Call Swift function from Objective-C class

I have an old Objective-C project and I want to call new Swift function and object, I have create the file "<ProjectName>-Bridging-Header.h" and "<ProjectName>-Swift.h"

was easy for me call the function from Swift to Objective-C but I have a problem for reverse.

So I have create a simple class "System.Swift"

import Foundation

@objc class System : NSObject {

    @objc func printSome() {
        println("Print line System");
    }
    
}

now I have try to follow the documentation here and inside the <...>-Swift.h file I have write this

@class System;

@interface System : NSObject

-(void)printSome;

@end

and I have import it inside my Objective-C Class. At this point inside my Objective C class (currently UIViewController) of my Objective-C code I have try to call "printSome" method:

- (void)viewDidLoad
{
    [super viewDidLoad];
    System * sis = [[System alloc] init];
    [sis printSome];
    //any additional setup after loading the view from its nib.
}

now I have the following Error:

Undefined symbols for architecture i386: "OBJC_CLASS$_System", referenced from: objc-class-ref in "ObjectiveC_Class_That_Call_Swift_Object".o ld: symbol(s) not found for architecture i386 clang: error: linker command failed with exit code 1 (use -v to see invocation)

Problem Solved, I previously create and included a new .h file in my Objective-C class named <ProductModuleName>-Swift.h but, as i discovered later, this step is not necessary because the compiler creates the necessary file invisible.

Simply include <ProductModuleName>-Swift.h in your class and it should work.

It is strange but will work after we do:

1. Add @objc to your Swift-class ("MySwiftClass").

2. Add in Obj-C, i.e. the .m file:

   #import "(ProjectName)-Swift.h"
   

3. Declare in header .h

   @class MySwiftClass;
   

Compiler will generate the interface for @objc marked class in MyModuleName-Swift.h file.

Auto-Generated Obj-C Example:

SWIFT_CLASS("_TtC10Project17220PLHelper")
@interface PLHelper

+ (void)notifyForDownloading:(NSDictionary *)userInfo;
- (instancetype)init OBJC_DESIGNATED_INITIALIZER;
@end

Assume We have ProjectName "MyFirstProjectOnSwift" and swift class name "mySwiftClass" and objectiveC class is "MyObjectiveCLass"

Following steps are:-

1. Add #import "MyFirstProjectOnSwift-Swift.h" in "MyObjectiveCLass.m"

2. Add @class mySwiftClass in MyObjectiveCLass.h;

3. Then in MyObjectiveCLass.m

   mySwiftClass *myClass = [mySwiftClass new]; {Call Like This in any method wherever you want to call swift method.}

4. [myClass methodName];

Check the -Swift.h file which has the import on your .m file in objective-C:

#import <YourProjectName-Swift.h>

Click on that line, and left-click - Jump To Definition.

This file should be included automatically, not manually.

Little additional tip for anyone stumbling upon this post and for hwhom the other answers do not work: you might also have to declare your Swift class "public".

Example:

@objc public class MySwiftClass: NSObject {