iOS ARC：使用 deinit 保留循环？

Question

iOS retain cycle:

The typical example is A has B, B has A.

So what if A has B & C, B has C?

Is it still retain cycle?

And is there a strong relationship that the deinit will not called by ARC when retain cycle exists?

Here is the example code:

class ViewModel{
    
    
    func doSth(){
        print("123")
    }
    
    
    deinit{
        print("ViewModel")
    }
}


class CustomView: UIView{
    
    var viewModel: ViewModel?
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        viewModel?.doSth()
    }
    
    
    deinit{
        print("CustomView")
    }
}


class ViewController: UIViewController {

    var viewModel = ViewModel()
    
    lazy var widget: CustomView = {
        let v = CustomView(frame: CGRect(origin: .zero, size: CGSize(width: 150, height: 150)))
        v.backgroundColor = UIColor.red
        v.viewModel = viewModel
        return v
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.addSubview(widget)
        
    }

    deinit{
        print("ViewController")
    }
}

when ViewController popped, three deinit method above are all called.

So there is no retain cycle.

Is it?

Answer 1

An easy way to figure out if you have a retain cycle is to draw a diagram. Use arrows for strong references, and dotted arrows for weak/unowned references.

If you can follow the solid arrows from an object back to that same object, you have a retain cycle.

In your case, where A owns B and C, and B also owns C, there are 2 owning references to C, but no "backwards" strong references. There is no way you get loop back to A or B, so there is no retain cycle.

Closures are another potential source of retain cycles that isn't obvious. If you have an "escaping" closure that references "self", and you hold a strong reference to that closure, you have a retain cycle because you have an owning reference to the closure and the closure, through it's self reference, has a strong reference to you.