Swift "Hello World" Unity plugin

HelloPlugin.cs

using System.Runtime.InteropServices;

// Put this C# file anywhere in Assets/

public static class HelloPlugin {
    /// Call into swift
    #if UNITY_IOS && !UNITY_EDITOR
    [DllImport("__Internal", EntryPoint = "LIBHelloWorld")]
    public static extern void HelloWorld();
    #else
    public static void HelloWorld() { }
    #endif
}

HelloPlugin.swift

// Put this Swift file in Assets/Plugins/iOS/

@_cdecl("LIBHelloWorld")
func libHelloWorld() {
    // do anything here
    print("Hello World!")
}

Passing parameters also works well.

While the @_cdecl is undocumented, I use this annotation a lot in production and have no issues to report.

Official support has been proposed in this thread.

Fixed compile error when building for iOS.

If you already have loose swift files building in your Xcode project, it just works.

With an empty unity project you may need to toggle an Xcode build setting so that swift files are built (a few years ago this was required).