ios - 在函数内部使用关键字 guard 是否会自动使函数的参数在 swift 中成为可选参数？

Question

我是编程新手，正在阅读 Start Developing iOS Apps (Swift) programming guide .我知道可选项有一个问号后缀，这意味着该值可以是 nil 或其他一些值。 Apple 的指南向我们介绍了如何使用 UIImagePickerController 及其委托(delegate)。 Apple 使用委托(delegate)方法 imagePickerControllerdidFinishPickingMediaWithInfo(_:)，它有一个字典作为参数。

func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]) {

        guard let selectedImage = info[UIImagePickerControllerOriginalImage] as? UIImage else {
            fatalError("Expected a dictionary containing an image, but was provided the following: \(info)")
        }
    }

Apple's UIImagePickerControllerDelegate documentation没有将此参数列为可选参数，我们为本教程创建的 UIImagePickerController 实例也不是可选参数，但本教程声明如下:

"This code accesses the original, unedited image from the info dictionary. It safely unwraps the optional returned by the dictionary and casts it as a UIImage object. The expectation is that the unwrapping and casting operations will never fail. If they do, it represents either a bug in your app that needs to be fixed at design time."

这是否意味着通过使用 guard 关键字，它会自动将变量变为可选变量？

Answer 1

没有。可选值源于尝试从字典中检索值。虽然 info 参数是 [String: Any] 类型的非可选字典，但下标引用 info[UIImagePickerControllerOriginalImage] 返回一个可选值类型 Any?。

来自 The Swift Programming Language (Swift 3.0.1) , "访问和修改字典":

Because it is possible to request a key for which no value exists, a dictionary’s subscript returns an optional value of the dictionary’s value type. If the dictionary contains a value for the requested key, the subscript returns an optional value containing the existing value for that key. Otherwise, the subscript returns nil:

if let airportName = airports["DUB"] {
    print("The name of the airport is \(airportName).")
} else {
    print("That airport is not in the airports dictionary.")
}
// Prints "The name of the airport is Dublin Airport."