如何在Swift中声明弱引用数组？

我想在Swift中存储一系列弱引用。数组本身不应该是一个弱引用-它的元素应该是。我认为可可NSPointerArray对此提供了非类型安全的版本。

创建通用包装，如下所示：

class Weak<T: AnyObject> {
  weak var value : T?
  init (value: T) {
    self.value = value
  }
}

将此类的实例添加到您的数组。

class Stuff {}
var weakly : [Weak<Stuff>] = [Weak(value: Stuff()), Weak(value: Stuff())]

定义时Weak，可以使用struct或class。

另外，为了帮助获取数组内容，您可以按照以下方式进行操作：

extension Array where Element:Weak<AnyObject> {
  mutating func reap () {
    self = self.filter { nil != $0.value }
  }
}

AnyObject上面的用法应替换为T-但我不认为当前的Swift语言允许这样定义扩展。

您可以将NSHashTable与weakObjectsHashTable一起使用。 NSHashTable<ObjectType>.weakObjectsHashTable()

对于Swift 3： NSHashTable<ObjectType>.weakObjects()

NSHashTable类参考

在OS X v10.5和更高版本中可用。

在iOS 6.0及更高版本中可用。

参加聚会有点晚，但是请尝试我的聚会。我实现为集合而不是数组。

弱对象集

class WeakObject<T: AnyObject>: Equatable, Hashable {
    weak var object: T?
    init(object: T) {
        self.object = object
    }

    var hashValue: Int {
        if let object = self.object { return unsafeAddressOf(object).hashValue }
        else { return 0 }
    }
}

func == <T> (lhs: WeakObject<T>, rhs: WeakObject<T>) -> Bool {
    return lhs.object === rhs.object
}


class WeakObjectSet<T: AnyObject> {
    var objects: Set<WeakObject<T>>

    init() {
        self.objects = Set<WeakObject<T>>([])
    }

    init(objects: [T]) {
        self.objects = Set<WeakObject<T>>(objects.map { WeakObject(object: $0) })
    }

    var allObjects: [T] {
        return objects.flatMap { $0.object }
    }

    func contains(object: T) -> Bool {
        return self.objects.contains(WeakObject(object: object))
    }

    func addObject(object: T) {
        self.objects.unionInPlace([WeakObject(object: object)])
    }

    func addObjects(objects: [T]) {
        self.objects.unionInPlace(objects.map { WeakObject(object: $0) })
    }
}

用法

var alice: NSString? = "Alice"
var bob: NSString? = "Bob"
var cathline: NSString? = "Cathline"

var persons = WeakObjectSet<NSString>()
persons.addObject(bob!)
print(persons.allObjects) // [Bob]

persons.addObject(bob!)
print(persons.allObjects) // [Bob]

persons.addObjects([alice!, cathline!])
print(persons.allObjects) // [Alice, Cathline, Bob]

alice = nil
print(persons.allObjects) // [Cathline, Bob]

bob = nil
print(persons.allObjects) // [Cathline]

注意，WeakObjectSet不会采用String类型，而是NSString。因为，String类型不是AnyType。我的快速版本是Apple Swift version 2.2 (swiftlang-703.0.18.1 clang-703.0.29)。

可以从Gist获取代码。 https://gist.github.com/codelynx/30d3c42a833321f17d39

**新增于2017年11月

我将代码更新为Swift 4

// Swift 4, Xcode Version 9.1 (9B55)

class WeakObject<T: AnyObject>: Equatable, Hashable {
    weak var object: T?
    init(object: T) {
        self.object = object
    }

    var hashValue: Int {
        if var object = object { return UnsafeMutablePointer<T>(&object).hashValue }
        return 0
    }

    static func == (lhs: WeakObject<T>, rhs: WeakObject<T>) -> Bool {
        return lhs.object === rhs.object
    }
}

