Swift在Protocol中声明了Class Func

Question

I had following confusion. As far as I know the main difference between static and class keywords when declaring method is that the second one could be overridden in subclasses.

The problem

However when I declare a protocol in Swift 1.2 like this:

protocol MyProtocol
{
    class func dummyClassMethod()
}

compiler gives an error:

Class methods are only allowed within classes; use 'static' to declare a static method

The error is pretty descriptive as obviously MyProtocol is not a class, however I want to make a class func part of the protocol.

What I've tried

I've found that if I declare interface in protocol as static , compiler is happy and I could use this static method in all classes that adopt this protocol:

protocol MyProtocol
{
    static func dummyClassMethod()
}

The question

So my question basically is is this right? This declaration states that my class method cannot be overridden in children, however in my implementation I could write and use the following:

class ClassA: MyProtocol
{
    class func dummyClassMethod() {

    }
}

class ClassB: ClassA
{
    override class func dummyClassMethod() {

    }
}

and now my dummyClassMethod is not static anymore...

1. Compiler is Ok and everything works - but why?

2. Is it specific to the fact that interface itself is static, however it's implementation is not?

3. Is there a better alternative for class func in protocols?

Objective-C solution

In ObjC this is pretty easy and compile & run flawlessly:

@protocol MyProtocol 

+(void)dummyClassMethod;

@end

Answer 1

You can review Apple's Documentation (subsection Method Requirements ).

There says:

As with type property requirements, you always prefix type method requirements with the static keyword when they are defined in a protocol. This is true even though type method requirements are prefixed with the class or static keyword when implemented by a class

In practice, You can do it as follow:

First, declare your protocol:

protocol SomeProtocol {
    static func someMethod()
}

Then, in your class you've 2 options :

First:

class SomeClass : SomeProtocol {
    class func someMethod()
}

Second:

class SomeClass : SomeProtocol {
    static func someMethod()
}

I hope, this may clarify your doubt..

Answer 2

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol doesn't actually provide an implementation for any of these requirements—it only describes what an implementation will look like. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements.

After this protocol definition it becomes reasonable that

As with type property requirements, you always prefix type method requirements with the static keyword when they are defined in a protocol. This is true even though type method requirements are prefixed with the class or static keyword when implemented by a class...

Answer 3

To make protocol method static and final implement that method with static keyword

class ClassA: MyProtocol{

    static func dummyClassMethod() {

    }
} 

and now you cant override dummyClassMethod function anymore. If you want to prevent overriding only you must declare protocol method as final. About class functions, they were not fully supported in Swift 1.0 and now in Swift 1.2 I think that they are moving towards static functions