Write Unity IOS plugin in Swift code

How to call Unity methods

Unity interface functions are defined in UnityInterface.h in Xcode project built by Unity. This header file is imported in UnitySwift-Bridging-Header.h, so you can call the functions directly in your Swift codes.

To call Unity methods, use UnitySendMessage function like below:

    //  Example.swift

import Foundation

class Example : NSObject {
    static func callUnityMethod(_ message: String) {
        // Call a method on a specified GameObject.
        UnitySendMessage("CallbackTarget", "OnCallFromSwift", message)
    }
}

How to access Swift classes from Unity

Step 1: Create your Swift classes.

//  Example.swift

import Foundation

class Example : NSObject {
    static func swiftMethod(_ message: String) {
        print("\(#function) is called with message: \(message)")
    }
}

Step 2: Include "unityswift-Swift.h" and define C functions to wrap Swift classes in .mm file (Objective-C++).

//  Example.mm

#import <Foundation/Foundation.h>
#import "unityswift-Swift.h"    // Required
                                // This header file is generated automatically when Xcode build runs.

extern "C" {
    void _ex_callSwiftMethod(const char *message) {
        // You can access Swift classes directly here.
        [Example swiftMethod:[NSString stringWithUTF8String:message]];
    }
}

Step 3: Create interface class to call exported C functions from C#.

// Example.cs

using System.Runtime.InteropServices;

public class Example {
    #if UNITY_IOS && !UNITY_EDITOR
    [DllImport("__Internal")]
    private static extern void _ex_callSwiftMethod(string message);
    #endif

    // Use this method to call Example.swiftMethod() in Example.swift
    // from other C# classes.
    public static void CallSwiftMethod(string message) {
        #if UNITY_IOS && !UNITY_EDITOR
        _ex_callSwiftMethod(message);
        #endif
    }
}

Step 4: Call the method from your C# code.

Example.CallSwiftMethod("Hello, Swift!");

The file names of UnitySwift-Bridging-Header.h and unityswift-Swift.h are defined in "Objective-C Bridging Header" entry and "Objective-C Generated Interface Header Name" entry in Build Settings. These settings and other settings about Swift compiler are set automatically by PostProcesser when the Unity build runs.

Requirements

iOS 7 or later

Compatibility

Unity 5.3.5f1 Xcode 7.3.1

As top-level Swift is simply not accessible from Unity, the "workaround" for Swift is to write an Objective-C wrapper class around it, and access that.

Depending on the amount and complexity of your Swift code that might still be the most optimal approach.