class WeakObjectSet<T: AnyObject> {
    var objects: Set<WeakObject<T>>

    init() {
        self.objects = Set<WeakObject<T>>([])
    }

    init(objects: [T]) {
        self.objects = Set<WeakObject<T>>(objects.map { WeakObject(object: $0) })
    }

    var allObjects: [T] {
        return objects.flatMap { $0.object }
    }

    func contains(_ object: T) -> Bool {
        return self.objects.contains(WeakObject(object: object))
    }

    func addObject(_ object: T) {
        self.objects.formUnion([WeakObject(object: object)])
    }

    func addObjects(_ objects: [T]) {
        self.objects.formUnion(objects.map { WeakObject(object: $0) })
    }
}

正如gokeji所述，我发现NSString不会根据使用的代码进行释放。我挠头，按如下方式编写MyString类。

// typealias MyString = NSString
class MyString: CustomStringConvertible {
    var string: String
    init(string: String) {
        self.string = string
    }
    deinit {
        print("relasing: \(string)")
    }
    var description: String {
        return self.string
    }
}

然后更换NSString与MyString这样的。然后奇怪地说它可行。

var alice: MyString? = MyString(string: "Alice")
var bob: MyString? = MyString(string: "Bob")
var cathline: MyString? = MyString(string: "Cathline")

var persons = WeakObjectSet<MyString>()

persons.addObject(bob!)
print(persons.allObjects) // [Bob]

persons.addObject(bob!)
print(persons.allObjects) // [Bob]

persons.addObjects([alice!, cathline!])
print(persons.allObjects) // [Alice, Cathline, Bob]

alice = nil
print(persons.allObjects) // [Cathline, Bob]

bob = nil
print(persons.allObjects) // [Cathline]

然后我发现一个奇怪的页面可能与此问题有关。

弱引用保留释放的NSString（仅限XC9 + iOS Sim）

https://bugs.swift.org/browse/SR-5511

它说的是问题，RESOLVED但我想知道这是否仍然与此问题有关。无论如何，MyString或NSString之间的行为差​​异不在此上下文范围内，但是如果有人发现了此问题，我将不胜感激。

这不是我的解决方案。我在Apple开发者论坛上找到了它。

@GoZoner有一个很好的答案，但是它会使Swift编译器崩溃。

这是一个弱对象容器的版本，不会使当前发布的编译器崩溃。

struct WeakContainer<T where T: AnyObject> {
    weak var _value : T?

    init (value: T) {
        _value = value
    }

    func get() -> T? {
        return _value
    }
}

然后，您可以创建以下容器的数组：

let myArray: Array<WeakContainer<MyClass>> = [myObject1, myObject2]

您可以通过创建一个包装器对象来保存弱指针来做到这一点。

struct WeakThing<T: AnyObject> {
  weak var value: T?
  init (value: T) {
    self.value = value
  }
}

然后在数组中使用它们

var weakThings = WeakThing<Foo>[]()

功能样式包装器如何？

class Class1 {}

func captureWeakly<T> (_ target:T) -> (() -> T?) where T: AnyObject {
    return { [weak target] in
        return target
    }
}

let obj1 = Class1()
let obj2 = Class1()
let obj3 = Class1()
let captured1 = captureWeakly(obj1)
let captured2 = captureWeakly(obj2)
let captured3 = captureWeakly(obj3)

只需调用返回的闭包来检查目标是否仍然存在。

let isAlive = captured1() != nil
let theValue = captured1()!

