How to call a Swift function with a callback from C

Question

I am trying to write an iOS plugin for Unity, which requires the use of a bridge with a C layer.

The plugin itself is written in Swift.

Currently my implementation works like so:

// Swift

@objc public class SomeClass {
 @objc public func SomeFunction() -> String {
 return "Hello"
 }
}

// C

extern "C" {
 void _someFunction() {
 // I can access the Swift file using "[SomeClass shared]"
 // This returns the "Hello" string correctly
 NSString *helloMessage = [[SomeClass shared] SomeFunction];
 }
}

This works, but now I need to make an async call which requires a callback from the Swift implementation.

// Swift

@objc public class SomeClass {
 @objc public func SomeFunction(completionHandler: (String) -> Void) {
 // Gets called after some async operations
 completionHandler("hello")
 }
}

Is it possible to call this Swift function from the C layer and how would you do so?

// C

extern "C" {
 void _someFunction() {
 // ??????
 }
}

Answer 1

If you set up bridging well then the call should be like

void _someFunction() {
 [[SomeClass shared] SomeFunctionWithCompletionHandler:^(NSString * _Nonnull value) {
 NSLog(@"Result: %@", value);
 }];
}

Btw, classes to be visible in Objective-C should be 'is-a' NSObject, like

@objcMembers public class SomeClass: NSObject {
 public static var shared = SomeClass()
 public func SomeFunction() -> String {
 return "Hello"
 }
 public func SomeFunction(completionHandler: (String) -> Void) {
 // Gets called after some async operations
 completionHandler("hello")
 }
}

Tested with Xcode 13.2 / iOS 15.2

• Elias Saarenpää: Thanks for replying! I have tried this implementation, but it throws an error "No visible @interface for ’SomeClass’ declares the selector ‘SomeFunction:’". Even though it could find the function I had previously without the 'completionHandler'.
• Elias Saarenpää: Never mind, I didn't write the 'WithCompletionHandler' part. Best use the Xcode autocomplete!