不能对“ViewController”类型的非可选值使用可选链接

Question

我对 swift 完全陌生，并且遵循有关创建待办事项列表应用程序的教程。只要我在self.createItem(name: text 中添加问号，就会收到错误Cannot use optional chaining on non-optional value of type 'ViewController'

@objc private func didTapAdd() {
        let alert = UIAlertController(title: "New Item", message: "Enter new Item", preferredStyle: .alert)
        alert.addTextField(configurationHandler: nil)
        alert.addAction(UIAlertAction(title: "Submit", style: .cancel, handler: {
            _ in
            guard let field = alert.textFields?.first, let text = field.text, !text.isEmpty else {
                return
            }
            self?.createItem(name: text)
        }))
        present(alert, animated: true)
        
    }

Answer 1

问题在于您没有将 self 变量声明为弱变量以使其成为可选变量。如下使用弱 self 使 ViewController 的 self 实例成为可选。然后你就可以使用问号了。

@objc private func didTapAdd() {
        let alert = UIAlertController(title: "New Item", message: "Enter new Item", preferredStyle: .alert)
        alert.addTextField(configurationHandler: nil)
        alert.addAction(UIAlertAction(title: "Submit", style: .cancel, handler: { [weak self] (_) in
            guard let field = alert.textFields?.first, let text = field.text, !text.isEmpty else {
                return
            }
            self?.createItem(name: text)
        }))
        present(alert, animated: true)
        
    }