user0000000 user0000000 - 1 month ago 17
Swift Question

Avoiding strong reference when passing a method to a function

When passing a method to a function that takes a closure, I can use either

someFunc(closure: someMethod) or
someFunc() { [unowned self] in self.someMethod() }`.

The first one is shorter but makes a strong reference. How can I use it while avoiding this strong reference?

Here is a demo with both the leaking one and the good one:
https://swiftlang.ng.bluemix.net/#/repl/581ccd3a0bdc661a6c566347

import Foundation

private var instanceCounter = 0

class Leak : NSObject {

override init() {
super.init()
instanceCounter += 1
}

deinit {
instanceCounter -= 1
}
}

class OnFunctionLeak : Leak {

override init() {
super.init()
_ = NotificationCenter.default.addObserver(forName: NSNotification.Name(rawValue: "OnFunctionLeak"),
object: nil,
queue: nil,
usingBlock: doNothing)
}

func doNothing(_ notif: Notification) { }

deinit {
NotificationCenter.default.removeObserver(self)
}
}

class OnClosureLeak : Leak {

override init() {
super.init()
_ = NotificationCenter.default.addObserver(forName: NSNotification.Name(rawValue: "OnFunctionLeak"),
object: nil,
queue: nil) { [unowned self] notif in
self.doNothing(notif)
}
}

func doNothing(_ notif: Notification) { }

deinit {
NotificationCenter.default.removeObserver(self)
}
}

var onFunctionLeak: OnFunctionLeak? = OnFunctionLeak()
onFunctionLeak = nil

//XCTAssertEqual(instanceCounter, 0)
print("instanceCounter: \(instanceCounter) == 0")

instanceCounter = 0
var onClosureLeak: OnClosureLeak? = OnClosureLeak()
onClosureLeak = nil

//XCTAssertEqual(instanceCounter, 0)
print("instanceCounter: \(instanceCounter) == 0")


The shorter choice is on line 26 and if I replace
doNothing
by
{ [unowned self] notif in self.doNothing(notif) }
, the strong reference is gone.

Any ideas?

Answer

How can I use it while avoiding this strong reference?

You can't.

Only an anonymous function defined inline (at the point of usage) can have a capture list (such as [unowned self]). Thus, only an anonymous function can provide the functionality you are asking for. A function defined with func simply cannot do it.

That's just a fact about Swift.

(There are probably underlying reasons for it; I suspect that the reasons have to do with storage. A func function is stored statically in some way. But an anonymous function defined inline is not; it comes into being at exactly the moment it is passed to the callee. But that's just a guess, and a rather vague guess at that.)