Using NSObject’s +load and +initialize from Swift, checkout the full story – https://medium.com/post-mortem/using-nsobjects-load-and-initialize-from-swift-f6f2c6d9aad0

Foo.m

#import <Foundation/Foundation.h>
#import <MyProject/MyProject-Swift.h>

// This was a more concise solution in Swift 4.2 and earlier. If you try this with Swift 5 you'd get "Swift class
// extensions and categories on Swift classes are not allowed to have +load methods" runtime error.

// @implementation Foo (private)
//     + (void)load { [self swiftyLoad]; }
//     + (void)initialize { [self swiftyInitialize]; }
// @end

// This is what you'd want to use with Swift 5, basically just a pure Objective-C defined class.

@interface Loader : NSObject
@end

@implementation Loader : NSObject
    + (void)load { [Foo swiftyLoad]; }
    + (void)initialize { [Foo swiftyInitialize]; }
@end

Foo.swift

import Foundation

public class Foo: NSObject {
    @objc public static func swiftyLoad() {
        Swift.print("Rock 'n' roll!")
    }

    @objc public static func swiftyInitialize() {
        Swift.print("Hello initialize!")
    }
}