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// 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.
import 'package:flutter/foundation.dart';
/// An immutable object that can provide functional copies of itself.
/// All implementers are expected to be immutable as defined by the annotation.
mixin Copyable {
/// Instantiates and returns a functionally identical object to oneself.
/// Outside of tests, this method should only ever be called by
/// [InstanceManager].
/// Subclasses should always override their parent's implementation of this
/// method.
Copyable copy();
/// 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) {
_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<Copyable>> _weakInstances =
<int, WeakReference<Copyable>>{};
final Map<int, Copyable> _strongInstances = <int, Copyable>{};
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(Copyable instance) {
assert(getIdentifier(instance) == null);
final int identifier = _nextUniqueIdentifier();
_addInstanceWithIdentifier(instance, identifier);
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(Copyable instance) {
final int? identifier = getIdentifier(instance);
if (identifier == null) {
return null;
_identifiers[instance] = null;
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 Copyable>(int 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 Copyable>(int identifier) {
final Copyable? weakInstance = _weakInstances[identifier]?.target;
if (weakInstance == null) {
final Copyable? strongInstance = _strongInstances[identifier];
if (strongInstance != null) {
final Copyable copy = strongInstance.copy();
_identifiers[copy] = identifier;
_weakInstances[identifier] = WeakReference<Copyable>(copy);
_finalizer.attach(copy, identifier, detach: copy);
return copy as T;
return strongInstance as T?;
return weakInstance as T;
/// Retrieves the identifier associated with instance.
int? getIdentifier(Copyable 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(Copyable instance, int identifier) {
assert(getIdentifier(instance) == null);
assert(identifier >= 0);
_addInstanceWithIdentifier(instance, identifier);
void _addInstanceWithIdentifier(Copyable instance, int identifier) {
_identifiers[instance] = identifier;
_weakInstances[identifier] = WeakReference<Copyable>(instance);
_finalizer.attach(instance, identifier, detach: instance);
final Copyable copy = instance.copy();
_identifiers[copy] = identifier;
_strongInstances[identifier] = copy;
/// Whether this manager contains the given [identifier].
bool containsIdentifier(int identifier) {
return _weakInstances.containsKey(identifier) ||
int _nextUniqueIdentifier() {
late int identifier;
do {
identifier = _nextIdentifier;
_nextIdentifier = (_nextIdentifier + 1) % _maxDartCreatedIdentifier;
} while (containsIdentifier(identifier));
return identifier;