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In WWDC 2014 session 403 Intermediate Swift and transcript, there was the following slide
The speaker said in that case, if we don't use [unowned self] there, it will be a memory leak. Does it mean we should always use [unowned self] inside closure?
On line 64 of ViewController.swift of the Swift Weather app, I don't use [unowned self]. But I update the UI by using some @IBOutlets like self.temperature and self.loadingIndicator. It may be OK because all @IBOutlets I defined are weak. But for safety, should we always use [unowned self]?
class TempNotifier { var onChange: (Int) -> Void = {_ in } var currentTemp = 72 init() { onChange = { [unowned self] temp in self.currentTemp = temp } } } 
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According to Apple's docs: “Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialisation.”
Using [weak self] is only required within situations in which capturing self strongly would end up causing a retain cycle, for example when self is being captured within a closure that's also ultimately retained by that same object.
In Swift, [weak self] prevents closures from causing memory leaks in your application. This is because when you use [weak self], you tell the compiler to create a weak reference to self. In other words, the ARC can release self from memory when necessary.
“Self” as a Type in Swift This “Self” has an uppercase “S”, which is how you can tell it apart from the lowercase self. In Swift, Self refers to a type – usually the current type in the current context. Just as lowercase self can mean any current object, uppercase Self can mean just about any current type.
No, there are definitely times where you would not want to use [unowned self]. Sometimes you want the closure to capture self in order to make sure that it is still around by the time the closure is called.
If you are making an asynchronous network request you do want the closure to retain self for when the request finishes. That object may have otherwise been deallocated but you still want to be able to handle the request finishing.
unowned self or weak self The only time where you really want to use [unowned self] or [weak self] is when you would create a strong reference cycle. A strong reference cycle is when there is a loop of ownership where objects end up owning each other (maybe through a third party) and therefore they will never be deallocated because they are both ensuring that each other stick around.
In the specific case of a closure, you just need to realize that any variable that is referenced inside of it, gets "owned" by the closure. As long as the closure is around, those objects are guaranteed to be around. The only way to stop that ownership, is to do the [unowned self] or [weak self]. So if a class owns a closure, and that closure captures a strong reference to that class, then you have a strong reference cycle between the closure and the class. This also includes if the class owns something that owns the closure.
In the example on the slide, TempNotifier owns the closure through the onChange member variable. If they did not declare self as unowned, the closure would also own self creating a strong reference cycle.
unowned and weak The difference between unowned and weak is that weak is declared as an Optional while unowned is not. By declaring it weak you get to handle the case that it might be nil inside the closure at some point. If you try to access an unowned variable that happens to be nil, it will crash the whole program. So only use unowned when you are positive that variable will always be around while the closure is around
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