Need clarification on “@objc class” in Swift

问题:

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
        }
    }
}

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

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.

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

回答1:

@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.

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).

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.

回答2:

On this page, it says

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.

So, the difference is that it applies implicitly to all members. Also, you'll be prevented from adding @objc to a generic class.