Swift closures causing strong retain cycle with self

closuresretain-cycleswift

I just want to know if I am understanding this correctly or not. So according to the apple docs when you create a closure as a property of a class instance and that closure references self(The class that created the closure property) this would cause a strong retain cycle an ultimately the class nor the closure will ever get released. So in laymen terms that means that if I have a class that has a property and that property is a closure, and once I assign the functionality of that closure within the class that declares the closure property that will cause a strong retain cycle. Heres a quick example of what I mean

class SomeViewController{
  let myClosure:()->Void

  public func someFunction(){
    ....bunch of code
    myClosure = {
       self.dismiss(blahBlahBlah)
    }
  }
}

This ultimately causes a retain cycle since the closure keeps a strong reference to self, which is the class that creates the closure property. Now to fix this according to apple I would define a capture list like so 

class SomeViewController{
  let myClosure:()->Void

  public func someFunction(){
    ....bunch of code
    myClosure = { [weak self] in
       self?.dismiss(blahBlahBlah)
    }
  }
}

Notice how I put the [weak self] before the in statement. This lets the closure know to only hold a weak reference to self and not a strong reference. IM supposed to use weak when self can out live the closure or unowned when the closure and self live for the same duration.

I got this information from here Automatic Reference Counting and in the Strong Reference Cycles for Closures section of that link, theres this sentence "A strong reference cycle can also occur if you assign a closure to a property of a class instance, and the body of that closure captures the instance" Im about 90% sure Im understanding this correctly but theres just that 10% of doubt. So do I have this correct?

The reason why im asking this is because I use callbacks for some of my buttons in my views. And those callbacks call to self but the self in that scenario is the view controller that is responding to the callback not the actual view itself. This is where im doubting myself because I from that sentence I highlighted I don't think I need to put [weak self] on all those button callbacks, but im just making sure. Heres an example of this

class SomeViewController {
    let someSubview:UIView

    override viewDidLoad() {
       //Some Subview has a button and in that view I just have some action that gets fired off calling the callback here in the view controller I don't need to use the [weak self] in this scenario because closure property is not in this class correct?
       someSubview.someButtonsCallback = {
       ....run code then 
       self?.dismiss(blahBlahBlah)
     }
 }

Best Answer

Yes, that can still cause a retain cycle.

The simplest retain cycle is 2 objects that each have strong references to each other, but 3-way and larger retain cycles are also possible.

In your case, you have view controller who's view contains a button (a strong reference.) The button has a strong reference to a closure. The closure strongly references the view controller using self. So the view owns the button. The button owns the closure. The closure owns the view controller. If you dismiss the view controller (say it was a modal) then it SHOULD be deallocated. However, since you have this 3-way retain cycle, it won't be deallocated. You should add a deinit method to your view controller with a print statement and try it.

The solution is to add a capture list (The [weak self] bit) just like you did in your first example.

Note that a common pattern is to add a capture list, and then map the weak variable to a strong variable inside the closure:

let myClosure = { [weak self] in 
  guard let strongSelf = self else { return }
  //...
  strongSelf.doSomething()
}

That way, if the closure is still active but the object that owns it was released, the guard statement at the beginning detects that self is nil and exits at the beginning of the closure. Otherwise you have to unwrap the optional every time you refer to it.

In certain cases it's also possible that the object in the capture list (self in these examples) could be deallocated in the middle of the closure being executed, which can cause unpredictable behavior. (Gory details: This is only when the closure is run on a different thread than the owner of the object in the capture list, but completion handlers are pretty commonly run on a background thread, so it does happen)

Imagine this:

let myClosure = { [weak self] in 

  self?.step1() //1

  //time-consuming code

  self?.property = newValue //2

  //more time-consuming code

  self?.doSomething() //3

  //even more time-consuming code

  self?.doSomethingElse() //4
}

With the code above, if the closure is run on a background thread, its possible that self would still be valid at step 1, but by the time you execute step 2, self has been deallocated. The same goes for steps 3 and 4. By adding the guard strongSelf = self else { return } at the beginning of the closure you test at the entry of the closure to make sure self is still valid, and if it is, you make the closure create a strong reference that only lives as long as the closure takes to run, and that prevents self from being deallocated while the closure code is executing.)

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