您可以将这些闭包存储到数组中。

let array1 = Array<() -> (Class1?)>([captured1, captured2, captured3])

您可以通过映射调用闭包来检索弱捕获的值。

let values = Array(array1.map({ $0() }))

实际上，您不需要执行闭合的功能。只需直接捕获一个对象。

let captured3 = { [weak obj3] in return obj3 }

我有相同的想法，用泛型创建弱容器。
结果，我为NSHashTable以下对象创建了包装器：

class WeakSet<ObjectType>: SequenceType {

    var count: Int {
        return weakStorage.count
    }

    private let weakStorage = NSHashTable.weakObjectsHashTable()

    func addObject(object: ObjectType) {
        guard object is AnyObject else { fatalError("Object (\(object)) should be subclass of AnyObject") }
        weakStorage.addObject(object as? AnyObject)
    }

    func removeObject(object: ObjectType) {
        guard object is AnyObject else { fatalError("Object (\(object)) should be subclass of AnyObject") }
        weakStorage.removeObject(object as? AnyObject)
    }

    func removeAllObjects() {
        weakStorage.removeAllObjects()
    }

    func containsObject(object: ObjectType) -> Bool {
        guard object is AnyObject else { fatalError("Object (\(object)) should be subclass of AnyObject") }
        return weakStorage.containsObject(object as? AnyObject)
    }

    func generate() -> AnyGenerator<ObjectType> {
        let enumerator = weakStorage.objectEnumerator()
        return anyGenerator {
            return enumerator.nextObject() as! ObjectType?
        }
    }
}

用法：

protocol MyDelegate : AnyObject {
    func doWork()
}

class MyClass: AnyObject, MyDelegate {
    fun doWork() {
        // Do delegated work.
    }
}

var delegates = WeakSet<MyDelegate>()
delegates.addObject(MyClass())

for delegate in delegates {
    delegate.doWork()
}

这不是最佳解决方案，因为WeakSet可以使用任何类型进行初始化，并且如果此类型不符合AnyObject协议，则应用程序将因详细原因而崩溃。但是我现在没有更好的解决方案。

原始的解决方案是这样定义的WeakSet：

class WeakSet<ObjectType: AnyObject>: SequenceType {}

但是在这种情况下WeakSet，无法使用协议初始化：

protocol MyDelegate : AnyObject {
    func doWork()
}

let weakSet = WeakSet<MyDelegate>()

目前，上述代码无法编译（Swift 2.1，Xcode 7.1）。
这就是为什么我放弃遵循AnyObject并添加fatalError()断言的附加防护措施的原因。

细节

• Swift 5.1，Xcode 11.3.1

解

struct WeakObject<Object: AnyObject> { weak var object: Object? }

选项1

@propertyWrapper
struct WeakElements<Collect, Element> where Collect: RangeReplaceableCollection, Collect.Element == Optional<Element>, Element: AnyObject {
    private var weakObjects = [WeakObject<Element>]()

    init(wrappedValue value: Collect) { save(collection: value) }

    private mutating func save(collection: Collect) {
        weakObjects = collection.map { WeakObject(object: $0) }
    }

    var wrappedValue: Collect {
        get { Collect(weakObjects.map { $0.object }) }
        set (newValues) { save(collection: newValues) }
    }
}

选项1的用法

class Class1 { // or struct
    @WeakElements var weakObjectsArray = [UIView?]() // Use like regular array. With any objects

    func test() {
        weakObjectsArray.append(UIView())
        weakObjectsArray.forEach { print($0) }
    }
}

选项2

struct WeakObjectsArray<Object> where Object: AnyObject {
    private var weakObjects = [WeakObject<Object>]()
}

extension WeakObjectsArray {
    typealias SubSequence = WeakObjectsArray<Object>
    typealias Element = Optional<Object>
    typealias Index = Int
    var startIndex: Index { weakObjects.startIndex }
    var endIndex: Index { weakObjects.endIndex }
    func index(after i: Index) -> Index { weakObjects.index(after: i) }
    subscript(position: Index) -> Element {
        get { weakObjects[position].object }
        set (newValue) { weakObjects[position] = WeakObject(object: newValue) }
    }
    var count: Int { return weakObjects.count }
    var isEmpty: Bool { return weakObjects.isEmpty }
}

extension WeakObjectsArray: RangeReplaceableCollection {
    mutating func replaceSubrange<C : Collection>( _ subrange: Range<Index>, with newElements: C) where Element == C.Element {
        weakObjects.replaceSubrange(subrange, with: newElements.map { WeakObject(object: $0) })
    }
}

