swift - 需要澄清 Swift 中的 "@objc class"

Question

我了解“@objc”属性对协议(protocol)的作用。我不明白的是为什么要将它添加到类中，例如在下面的示例中:

@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
        }
    }
}

这是它在 Apple 的 Swift 文档中的陈述:

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“@objc class..”的解释，只有“@objc protocol”。

Answer 1

关于 this page , 它说

If you apply the objc attribute to a class or protocol, it’s implicitly applied to the members of that class or protocol. The compiler also implicitly adds the objc attribute to a class that inherits from another class marked with the objc attribute. Protocols marked with the objc attribute can’t inherit from protocols that aren’t.

因此，不同之处在于它隐含地适用于所有成员。此外，您将无法将 @objc 添加到泛型类。