FeedTableViewController.swift

// ...

    var productListener: Product.Listener?
    
    fileprivate var products: [Product] = []
    {
        didSet
        {
            feedTableView.reloadData()
        }
    }
    
    override func viewWillAppear(_ animated: Bool)
    {
        super.viewWillAppear(animated)
        
        productListener = Product.listenToFeed
        {
            [unowned self] in self.products = $0
        }
    }
    
    override func viewWillDisappear(_ animated: Bool)
    {
        super.viewWillDisappear(animated)
        
        productListener?.remove()
    }
    
 // ...