选项2的用法

class Class2 { // or struct
    var weakObjectsArray = WeakObjectsArray<UIView>() // Use like regular array. With any objects

    func test() {
        weakObjectsArray.append(UIView())
        weakObjectsArray.forEach { print($0) }
    }
}

完整样本

不要忘记粘贴解决方案代码

import UIKit

class ViewController: UIViewController {

    @WeakElements var weakObjectsArray = [UIView?]()
    //var weakObjectsArray = WeakObjectsArray<UIView>()

    override func viewDidLoad() {
        super.viewDidLoad()
        addSubviews()
    }

    private func printArray(title: String) {
        DispatchQueue.main.async {
            print("=============================\n\(title)\ncount: \(self.weakObjectsArray.count)")
            self.weakObjectsArray.enumerated().forEach { print("\($0) \(String(describing: $1))") }
        }
    }
}

extension ViewController {

    private func createRandomRectangleAndAdd(to parentView: UIView) -> UIView {
        let view = UIView(frame: CGRect(x: Int.random(in: 0...200),
                                        y: Int.random(in: 60...200),
                                        width: Int.random(in: 0...200),
                                        height: Int.random(in: 0...200)))
        let color = UIColor(red: CGFloat.random(in: 0...255)/255,
                            green: CGFloat.random(in: 0...255)/255,
                            blue: CGFloat.random(in: 0...255)/255,
                            alpha: 1)
        view.backgroundColor = color
        parentView.addSubview(view)
        return view
    }

    private func addSubviews() {
        (0...1).forEach { _ in addView() }
        addButtons()
    }

    private func createButton(title: String, frame: CGRect, action: Selector) -> UIButton {
        let button = UIButton(frame: frame)
        button.setTitle(title, for: .normal)
        button.addTarget(self, action: action, for: .touchUpInside)
        button.setTitleColor(.blue, for: .normal)
        return button
    }

    private func addButtons() {
        view.addSubview(createButton(title: "Add",
                                     frame: CGRect(x: 10, y: 20, width: 40, height: 40),
                                     action: #selector(addView)))

        view.addSubview(createButton(title: "Delete",
                                     frame: CGRect(x: 60, y: 20, width: 60, height: 40),
                                     action: #selector(deleteView)))

        view.addSubview(createButton(title: "Remove nils",
                                     frame: CGRect(x: 120, y: 20, width: 100, height: 40),
                                     action: #selector(removeNils)))
    }

    @objc func deleteView() {
        view.subviews.first { view -> Bool in return !(view is UIButton) }?
            .removeFromSuperview()

        printArray(title: "First view deleted")
    }

    @objc func addView() {
        weakObjectsArray.append(createRandomRectangleAndAdd(to: view))
        printArray(title: "View addded")
    }

    @objc func removeNils() {
        weakObjectsArray = weakObjectsArray.filter { $0 != nil }
        printArray(title: "Remove all nil elements in weakArray")
    }
}

现有的WeakContainer示例很有帮助，但实际上并不能帮助一个人在现有的快速容器（如列表和字典）中使用弱引用。

如果要使用List方法（例如contains），则WeakContainer将需要实现Equatable。因此，我添加了代码以使WeakContainer相等。

如果您想在字典中使用WeakContainer，我还使其可哈希化，因此可以用作字典键。

我还将其重命名为WeakObject，以强调这仅适用于类类型，并将其与WeakContainer示例区分开来：

struct WeakObject<TYPE where TYPE:AnyObject> : Equatable, Hashable
{
    weak var _value : TYPE?
    let _originalHashValue : Int

    init (value: TYPE)
    {
        _value = value

        // We keep around the original hash value so that we can return it to represent this
        // object even if the value became Nil out from under us because the object went away.
        _originalHashValue = ObjectIdentifier(value).hashValue
    }

    var value : TYPE?
    {
        return _value
    }

    var hashValue: Int
    {
        return _originalHashValue
    }
}

func ==<T>(lhs: WeakObject<T>, rhs: WeakObject<T>) -> Bool
{
    if lhs.value == nil  &&  rhs.value == nil {
        return true
    }
    else if lhs.value == nil  ||  rhs.value == nil {
        return false
    }

