Nerd snipery

Playground.swift

//: Playground - noun: a place where people can play

import UIKit

class VC {
    var dataSource: DataSource?
    
    func viewDidLoad() {
        let items = dataSource?.numberOfItemsIn(section: 9) ?? -1
        print("There are \(items) items")
    }
    
    deinit {
        print("deinit on the VC")
    }
}

class DataSource {
    struct Hooks {
        var numberOfSectionsIn: ((_: Int) -> Int?)?
    }
    
    var hooks: Hooks?
    
    func numberOfItemsIn(section: Int) -> Int {
        guard hooks?.numberOfSectionsIn?(section) == nil else { return hooks!.numberOfSectionsIn!(section)! }
        return 99
    }
    
    deinit {
        hooks = nil
        print("deinit on the data source")
    }
}

class Frame {
    var dataSource: DataSource? = DataSource()
    let itemCount = 100
    
    init() {
        var hooks = DataSource.Hooks()
        // Right here I want to capture an unowned self
//        hooks.numberOfSectionsIn = numberOfSectionIn
        hooks.numberOfSectionsIn = disown(function: Frame.numberOfSectionIn)
        
        // But if I pass in a closure with an unowned self, there's no problem
        //        hooks.numberOfSectionsIn = { [unowned self] (section) -> Int? in
        //            return self.itemCount
        //        }
        
        dataSource?.hooks = hooks
    }
    
    private func disown<In, Out>(function: @escaping ((Frame) -> (In) -> Out)) -> ((In) -> Out) {
        let result: ((In) -> Out) =  { [unowned self] arg in
            let selflessFunc = function(self)
            return selflessFunc(arg)
        }
        return result
    }
    
    func numberOfSectionIn(_ section: Int) -> Int? {
        // The DataSource hook now has a strong reference to Frame
        return itemCount
    }
    
    deinit {
        dataSource = nil
        print("deinit on the frame")
    }
}

var frame: Frame? = Frame()
var vc: VC? = VC()
vc?.dataSource = frame!.dataSource
vc?.viewDidLoad()
vc = nil
frame = nil