需要澄清“@objc类”在Swift [英] Need clarification on "@objc class" in Swift

问题描述

我理解@objc属性对协议的作用。我不明白是为什么它被添加到类，如下面的例子：

I understand what the "@objc" attribute does for protocols. What I don't understand is why it is added to classes, such as in the following example:

@objc protocol CounterDataSource {
    optional func incrementForCount(count: Int) -> Int
    optional var fixedIncrement: Int { get }
}

@objc class Counter {
    var count = 0
    var dataSource: CounterDataSource?
    func increment() {
        if let amount = dataSource?.incrementForCount?(count) {
            count += amount
        } else if let amount = dataSource?.fixedIncrement {
            count += amount
        }
    }
}

这里是它对苹果的Swift文档说的：

Here is what it states on Apple's Swift documentation:

注意，@objc协议只能被类采用，或枚举。如果您将协议标记为@objc以指定可选要求，您将只能将该协议应用于类类型。

Note also that @objc protocols can be adopted only by classes, and not by structures or enumerations. If you mark your protocol as @objc in order to specify optional requirements, you will only be able to apply that protocol to class types.

没有解释@objc class ..，只有@objc protocol。

There is no explanation of "@objc class..", only "@objc protocol".

推荐答案

@objc 对类声明意味着注释的类将暴露给Objective-C运行时。虽然有其他隐含的差异，但我认为这是最重要的一个。

@objc on class declarations means the annotated class will be exposed to the Objective-C runtime. There are other implicit differences though, but I think this is the most important one.

您可以尝试从纯Swift类（不是NSObject的子类）中省略注释，然后您无法在Objective-C代码中使用此类将不会出现在您的Objective-C代码使用的gented头文件中）。

You can try to omit the annotation from a pure Swift class (not a subclass of NSObject), then you are unable to use this class in Objective-C code (it won't appears in the genrated header file used by your Objective-C code).

注意，任何继承 NSObject 或其任何子类的类都会自动）标记为 @objc 。因此，将 UIViewController 的子类显式标记为 @objc 没有意义，因为Swift编译器为你。

Note that any classes inheriting NSObject or any of its subclasses will automatically(implicitly) be marked @objc. So it does not make sense to explicitly mark say a subclass of UIViewController as @objc, because the Swift compiler does it for you.

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