    // If the objects are the same, then we are good to go
    return lhs.value! === rhs.value!
}

这使您可以做一些很酷的事情，例如使用弱引用字典：

private var m_observerDict : Dictionary<WeakObject<AnyObject>,FLObservationBlock> = Dictionary()

func addObserver( observer:AnyObject, block:FLObservationBlock )
{
    let weakObserver = WeakObject(value:observer)
    m_observerDict[weakObserver] = block
}


func removeObserver( observer:AnyObject )
{
    let weakObserver = WeakObject(value:observer)
    m_observerDict.removeValueForKey(weakObserver)
}

以下是如何让@ GoZoner最伟大的回答符合Hashable，因此它可以被索引在容器中的对象，如：Set，Dictionary，Array等。

private class Weak<T: AnyObject>: Hashable {
    weak var value : T!
    init (value: T) {
       self.value = value
    }

    var hashValue : Int {
       // ObjectIdentifier creates a unique hashvalue for objects.
       return ObjectIdentifier(self.value).hashValue
    }
}

// Need to override so we can conform to Equitable.
private func == <T>(lhs: Weak<T>, rhs: Weak<T>) -> Bool {
    return lhs.hashValue == rhs.hashValue
}

由于NSPointerArray已经自动处理了大部分此类操作，因此我通过为其创建类型安全的包装程序解决了该问题，从而避免了其他答案中的许多重复步骤：

class WeakArray<T: AnyObject> {
    private let pointers = NSPointerArray.weakObjects()

    init (_ elements: T...) {
        elements.forEach{self.pointers.addPointer(Unmanaged.passUnretained($0).toOpaque())}
    }

    func get (_ index: Int) -> T? {
        if index < self.pointers.count, let pointer = self.pointers.pointer(at: 0) {
            return Unmanaged<T>.fromOpaque(pointer).takeUnretainedValue()
        } else {
            return nil
        }
    }
    func append (_ element: T) {
        self.pointers.addPointer(Unmanaged.passUnretained(element).toOpaque())
    }
    func forEach (_ callback: (T) -> ()) {
        for i in 0..<self.pointers.count {
            if let element = self.get(i) {
                callback(element)
            }
        }
    }
    // implement other functionality as needed
}

用法示例：

class Foo {}
var foo: Foo? = Foo()
let array = WeakArray(foo!)
print(array.get(0)) // Optional(Foo)
foo = nil
DispatchQueue.main.async{print(array.get(0))} // nil

前面需要做更多的工作，但是其余代码中的用法更干净。如果您想使其更像数组，甚至可以实现下标，将其设置为SequenceType等（但我的项目仅需要append，forEach所以我手边没有确切的代码）。

相同问题的另一种解决方案……这个问题的重点是存储对对象的弱引用，但也允许您存储结构。

[我不确定它有多有用，但是确实花了一些时间才能正确使用语法]

class WeakWrapper : Equatable {
    var valueAny : Any?
    weak var value : AnyObject?

    init(value: Any) {
        if let valueObj = value as? AnyObject {
            self.value = valueObj
        } else {
            self.valueAny = value
        }
    }

    func recall() -> Any? {
        if let value = value {
            return value
        } else if let value = valueAny {
            return value
        }
        return nil
    }
}


func ==(lhs: WeakWrapper, rhs: WeakWrapper) -> Bool {
    return ObjectIdentifier(lhs) == ObjectIdentifier(rhs)
}



class Stuff {}
var weakArray : [WeakWrapper] = [WeakWrapper(value: Stuff()), WeakWrapper(value: CGRectZero)]

extension Array where Element : WeakWrapper  {

    mutating func removeObject(object: Element) {
        if let index = self.indexOf(object) {
            self.removeAtIndex(index)
        }
    }

    mutating func compress() {
        for obj in self {
            if obj.recall() == nil {
                self.removeObject(obj)
            }
        }
    }


