WeakReferences.swift

import Cocoa

class TestObject {
    init() {
        print("Init \(self)")
    }
    
    deinit {
        print("Deinit \(self)")
    }
}

struct WeakReference<T: AnyObject> {
    weak var reference: T?
}

class Object {
    private var _objects: [WeakReference<TestObject>] = []
    var objects: [TestObject] {
        set(newValue) { self._objects = newValue.map({ WeakReference(reference: $0) }) }
        get { return self._objects.flatMap({ $0.reference }) }
    }
}

let object = Object()

var testObject: TestObject? = TestObject()

object.objects = [testObject!]

print("BEFORE: \(object.objects)")

testObject = nil

print("AFTER: \(object.objects)")

object.objects = []

print("Finish")