我想我应该为视图控制器添加一些具体的例子。很多解释，不仅仅是在Stack Overflow上，真的很好，但我用现实世界的例子工作得更好(@drewag在这方面有一个很好的开始):

If you have a closure to handle a response from a network requests use weak, because they are long lived. The view controller could close before the request completes so self no longer points to a valid object when the closure is called. If you have closure that handles an event on a button. This can be unowned because as soon as the view controller goes away, the button and any other items it may be referencing from self goes away at the same time. The closure block will also go away at the same time. class MyViewController: UIViewController { @IBOutlet weak var myButton: UIButton! let networkManager = NetworkManager() let buttonPressClosure: () -> Void // closure must be held in this class. override func viewDidLoad() { // use unowned here buttonPressClosure = { [unowned self] in self.changeDisplayViewMode() // won't happen after vc closes. } // use weak here networkManager.fetch(query: query) { [weak self] (results, error) in self?.updateUI() // could be called any time after vc closes } } @IBAction func buttonPress(self: Any) { buttonPressClosure() } // rest of class below. }

以下是来自苹果开发者论坛的精彩语录:

无主vs无主(安全)vs无主(不安全)

unowned(safe) is a non-owning reference that asserts on access that the object is still alive. It's sort of like a weak optional reference that's implicitly unwrapped with x! every time it's accessed. unowned(unsafe) is like __unsafe_unretained in ARC—it's a non-owning reference, but there's no runtime check that the object is still alive on access, so dangling references will reach into garbage memory. unowned is always a synonym for unowned(safe) currently, but the intent is that it will be optimized to unowned(unsafe) in -Ofast builds when runtime checks are disabled.

无主vs弱

unowned actually uses a much simpler implementation than weak. Native Swift objects carry two reference counts, and unowned references bump the unowned reference count instead of the strong reference count. The object is deinitialized when its strong reference count reaches zero, but it isn't actually deallocated until the unowned reference count also hits zero. This causes the memory to be held onto slightly longer when there are unowned references, but that isn't usually a problem when unowned is used because the related objects should have near-equal lifetimes anyway, and it's much simpler and lower-overhead than the side-table based implementation used for zeroing weak references.

更新:在现代Swift中，weak内部使用与un物主相同的机制。所以这个比较是不正确的，因为它比较了Objective-C的弱和Swift的unonwed。

原因

What is the purpose of keeping the memory alive after owning references reach 0? What happens if code attempts to do something with the object using an unowned reference after it is deinitialized? The memory is kept alive so that its retain counts are still available. This way, when someone attempts to retain a strong reference to the unowned object, the runtime can check that the strong reference count is greater than zero in order to ensure that it is safe to retain the object. What happens to owning or unowned references held by the object? Is their lifetime decoupled from the object when it is deinitialized or is their memory also retained until the object is deallocated after the last unowned reference is released? All resources owned by the object are released as soon as the object's last strong reference is released, and its deinit is run. Unowned references only keep the memory alive—aside from the header with the reference counts, its contents is junk.

兴奋,是吗?

如果self可以在闭包中为nil，请使用[weak self]。

如果self在闭包中永远不会为nil，请使用[无主的self]。

Apple Swift文档中有一个很好的章节，用图片解释了在闭包中使用strong、weak和u主的区别:

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/AutomaticReferenceCounting.html

如果以上都说不通:

博士tl;

就像一个隐式打开的可选选项，如果你能保证 引用在它的使用点不会为nil，使用无主。 如果不是，那么你应该使用weak。

解释:

我检索了以下在:弱无主链接。从我收集到的，无主self不能为nil，但弱self可以，并且无主self可以导致悬空指针…这在Objective-C中是臭名昭著的。希望能有所帮助

“无主弱引用和无主引用行为相似，但并不相同。”

Unowned references, like weak references, do not increase the retain count of the object being referred. However, in Swift, an unowned reference has the added benefit of not being an Optional. This makes them easier to manage rather than resorting to using optional binding. This is not unlike Implicitly Unwrapped Optionals . In addition, unowned references are non-zeroing. This means that when the object is deallocated, it does not zero out the pointer. This means that use of unowned references can, in some cases, lead to dangling pointers. For you nerds out there that remember the Objective-C days like I do, unowned references map to unsafe_unretained references.

这就是让人有点困惑的地方。

弱引用和无主引用都不会增加保留计数。

它们都可以用来打破保留循环。那么我们什么时候使用它们呢?

苹果公司的文件显示:

“当一个引用在其生命周期的某个时间点变成nil是有效的时，就使用弱引用。相反，当你知道在初始化过程中设置的引用永远不会为nil时，使用无主引用。”

根据Apple-doc

弱引用总是可选类型，并且是自动的 当它们引用的实例被释放时变为nil。 如果捕获的引用永远不会变成nil，那么它应该始终被捕获为无主引用，而不是弱引用

的例子,

    // if my response can nil use  [weak self]
      resource.request().onComplete { [weak self] response in
      guard let strongSelf = self else {
        return
      }
      let model = strongSelf.updateModel(response)
      strongSelf.updateUI(model)
     }

    // Only use [unowned self] unowned if guarantees that response never nil  
      resource.request().onComplete { [unowned self] response in
      let model = self.updateModel(response)
      self.updateUI(model)
     }

更新11/2016

我写了一篇关于扩展这个答案的文章(通过研究SIL来理解ARC做了什么)，请在这里查看。

原来的答案

前面的答案并没有给出什么时候使用一种而不是另一种的直接规则，所以让我补充一些东西。

无主或弱的讨论归结为变量和引用它的闭包的生命周期问题。

场景

你可以有两个可能的场景:

The closure have the same lifetime of the variable, so the closure will be reachable only until the variable is reachable. The variable and the closure have the same lifetime. In this case you should declare the reference as unowned. A common example is the [unowned self] used in many example of small closures that do something in the context of their parent and that not being referenced anywhere else do not outlive their parents. The closure lifetime is independent from the one of the variable, the closure could still be referenced when the variable is not reachable anymore. In this case you should declare the reference as weak and verify it's not nil before using it (don't force unwrap). A common example of this is the [weak delegate] you can see in some examples of closure referencing a completely unrelated (lifetime-wise) delegate object.

实际使用

那么，大多数时候你会/应该使用哪种呢?

引用乔·格罗夫在推特上的话:

无主更快，并且允许不可变性和非可选性。 如果你不需要弱，就不要用它。

你会在这里找到更多关于无主内部工作的信息。

*通常也称为无主(safe)，表示在访问无主引用之前执行运行时检查(导致无效引用崩溃)。