Swift嵌套的非可选结构提供了可选

问题描述

我有以下代码:

struct Product {
    var image: URL!
    var title: String!
    var price: Price!
    var rating: Float!
    var url: URL!
}

struct Price {
    var value: Double!
    var currency: String!  // should be enum
}

我后来用

初始化Product:

product = Product(
    image: URL(string: "...")!,
    title: "...",
    price: Price(
        value: 5.99,
        currency: "CAD"
    ),
    rating: 4.5,
    url: URL(string: "...")!
)

在运行时，product.price的类型为Price?，我发现这很奇怪，因为它是隐式展开的.

During runtime, product.price is of type Price? I find this weird since it's implicitly unwrapped.

我尝试给Price一个init()方法，结果相同.我也尝试在Product定义中使用var price: Price! = Price(value: 0, currency: "CAD")，结果相同. (我向Price添加了一个成员初始化器.)

I've tried giving Price an init() method, with the same results. I've also tried using var price: Price! = Price(value: 0, currency: "CAD") in the Product definition, with the same results. (I add a memberwise initializer to Price.)

这是怎么回事?

推荐答案

否，您明确将其设置为可选:

No, you explicitly set it to be optional:

struct Product {
  var image: URL! // <-- Explicitly marked as optional via '!'
}

如果您希望它不是可选的，请不要通过!或?将其标记为可选:

if you want it to be non-optional, do not mark it as optional via ! or ?:

struct Product {
  var image: URL // <-- not marked as optional
}

!和?都是可选的.唯一的区别是，后者需要显式展开(if let)，而前者需要自动展开(如果使用不正确，可能会导致崩溃).

Both ! and ? are optionals. The only difference being that the latter needs to be explicitly unwrapped (if let) while the former is automatically unwrapped (potentially leading to crashes if used incorrectly).

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