如何在结构中解开 swift 选项 [英] how to unwrap swift optionals within a struct

问题描述

我正在学习 swift 并找到了一个在结构中包含可选属性的示例.当我尝试为可选项设置一个值时，我发现它为零.

I'm learning swift and found an example that contains an optional property within a struct. When I try to set a value to the optional I find that it is nil.

struct Price{
    var USD: Double = 0.0
    var CAD: Double = 0.0
}
struct Item{
    var name: String = "not defined"
    var price: Price?
}


var purchase:Item = Item()
purchase.name = "lampshade"
purchase.price?.USD = 19.2


print("purchase name is \(purchase.name), purchase price is \(purchase.price?.USD)")

收益

purchase name is lampshade, purchase price is nil

如果我尝试 (purchase.price.USD)，我会收到错误消息可选类型‘价格’的值?"必须解包以引用封装基类型价格"的成员美元"

if I try (purchase.price.USD) I get an error "Value of optional type 'Price?' must be unwrapped to refer to member 'USD' of wrapped base type 'Price'

如何解开价格以设置美元价值?

How can I unwrap price in order to set a value of USD?

推荐答案

如果您没有为 Optional 赋值，则不能直接设置 Optional 的属性> 本身，由于可选链接，usd 的 setter 不会被调用.

You cannot directly set a property of an Optional if you haven't assigned a value to the Optional itself, since due to the optional chaining, the setter of usd won't be called.

相反，您需要为 purchase.price 分配一个 Price.

Instead, you need to assign a Price to purchase.price.

var purchase:Item = Item()
purchase.name = "lampshade"
purchase.price = Price(USD: 19.2)

或者如果你想分配一个空的"price，那么可选的链接就可以工作，因为现在price 不是nil.

Or if you want to assign an "empty" price, then optional chaining on that works, since now price is not nil.

var purchase:Item = Item()
purchase.name = "lampshade"
purchase.price = Price()
purchase.price?.USD = 19.2

此外，您应该尝试在默认情况下使属性不可变(let)，并且仅在初始化后确实需要更改属性时才使属性可变(var).您还应该只向属性添加默认值，因为它们具有默认值是有意义的.例如，name 不应该有一个，而应该是不可变的，它的值在 init 中设置.

Also, you should try to make properties immutable immutable (let) by default and only make properties mutable (var) if they really need to change after initialisation. You should also only add default values to properties where it makes sense for them to have a default value. For instance, name shouldn't have one, but rather, should be immutable with its value being set in the init.

struct Item{
    let name: String
    var price: Price?
}


var purchase = Item(name: "lampshade")
purchase.price = Price(USD: 19.2)

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