Unit testing memory leaks

DeallocationSentinel.swift

//
//  DeallocationSentinel.swift
//
//  Created by Rogerio de Paula Assis on 7/14/19.
//  Extracted from this CCH Melbourne talk: https://www.youtube.com/watch?v=514rJ1efv84
//

public func expectDeallocation(
    of object: AnyObject,
    using deallocationExpectations: inout [XCTestExpectation]
) {
    let expectation = XCTestExpectation(description: "Object \(object) deallocated")
    objc_setAssociatedObject(
        object,
        &deallocatedExpectationKey,
        DeallocationSentinel(deallocClosure: expectation.fulfill),
        .OBJC_ASSOCIATION_RETAIN
    )
    deallocationExpectations.append(expectation)
}

// MARK: Private

private var deallocatedExpectationKey = 0
private final class DeallocationSentinel {
    
    private let deallocClosure: () -> Void
    
    init(deallocClosure: @escaping () -> Void) {
        self.deallocClosure = deallocClosure
    }
    
    deinit {
        deallocClosure()
    }
}

DeallocTests.swift

//
//  DeallocTests.swift
//
//  Created by Rogerio de Paula Assis on 7/14/19.
//  Extracted from this CCH Melbourne talk: https://www.youtube.com/watch?v=514rJ1efv84
//

import XCTest

class DeallocTests: XCTestCase {
    
    private var deallocationExpectations: [XCTestExpectation] = []
    
    var subject: ViewModel!
    
    override func setUp() {
        subject = ViewModel()
        let vc = ViewController(viewModel: subject)
        subject.vc = vc
        expectDeallocation(of: subject, using: deallocationExpectations)
    }
    
    override func tearDown() {
        wait(for: self.deallocatioExpectations, timeout: 1)
    }
    
    func testDeallocation() {
        XCTAssert(subject != nil)
        subject = nil
    }
}

Dummy.swift

// Dummy classes created to simulate a retain cycle for the test case above
// Add/remove weak from view model to verify different test result

class ViewModel {
    /* weak */ var vc: ViewController?
}

class ViewController: UIViewController {
    let viewModel: ViewModel
    
    init(viewModel :ViewModel) {
        self.viewModel = viewModel
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}