swift - Do capture lists of inner closures need to redeclare `self` as `weak` or `unowned`?

Question

If I have a closure passed to a function like this:

 someFunctionWithTrailingClosure { [weak self] in
     anotherFunctionWithTrailingClosure { [weak self] in 
         self?.doSomething()
     }
 }

If I declare self as [weak self] in someFunctionWithTrailingClosure's capture list without redeclaring it as weak again in the capture list of anotherFunctionWithTrailingClosure self is already becoming an Optional type but is it also becoming a weak reference as well?

Thanks!

Answer 1

The [weak self] in anotherFunctionWithTrailingClosure is not needed.

You can empirically test this:

class Experiment {
    func someFunctionWithTrailingClosure(closure: @escaping () -> Void) {
        print("starting", #function)
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            closure()
            print("finishing", #function)
        }
    }

    func anotherFunctionWithTrailingClosure(closure: @escaping () -> Void) {
        print("starting", #function)
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            closure()
            print("finishing", #function)
        }
    }

    func doSomething() {
        print(#function)
    }

    func testCompletionHandlers() {
        someFunctionWithTrailingClosure { [weak self] in
            self?.anotherFunctionWithTrailingClosure { // [weak self] in
                self?.doSomething()
            }
        }
    }

    // go ahead and add `deinit`, so I can see when this is deallocated

    deinit {
        print("deinit")
    }
}

And then:

func performExperiment() {
    DispatchQueue.global().async {
        let obj = Experiment()

        obj.testCompletionHandlers()

        // sleep long enough for `anotherFunctionWithTrailingClosure` to start, but not yet call its completion handler

        Thread.sleep(forTimeInterval: 1.5)
    }
}

If you do this, you will see that doSomething is never called and that deinit is called before anotherFunctionWithTrailingClosure calls its closure.

That having been said, I might still be inclined to use the [weak self] syntax on anotherFunctionWithTrailingClosure to make my intent explicit.