==` while this was a |
| // HashMap). |
| final Expando<int> _identifiers = Expando<int>(); |
| final Map<int, WeakReference<Object>> _weakInstances = |
| <int, WeakReference<Object>>{}; |
| final Map<int, Object> _strongInstances = <int, Object>{}; |
| final Map<int, Function> _copyCallbacks = <int, Function>{}; |
| late final Finalizer<int> _finalizer; |
| int _nextIdentifier = 0; |
| |
| /// Called when a weak referenced instance is removed by [removeWeakReference] |
| /// or becomes inaccessible. |
| late final void Function(int) onWeakReferenceRemoved; |
| |
| /// Adds a new instance that was instantiated by Dart. |
| /// |
| /// In other words, Dart wants to add a new instance that will represent |
| /// an object that will be instantiated on the host platform. |
| /// |
| /// Throws assertion error if the instance has already been added. |
| /// |
| /// Returns the randomly generated id of the [instance] added. |
| int addDartCreatedInstance<T extends Object>( |
| T instance, { |
| required T Function(T original) onCopy, |
| }) { |
| final int identifier = _nextUniqueIdentifier(); |
| _addInstanceWithIdentifier(instance, identifier, onCopy: onCopy); |
| return identifier; |
| } |
| |
| /// Removes the instance, if present, and call [onWeakReferenceRemoved] with |
| /// its identifier. |
| /// |
| /// Returns the identifier associated with the removed instance. Otherwise, |
| /// `null` if the instance was not found in this manager. |
| /// |
| /// This does not remove the the strong referenced instance associated with |
| /// [instance]. This can be done with [remove]. |
| int? removeWeakReference(Object instance) { |
| final int? identifier = getIdentifier(instance); |
| if (identifier == null) { |
| return null; |
| } |
| |
| _identifiers[instance] = null; |
| _finalizer.detach(instance); |
| onWeakReferenceRemoved(identifier); |
| |
| return identifier; |
| } |
| |
| /// Removes [identifier] and its associated strongly referenced instance, if |
| /// present, from the manager. |
| /// |
| /// Returns the strong referenced instance associated with [identifier] before |
| /// it was removed. Returns `null` if [identifier] was not associated with |
| /// any strong reference. |
| /// |
| /// This does not remove the the weak referenced instance associtated with |
| /// [identifier]. This can be done with [removeWeakReference]. |
| T? remove<T extends Object>(int identifier) { |
| _copyCallbacks.remove(identifier); |
| return _strongInstances.remove(identifier) as T?; |
| } |
| |
| /// Retrieves the instance associated with identifier. |
| /// |
| /// The value returned is chosen from the following order: |
| /// |
| /// 1. A weakly referenced instance associated with identifier. |
| /// 2. If the only instance associated with identifier is a strongly |
| /// referenced instance, a copy of the instance is added as a weak reference |
| /// with the same identifier. Returning the newly created copy. |
| /// 3. If no instance is associated with identifier, returns null. |
| /// |
| /// This method also expects the host `InstanceManager` to have a strong |
| /// reference to the instance the identifier is associated with. |
| T? getInstanceWithWeakReference<T extends Object>(int identifier) { |
| final T? weakInstance = _weakInstances[identifier]?.target as T?; |
| |
| if (weakInstance == null) { |
| final T? strongInstance = _strongInstances[identifier] as T?; |
| if (strongInstance != null) { |
| // This cast is safe since it matches the argument type for |
| // _addInstanceWithIdentifier, which is the only place _copyCallbacks |
| // is populated. |
| final T Function(T) copyCallback = |
| _copyCallbacks[identifier]! as T Function(T); |
| final T copy = copyCallback(strongInstance); |
| _identifiers[copy] = identifier; |
| _weakInstances[identifier] = WeakReference<Object>(copy); |
| _finalizer.attach(copy, identifier, detach: copy); |
| return copy; |
| } |
| return strongInstance; |
| } |
| |
| return weakInstance; |
| } |
| |
| /// Retrieves the identifier associated with instance. |
| int? getIdentifier(Object instance) { |
| return _identifiers[instance]; |
| } |
| |
| /// Adds a new instance that was instantiated by the host platform. |
| /// |
| /// In other words, the host platform wants to add a new instance that |
| /// represents an object on the host platform. Stored with [identifier]. |
| /// |
| /// Throws assertion error if the instance or its identifier has already been |
| /// added. |
| /// |
| /// Returns unique identifier of the [instance] added. |
| void addHostCreatedInstance<T extends Object>( |
| T instance, |
| int identifier, { |
| required T Function(T original) onCopy, |
| }) { |
| _addInstanceWithIdentifier(instance, identifier, onCopy: onCopy); |
| } |
| |
| void _addInstanceWithIdentifier<T extends Object>( |
| T instance, |
| int identifier, { |
| required T Function(T original) onCopy, |
| }) { |
| assert(!containsIdentifier(identifier)); |
| assert(getIdentifier(instance) == null); |
| assert(identifier >= 0); |
| _identifiers[instance] = identifier; |
| _weakInstances[identifier] = WeakReference<Object>(instance); |
| _finalizer.attach(instance, identifier, detach: instance); |
| |
| final Object copy = onCopy(instance); |
| _identifiers[copy] = identifier; |
| _strongInstances[identifier] = copy; |
| _copyCallbacks[identifier] = onCopy; |
| } |
| |
| /// Whether this manager contains the given [identifier]. |
| bool containsIdentifier(int identifier) { |
| return _weakInstances.containsKey(identifier) || |
| _strongInstances.containsKey(identifier); |
| } |
| |
| int _nextUniqueIdentifier() { |
| late int identifier; |
| do { |
| identifier = _nextIdentifier; |
| _nextIdentifier = (_nextIdentifier + 1) % _maxDartCreatedIdentifier; |
| } while (containsIdentifier(identifier)); |
| return identifier; |
| } |
| } |