Swift 中的协议扩展如何工作？

Question

I am starting a new iOS Swift project. I need to add some functionality that all of the project classes should inherit. Unlike Objective C not all classes in Swift are inherited from any particular class ie NSObject . For example,

I cannot create extension of Any or AnyObject class, like I can for NSObject . For example, below code is OK.

extension NSObject {

    @objc var classTag: String {
        return String(describing: type(of: self))
    }
}

But, extension on AnyObject give compiler error,

Non-nominal type 'AnyObject' cannot be extended

So, I have decided to make all swift classes in my project to inherit from my BaseClass which is actually inherited from NSObject .

class BaseClass: NSObject {

    @objc var classTag: String {
        return String(describing: type(of: self))
    }
}

And other classes ie

class OtherClass: BaseClass {
    // have classTag
}

Before I make a policy for my team, to inherit all classes from BaseClass , I want to know if there are some disadvantages of inheriting all the classes from NSObject by default.

Answer 1

I don't think there is even a need to inherit from NSObject if you are making a BaseClass to be inherited by other classes.

You can simply add classTag in the BaseClass itself, ie

class BaseClass {
    var classTag: String {
        return String(describing: type(of: self))
    }
}

class SubClass: BaseClass {
    func someFunc() {
        print(self.classTag)
    }
}

Another option can be to use protocol and protocol extension and provide the default definition of classTag , ie

protocol SomeProtocol {
    var classTag: String { get }
}

extension SomeProtocol {
    var classTag: String {
        return String(describing: type(of: self))
    }
}

Then, you can conform SomeProtocol to the classes wherever required, ie

class SubClass: SomeProtocol {
    func someFunc() {
        print(self.classTag)
    }
}

In any case, inheriting from NSObject is unnecessary since you don't need any NSObject specific functionality for that.

Answer 2

• Classes inherited from NSObject cannot be final . While whole module optimization turned on, Swift will add final when possible but not in objective c classes. This is micro optimization and I really won't consider unless you have really special case.
• When considering using copy semantics (using structs) you will not be able to extend your structs by inheritance.
• You will add much noise in auto complete in IDE.
• Using protocols (POP) can give you more flexibility/power. Like conditional conformance etc... And you can extend NSObject if needed.

Consider using protocol extension and maybe structs instead of NSObject .

protocol Base {
  func work()
}

extension Base {
  func work() {...}
}

struct AppState: Base {}

AppState().work()