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 的指南正在指导我们如何使用 UIImagePickerController 及其委托。 Apple 使用委托方法 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 未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，但教程说明如下：

"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

根据 Apple 文档

A guard statement is used to transfer program control out of a scope if one or more conditions aren’t met.

和

The value of any condition in a guard statement must be of type Bool or a type bridged to Bool. The condition can also be an optional binding declaration.

所以guard保证变量总是包含一个值，否则控制将被转移到范围之外。

It does not make any other variable optional.

Answer 2

一句话，没有

guard 语句断言某个布尔表达式为真，或者强制退出当前作用域。它通常与 let 一起使用来解包一个可选的，如果可选包含一个值则继续或者如果它是 nil 则退出但它不必这样做。您也可以使用这样的代码：

guard x == 3 else {
  return
}

或计算结果为布尔值 (true/false) 的任何表达式。

Answer 3

没有。可选值源于尝试从字典中检索值。虽然 info 参数是 non-optional 类型 [String: Any] 的字典，但下标引用 info[UIImagePickerControllerOriginalImage] returns 是 Any? 类型的可选值。

来自 The Swift Programming Language (Swift 3.0.1)、"Accessing and Modifying a Dictionary":

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."

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？

Does using the keyword guard inside a function automatically make a function's parameter optional in swift?

uiimagepickercontroller

ios

swift

apple-documentation