我想在Swift中存储一个弱引用数组。数组本身不应该是弱引用——它的元素应该是。我认为Cocoa NSPointerArray提供了一个非类型安全的版本。
当前回答
我基于@Eonil的工作，因为我喜欢闭包弱绑定策略，但我不想为变量使用函数操作符，因为这感觉非常违反直觉
相反，我所做的是:
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
}
}
}
这样你可以做一些事情，比如:
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
其他回答
函数式样式包装器呢?
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()!
你可以把闭包存储到一个数组中。
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，你不需要一个函数来创建闭包。直接捕获一个对象。
let captured3 = { [weak obj3] in return obj3 }
这不是我的解决方案。我是在苹果开发者论坛上找到它的。
@GoZoner有一个很好的答案，但它崩溃了Swift编译器。
下面是一个弱对象容器的版本，它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
}
}
然后你可以创建一个这些容器的数组:
let myArray: Array<WeakContainer<MyClass>> = [myObject1, myObject2]
这是一个类型安全的集合，用于保存弱对象的容器。当它被访问时，它也会自动移除nil容器/包装器。
例子:
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 }
}
}
现有的WeakContainer示例很有帮助，但它并不能真正帮助在现有的快速容器(如列表和字典)中使用弱引用。
如果您想使用List方法，如contains，那么WeakContainer将需要实现Equatable。因此，我添加了允许WeakContainer是相等的代码。
如果您想在字典中使用WeakContainer，我还将其设置为可哈希的，以便可以将其用作字典键。
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!
}
这允许你做一些很酷的事情，比如使用弱引用字典:
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)
}
现在参加派对有点晚了，但试试我的吧。我实现为一个集合而不是数组。
WeakObjectSet
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) })
}
}
使用
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不会接受String类型，而是接受NSString类型。因为，String类型不是AnyType。我的swift版本是Apple swift version 2.2 (swiftlang-703.0.18.1 clang-703.0.29)。
代码可以从Gist中获取。 https://gist.github.com/codelynx/30d3c42a833321f17d39
**于2017年11月添加
我把代码更新为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提到的，我算出NSString不会被释放基于代码在使用。 我抓了抓头，写了MyString类，如下所示。
// 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
}
}
然后像这样用MyString替换NSString。那么说它有效就很奇怪了。
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]
然后我发现了一个奇怪的页面可能与这个问题有关。
弱引用保留已释放的NSString(仅限XC9 + iOS Sim)
https://bugs.swift.org/browse/SR-5511
上面说问题已经解决了，但我想知道这是否和这个问题有关。 无论如何，MyString或NSString之间的行为差异超出了这个上下文，但如果有人能解决这个问题，我会很感激。
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