}

weakArray[0].recall()
weakArray[1].recall() == nil
weakArray.compress()
weakArray.count

您可以在周围创建包装器Array。或使用此库https://github.com/NickRybalko/WeakPointerArray let array = WeakPointerArray<AnyObject>() 类型安全。

其他答案涵盖了泛型的角度。以为我会分享一些涵盖nil角度的简单代码。

我想要Label应用程序中当前存在的所有s 的静态数组（偶尔读取），但不想看到nils以前的旧数组。

没什么，这是我的代码...

public struct WeakLabel {
    public weak var label : Label?
    public init(_ label: Label?) {
        self.label = label
    }
}

public class Label : UILabel {
    static var _allLabels = [WeakLabel]()
    public static var allLabels:[WeakLabel] {
        get {
            _allLabels = _allLabels.filter{$0.label != nil}
            return _allLabels.filter{$0.label != nil}.map{$0.label!}
        }
    }
    public required init?(coder: NSCoder) {
        super.init(coder: coder)
        Label._allLabels.append(WeakLabel(self))
    }
    public override init(frame: CGRect) {
        super.init(frame: frame)
        Label._allLabels.append(WeakLabel(self))
    }
}

我基于@Eonil的工作，因为我喜欢闭包弱绑定策略，但是我不想对变量使用函数运算符，因为它感觉非常反直观

相反，我所做的如下：

class Weak<T> where T: AnyObject {
    fileprivate var storedWeakReference: ()->T? = { return nil }

    var value: T? {
        get {
            return storedWeakReference()
        }
    }

    init(_ object: T) {
        self.storedWeakReference = storeWeakReference(object)
    }

    fileprivate func storeWeakReference<T> (_ target:T) -> ()->T? where T: AnyObject {
        return { [weak target] in
            return target
        }
    }
}

这样，您可以执行以下操作：

var a: UIViewController? = UIViewController()
let b = Weak(a)
print(a) //prints Optional(<UIViewController: 0xSomeAddress>)
print(b.value) //prints Optional(<UIViewController: 0xSomeAddress>)
a = nil
print(a) //prints nil
print(b.value) //prints nil

这是我的解决方案：

• 释放数组时清理数组，因为WeakObjectSet正在存储而不是从WeakObject中退出
• 解决在Set中发现重复元素时的致命错误

-

// MARK: - WeakObjectSet 

public class WeakObject<T: AnyObject>: Equatable, Hashable {

    // MARK: Public propreties

    public weak var object: T?
    public var hashValue: Int {
        return self.identifier.hashValue
    }

    // MARK: Private propreties

    private let identifier: ObjectIdentifier

    // MARK: Initializer

    public init(object: T) {
        self.identifier = ObjectIdentifier(object)
        self.object = object
    }

    public static func == (lhs: WeakObject<T>, rhs: WeakObject<T>) -> Bool {
        return lhs.identifier == rhs.identifier
    }
}

// MARK: - WeakObjectSet

public class WeakObjectSet<T: AnyObject> {

    // MARK: Public propreties

    public var allObjects: [T] {
        return allSetObjects.compactMap { $0.object }
    }

    // MARK: Private propreties

    private var objects: Set<WeakObject<T>>
    private var allSetObjects: Set<WeakObject<T>> {
        get {
            objects = self.objects.filter { $0.object != nil }
            return objects
        }
        set {
            objects.formUnion(newValue.filter { $0.object != nil })
        }
    }

    // MARK: Initializer

    public init() {
        self.objects = Set<WeakObject<T>>([])
    }

    public init(objects: [T]) {
        self.objects = Set<WeakObject<T>>(objects.map { WeakObject(object: $0) })
    }

