Unowned self in a closure in a closure

Question

If I have a closure in another closure is it enough to use unowned/weak once in the outer closure to avoid retain cycles?

Example:

foo.aClosure({[unowned self] (allowed: Bool) in
            if allowed {
                self.doStuff()

                self.something.anotherClosure({ (s:String) -> (Void) in
                    self.doSomethingElse(s)
                })   
            }
        })

Answer 1

Only declaring weak or unowned self in the capture list of the outer closure is enough to avoid retain cycles if you don't create a strong reference to self within the outer closure (e.g. by doing: guard let strongSelf = self else { return }).

If you do create a strong reference within the closure, you must add a capture list to the inner closure to ensure that it captures your strong reference to self weakly.

Here are some examples:

import Foundation
import PlaygroundSupport

class SomeObject {
    typealias OptionalOuterClosure = ((Int) -> Void)?
    typealias InnerClosure = () -> Void

    var outerClosure: OptionalOuterClosure

    func setup() {
        // Here are several examples of the outer closure that you can easily switch out below
        // All of these outer closures contain inner closures that need references to self

        // optionalChecks
        //  - has a capture list in the outer closure
        //  - uses the safe navigation operator (?) to ensure that self isn't nil
        // this closure does NOT retain self, so you should not see the #2 calls below
        let optionalChecks: OptionalOuterClosure = { [weak self] callNumber in
            print("outerClosure \(callNumber)")

            self?.delayCaller { [weak self] in
                print("innerClosure \(callNumber)")
                self?.doSomething(callNumber: callNumber)
            }
        }

        // copiedSelfWithInnerCaptureList
        //  - has a capture list in the outer closure
        //  - creates a copy of self in the outer closure called strongSelf to ensure that self isn't nil
        //  - has a capture list in the inner closure
        //  - uses the safe navigation operator (?) to ensure strongSelf isn't nil
        // this closure does NOT retain self, so you should not see the #2 calls below
        let copiedSelfWithInnerCaptureList: OptionalOuterClosure = { [weak self] callNumber in
            guard let strongSelf = self else { return }
            print("outerClosure \(callNumber)")

            strongSelf.delayCaller { [weak strongSelf] in
                print("innerClosure \(callNumber)")
                strongSelf?.doSomething(callNumber: callNumber)
            }
        }

        // copiedSelfWithoutInnerCaptureList
        //  - has a capture list in the outer closure
        //  - creates a copy of self in the outer closure called strongSelf to ensure that self isn't nil
        //  - does NOT have a capture list in the inner closure and does NOT use safe navigation operator
        // this closure DOES retain self, so you should see the doSomething #2 call below
        let copiedSelfWithoutInnerCaptureList: OptionalOuterClosure = { [weak self] callNumber in
            guard let strongSelf = self else { return }
            print("outerClosure \(callNumber)")

            strongSelf.delayCaller {
                print("innerClosure \(callNumber)")
                strongSelf.doSomething(callNumber: callNumber)
            }
        }

        // retainingOuterClosure
        //  - does NOT have any capture lists
        // this closure DOES retain self, so you should see the doSomething #2 call below
        let retainingOuterClosure: OptionalOuterClosure = { callNumber in
            print("outerClosure \(callNumber)")

            self.delayCaller {
                print("innerClosure \(callNumber)")
                self.doSomething(callNumber: callNumber)
            }
        }

        // Modify which outerClosure you would like to test here
        outerClosure = copiedSelfWithInnerCaptureList
    }

    func doSomething(callNumber: Int) {
        print("doSomething \(callNumber)")
    }

    func delayCaller(closure: @escaping InnerClosure) {
        delay(seconds: 1, closure: closure)
    }

    deinit {
        print("deinit")
    }
}

// Handy delay method copied from: http://alisoftware.github.io/swift/closures/2016/07/25/closure-capture-1/
func delay(seconds: Int, closure: @escaping () -> Void) {
    let time = DispatchTime.now() + .seconds(seconds)
    DispatchQueue.main.asyncAfter(deadline: time) {
        print("🕑")
        closure()
    }
}

var someObject: SomeObject? = SomeObject()
someObject?.setup()

// Keep a reference to the outer closure so we can later test if it retained someObject
let copiedOuterClosure = someObject!.outerClosure!

// Call the outer closure once just to make sure it works
copiedOuterClosure(1)

// Wait a second before we destroy someObject to give the first call a chance to work
delay(seconds: 1) {
    // Run the outerClosure again to check if we retained someObject
    copiedOuterClosure(2)

    // Get rid of our reference to someObject before the inner closure runs
    print("de-referencing someObject")
    someObject = nil
}

// Keep the main run loop going so our async task can complete (need this due to how playgrounds work)
PlaygroundPage.current.needsIndefiniteExecution = true