Swift 3 & iOS 10 false memory leak bug

问题:

There seems to be a (false) memory leak bug in Xcode 8 when using with iOS 10 & Swift 3.

The following code reports a memory leak in Instruments and the Xcode 8 memory debugger:

class SomeClass: NSObject {
    var view: SomeView!

    deinit {
        print("SomeClass deinit")
    }
}

class SomeView: UIView {
    weak var reference: SomeClass?

    deinit {
        print("SomeView deinit")
    }
}

class ViewController: UIViewController {
    var someProperty: SomeClass?

    override func viewDidLoad() {
        super.viewDidLoad()

        let c = SomeClass()
        let v = SomeView()
        c.view = v
        v.reference = c

        someProperty = c
    }
}

回答1:

I've tried different variations to confirm it's indeed a bug and my findings are:

1. When you don't assign c in the sample code to someProperty, both instances will print the string in their respective deinits. A true strong reference cycle won't deinit.
2. When SomeClass doesn't inherit from NSObject, this bug doesn't happen.
3. When using Swift 2.2, this doesn't happen.
4. When using iOS 9-, this doesn't happen.
5. Once someProperty is set to nil somewhere in the code, both instances are deinited. Xcode 8 memory debugger confirms there are no memory leaks. However in Instruments, this change isn't reflected--rightfully so, since a real memory leak probably won't be resolved.

FYI, this doesn't happen only when it's assigned to a property of UIViewController. I originally found out about this behavior in a singleton object.

回答2:

Seems fixed with iOS 10.3 (first beta) and Xcode 8.3 (first beta).