How to define a callback in C

[cihadturhan]

Hi @jwtan,

Thank you for sharing this, I spent countless hours to fix Unity + Swift problem. Without your explanation I couldn't understand what are the problems. I wish I could've found this repo earlier.
I think you should share this on unity forums because this is a hidden gem here :)

So, I have a project that I want to integrate with a swift library but I can't use the callback on swift:

I create a header to define my callback

// SwiftToUnityBridge.h
#ifndef SwiftToUnityBridge_h
#define SwiftToUnityBridge_h

#ifdef __cplusplus // gives error without it
extern "C" {
#endif

  typedef void (*SwiftCallback)(); //<- I define the callback here

#ifdef __cplusplus
}
#endif

#endif

Then, add a callback on mm file

// SwiftToUnityBridge.mm
#import <UnityFramework/UnityFramework-Swift.h>

extern "C"
{
    void UnityOnStart()
    {
        [[SwiftToUnity shared]   UnityOnStart];
    }

+    void CallWithACallback(SwiftCallback callback)
+    {
+        [[SwiftToUnity shared] callWithCallback: callback];
+    }
}

and, define a function on swift

// SwiftToUnity.swift
import Foundation

@objc public class SwiftToUnity: NSObject
{
    @objc public static let shared = SwiftToUnity()

    @objc public func UnityOnStart()
    {
        UnitySendMessage("Cube", "OnMessageReceived", "Hello World!");
    }
    
+    @objc public func call(callback: @escaping SwiftCallback)
+    {
+        callback();
+    }
}

And moved the header from project to public (I don't know if I should do this)

After that, I added the header to UnityFramework.modulemap

framework module UnityFramework {
  umbrella header "UnityFramework.h"

  export *
  module * { export * }

  module UnityInterface {
      header "UnityInterface.h"
+      header "SwiftToUnityBridge.h"

      export *
  }
}

But I can't find out why it still doesn't let me use the callback

Generated UnityFramework-Swift.h:

Thank you for sharing this.