xBACP

A few days ago while reading the Swift 2.2 documentation, I came upon the section discussing Type Methods. Swift allows one to define methods for instances of a type and the types themselves. The types can be classes, structures, or enumerations, etc.

Two keywords are associated with defining and declaring type methods:

• static
• class

Which one to choose when dealing with classes comes down to whether or not one wants to allow a subclass to override the method. The following examples illustrate the use of static and class for type methods:

import Foundation

class SomeClass {
    
    static func someClassMethod() {
    
    }
}

class AnotherClass {
    
    class func someClassMethod() {
        
    }
}

class SubClass: AnotherClass {
    
    override class func someClassMethod() {
        
    }
    // compiles fine
}

The code above does not tick off the compiler. The following code will:

import Foundation

class SomeClass {
    
    static func someClassMethod() {
    
    }
}

class AnotherClass {
    
    class func someClassMethod() {
        
    }
}

class SubClass: SomeClass {
    
    override static func someClassMethod() {
        
    }
    // compiler error message: class method overrides a 'final' class method
}

At the end of the day it comes down to design choice. If one does not intend for subclasses to muck around with the innards of the class-level methods, make them static. Otherwise, consider using class to allow the flexibility to override the parent implementation or add to the method like the following snippet:

import Foundation

class SomeClass {
    
    static func someClassMethod() {
    
    }
}

class AnotherClass {
    
    class func someClassMethod() {
        
    }
}

class SubClass: AnotherClass {
    
    override class func someClassMethod() {
        
        super.someClassMethod()
        
        // more code here...
    }
}