首先，您应该知道什么是Optional值。 详细信息请参见《Swift编程语言》。

其次，您应该知道可选值有两个状态。一个是全值，另一个是空值。因此，在实现一个可选值之前，应该检查它是哪个状态。

你可以用if let…或者守卫让…Else等等。

还有一种方法，如果你不想在实现之前检查变量的状态，你也可以使用var buildingName = buildingName ??“buildingName”。

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

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。

如果在CollectionView中出现此错误，请尝试创建CustomCell文件和Custom xib。

在mainVC的ViewDidLoad()中添加此代码。

    let nib = UINib(nibName: "CustomnibName", bundle: nil)
    self.collectionView.register(nib, forCellWithReuseIdentifier: "cell")

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.

Swift 5.7 +

如果让简写的影子一个现有的可选变量

以上回答清楚地解释了为什么会出现这个问题以及如何处理这个问题。但是从swift 5.7+开始，有了一种新的方式来处理这个问题。

是我的变量：整数？

以前

if let myVariable = myVariable {
    //this part get executed if the variable is not nil
}else{
    //this part get executed if the variable is nil
}

now

现在我们可以省略表达式的右边。

if let myVariable {
    //this part get executed if the variable is not nil
}else{
    //this part get executed if the variable is nil
}

以前，我们必须重复引用标识符两次，这可能导致这些可选绑定条件过于冗长，特别是在使用冗长的变量名时。

但是现在有一种简写语法，可以通过省略表达式的右边来实现可选绑定。

同样的事情也适用于guard let语句。

详情如下:

if-let简写的建议