UIViewControllerReferenceCycleTests.swift

//
//  UIViewControllerReferenceCycleTests.swift
//  TargetTests
//

import UIKit
import XCTest

@testable import Target

final class UIViewControllerReferenceCycleTests: XCTestCase {
    private func _testViewController(_ builder: @escaping @autoclosure () -> UIViewController) {
        let exp = expectation(description: "viewController should be deallocated.")
        DispatchQueue.main.async {
            let viewController = builder()
            viewController.loadViewIfNeeded()
            viewController.deallocNotifier.observe {
                exp.fulfill()
            }
        }
        wait(for: [exp], timeout: 1.0)
    }
    
    func testViewController() {
        _testViewController(ViewController())
    }
}

private final class DeallocNotifier {
    private var _observers: [() -> Void] = []

    func observe(_ observer: @escaping () -> Void) {
        _observers.append(observer)
    }

    deinit {
        _observers.forEach { observer in
            observer()
        }
    }
}

private var deallocNotifierContext: UInt8 = 0

extension NSObject {
    fileprivate var deallocNotifier: DeallocNotifier {
        /// This is a technique to use getter as if this object has additional property
        if let notifier = objc_getAssociatedObject(self, &deallocNotifierContext) as? DeallocNotifier {
            return notifier
        }
        let notifier = DeallocNotifier()
        objc_setAssociatedObject(
            self,
            &deallocNotifierContext,
            notifier,
            .OBJC_ASSOCIATION_RETAIN_NONATOMIC
        )
        return notifier
    }
}