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 @IBOutlet
s like self.temperature
and self.loadingIndicator
. It may be OK because all @IBOutlet
s 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
}
}
}
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