有什么区别：和=在swift中 [英] What's the difference between : and = in swift

问题描述

很抱歉，如果标题相当令人困惑，但我很想知道这两行之间的区别：

Sorry if the title is rather confusing, but I'm curious to know the difference between these two lines:

var title = String()
var title: String

是否正在初始化一个，只有一个是声明？哪个更正确？

Is one being initialized and one only be declared? Which is more correct?

例如，如果我有一个结构，我应该使用另一个吗？

For example, if I have a struct should I use one of the other?

所以我问这个的原因是因为我正在学习如何从网址中获取一些JSON然后在我的应用中显示它。这样做的一种新方法是使用Decodable。所以，我在模型类中有一个结构，如下所示：

So the reason I ask this is because I'm learning about how to grab some JSON from a url and then display it in my app. One of the new ways of doing so is using Decodable. So, I have a struct in a model class like so:

struct Videos: Decodable {

    var title = String()
    var number_of_views : Int
    var thumbnail_image_name: String
    var channel: Channel
    var duration: Int

}

在另一个课程中我有这个：

In another class I have this:

URLSession.shared.dataTask(with: url){(data,response,error) in

            if(error != nil){
                print(error!)
                return
            }

        guard let data = data else { return }


        do{
            self.Videos2 = try JSONDecoder().decode([Videos].self, from: data)

            //self.collectionView?.reloadData()

        }catch let jsonErr{
            print(jsonErr)
        }

    }.resume()

那么，我应该在结构中声明或初始化变量吗？我假设我应该像这样声明它们：
var title：String？
这是我的结构中正确的语法吗？

So, should I declare or initialize the variables in my struct? I'm assuming I should just declare them like so: var title: String? Would that be the correct syntax in my struct?

更新：
我明白这个问题比我最初提出的要宽泛得多。对此我感到很抱歉，但是非常感谢你所有的答案，这些答案为我澄清了很多。

UPDATE: I understand this question was more broad then I originally proposed it to be. I'm sorry about that, but thank you so much for all your great answers that clarified a lot up for me.

推荐答案

区别在于：定义变量的类型，而 = 为变量指定一个实际值。

The difference is that : defines the type of your variable, whereas = assigns an actual value to the variable.

所以：

var title = String()

这调用 String 类型的初始值设定项，创建一个新的 String 实例。然后它将此值分配给 title 。 title 的类型被推断为 String ，因为您正在分配类型的对象字符串到它;但是，您也可以将此行显式写为：

This calls the initializer of the String type, creating a new String instance. It then assigns this value to title. The type of title is inferred to be String because you're assigning an object of type String to it; however, you could also write this line explicitly as:

var title: String = String()

这意味着你要声明一个 title 类型<$ c $的变量c> String ，并为其指定一个新的 String 。

This would mean you are declaring a title variable of type String, and assigning a new String to it.

var title: String

这只是说你要定义一个类型的变量字符串。但是，您没有为其分配值。在使用它之前你需要为这个变量赋值，或者你会得到一个编译错误（如果这是一个属性而不仅仅是一个变量，你需要在你到达类型的末尾之前分配它） init（）方法，除非它后面的？是可选的，在这种情况下它被隐式初始化为 nil ）。

This simply says you're defining a variable of type String. However, you are not assigning a value to it. You will need to assign something to this variable before you use it, or you will get a compile error (and if this is a property rather than just a variable, you'll need to assign it before you get to the end of your type's init() method, unless it's optional with ? after it, in which case it gets implicitly initialized to nil).

编辑：对于你的例子，我可能会使用声明所有变量和：，假设您的JSON为所有这些属性提供值。然后， Decodable 生成的初始化程序应在创建对象时设置所有属性。所以，类似于：

For your example, I'd probably declare all the variables using let and :, assuming that your JSON provides values for all of those properties. The initializer generated by Decodable should then set all the properties when you create the object. So, something like:

struct Videos: Decodable {
    let title: String
    let number_of_views : Int
    let thumbnail_image_name: String
    let channel: Int
    let duration: Int
}

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