简单地说 您正在尝试使用可选变量的值为nil。 快速修复可以使用保护或如果让而不是强制打开像放!在变量的末尾

我在从表视图控制器到视图控制器进行segue时遇到了这个错误，因为我忘记在主故事板中为视图控制器指定自定义类名。

一些简单的东西值得检查，如果其他看起来都没问题

This is because you are trying to use a value which can possible be nil, but you decided you don't want to have to check it, but instead assume its set when you uses it and define it as !, there are different philosophies on use of variable set as force unwrap, some people are against there use at all, I personal think they are ok for things that will crash all the time and are simple to reason about, usually references to resource, like outlets to xib files, or uses of images with you app that are part of your assets, if these are not set up properly, you app is going to crash straight away, for a very obvious reason, you can get into difficult when the order of objects being created can be uncertain, and trying to reason solutions to this can be difficult, it usually means a bad design as even it you make them optional, calls to you optional variable may not ever be executed, some projects can demand use of force unwraps for security reasons, things like banking apps, because they want the app to crash rather then continue to work in an unplanned way.

如果我的情况下，我设置一个变量UILabel为nil。

所以我修复了它，之后它就没有抛出错误了。

代码片段

class ResultViewController: UIViewController {

    @IBOutlet weak var resultLabel: UILabel!
    var bmiValue=""
    override func viewDidLoad() {
        super.viewDidLoad()
        print(bmiValue)
        resultLabel.text=bmiValue //where bmiValue was nil , I fixed it and problem was solved

    }
    
    @IBAction func recaculateBmi(_ sender: UIButton) {
        self.dismiss(animated: true, completion: nil)
    }
    

}

错误EXC_BAD_INSTRUCTION和致命错误:在隐式地展开Optional值时意外地发现nil，在声明@IBOutlet但没有连接到故事板时出现的最多。

你还应该了解其他答案中提到的可选选项是如何工作的，但这是唯一一次出现在我身上。

Xcode 12 iOS 14 Swift 5

我的问题是导航类型，因为我直接调用vie控制器，而没有实例化故事板，这意味着数据还没有从故事板设置。

当你导航时，用

let homeViewController = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "home") as? HomeEventsViewController
    homeViewController?.modalTransitionStyle = .crossDissolve
    homeViewController?.modalPresentationStyle = .fullScreen
    view.present(homeViewController ?? UIViewController(), animated: true, completion: nil)

希望它能起作用:-)