    // MARK: Public function

    public func contains(_ object: T) -> Bool {
        return self.allSetObjects.contains(WeakObject(object: object))
    }

    public func addObject(_ object: T) {
        self.allSetObjects.insert(WeakObject(object: object))
    }

    public func addObjects(_ objects: [T]) {
        objects.forEach { self.allSetObjects.insert(WeakObject(object: $0)) }
    }
}

这是一个类型安全的集合，其中包含弱对象的容器。当访问它时，它也会自动删除零个容器/包装器。

例：

protocol SomeDelegate: class {
    func doSomething()
}

class SomeViewController: UIViewController {
    var delegates: WeakCollection<SomeDelegate> = []

    func someFunction(delegate: SomeDelegate) {
        delegates.append(delegate)
    }

    func runDelegates() {
        delegates.forEach { $0.doSomething() }
    }
}

自定义集合 https://gist.github.com/djk12587/46d85017fb3fad6946046925f36cefdc

import Foundation

/**
 Creates an array of weak reference objects.
 - Important:
    Because this is an array of weak objects, the objects in the array can be removed at any time.
    The collection itself will handle removing nil objects (garbage collection) via the private function cleanUpNilContainers()
 */

class WeakCollection<T>: RangeReplaceableCollection, ExpressibleByArrayLiteral {
    typealias Index = Int
    typealias Element = T
    typealias Iterator = IndexingIterator<[Element]>

    private var weakContainers: [WeakReferenceContainer]

    required convenience init(arrayLiteral: Element...) {
        self.init()
        self.weakContainers = WeakCollection.createWeakContainers(from: arrayLiteral)
    }

    required init() {
        weakContainers = []
    }

    required init<S>(_ elements: S) where S: Sequence, WeakCollection.Element == S.Element {
        self.weakContainers = WeakCollection.createWeakContainers(from: elements)
    }

    static private func createWeakContainers<S>(from weakCollection: S) -> [WeakReferenceContainer] where S: Sequence,
        WeakCollection.Element == S.Element {
            return weakCollection.compactMap { WeakReferenceContainer(value: $0 as AnyObject) }
    }

    func append<S>(contentsOf newElements: S) where S: Sequence, WeakCollection.Element == S.Element {
        self.weakContainers.append(contentsOf: WeakCollection.createWeakContainers(from: newElements))
    }

    var startIndex: Index {
        return references.startIndex
    }

    var endIndex: Index {
        return references.endIndex
    }

    func replaceSubrange<C, R>(_ subrange: R, with newElements: C) where
        C: Collection, R: RangeExpression, WeakCollection.Element == C.Element, WeakCollection.Index == R.Bound {
            weakContainers.replaceSubrange(subrange, with: WeakCollection.createWeakContainers(from: newElements))
    }

    func index(after i: Int) -> Int {
        return references.index(after: i)
    }

    func makeIterator() -> IndexingIterator<[Element]> {
        return references.makeIterator()
    }

    subscript(index: Int) -> Element {
        get {
            return references[index]
        }
        set {
            weakContainers[index] = WeakReferenceContainer(value: newValue as AnyObject)
        }
    }
}

extension WeakCollection {
    private class WeakReferenceContainer {
        private(set) weak var value: AnyObject?

        init(value: AnyObject?) {
            self.value = value
        }
    }

    private func cleanUpNilContainers() {
        weakContainers = weakContainers.compactMap { $0.value == nil ? nil : $0 }
    }

    private var references: [Element] {
        cleanUpNilContainers()
        return weakContainers.compactMap { $0.value as? T }
    }
}

怎么样一个功能性的做法？

let observers = [() -> Observer?]()

observers.append({ [weak anObserver] in return anObserver })

这是主要思想，然后添加任何便捷逻辑来跟踪阵列中的内容。例如，可以考虑使用使用键作为字典的字典作为查找其中内容的方法。