XML解析器出现Swift错误,无法转换RSSItem类型的值?

我一直在学习一个关于在Swift中创建XML解析器的老教程 https://www.youtube.com/watch?v=fP69LI5bZlg . 其中,对于完成处理程序,他有:

func parserDidEndDocument(_ parser: XMLParser) {
        parserCompletionHandler?(rssItems)
    }

问题是它在rssItems上抛出了一个错误并说: Cannot convert value of type '[RSSItem]' to expected argument type 'RSSItem'

自本教程以来,Swift发生了怎样的变化,我怎样才能让它正常工作?下面是我的全部代码,以提供上下文。

struct RSSItem {
    var title: String
    var description: String
    var link: String
    var pubDate: String
}

// download xml from the internet

class FeedParser: NSObject, XMLParserDelegate
{
    private var rssItems: [RSSItem] = []
    private var currentElement = ""
    private var currentTitle: String = ""
    private var currentDescription: String = ""
    private var currentPubDate: String = ""
    private var currentLink: String = ""
    private var parserCompletionHandler: ((RSSItem) -> Void)?
    
    func parseFeed(url: String, completionHandler: ((RSSItem) -> Void)?)
    {
        self.parserCompletionHandler = completionHandler
        let request = URLRequest(url: URL(string: url)!)
        let urlSession = URLSession.shared
        let task = urlSession.dataTask(with: request) { (data, response, error) in
            guard let data = data else {
                if let error = error {
                    print(error.localizedDescription)
                }
                return
            }
            let parser = XMLParser(data: data)
            parser.delegate = self
            parser.parse()
            
        }
        
        task.resume()
    }
    func parser(_ parser: XMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [String : String] = [:]) {
        if currentElement == "item" {
            currentTitle = ""
            currentDescription = ""
            currentPubDate = ""
            currentLink = ""
        }
    }
    func parser(_ parser: XMLParser, foundCharacters string: String) {
        switch currentElement {
        case "title": currentTitle += string
        case "description": currentDescription += string
        case "pubDate" : currentPubDate += string
        case "link" : currentLink += string
        default: break
        }
    }
    func parser(_ parser: XMLParser, didEndElement elementName: String, namespaceURI: String?, qualifiedName qName: String?) {
        if elementName == "item" {
            let rssItem = RSSItem(title: currentTitle, description: currentDescription, link: currentLink, pubDate: currentPubDate)
            self.rssItems.append(rssItem)
        }
    }
    func parserDidEndDocument(_ parser: XMLParser) {
        parserCompletionHandler?(rssItems)
    }
    func parser(_ parser: XMLParser, parseErrorOccurred parseError: Error) {
        print(parseError.localizedDescription)
    }
}