Create callbacks between Unity C#, JavaScript, and C/C++/C# code

Callbacks are an important part of web development because they let your Unity project communicate better with your JavaScript browser, JavaScript plug-inA set of code created outside of Unity that creates functionality in Unity. There are two kinds of plug-ins you can use in Unity: Managed plug-ins (managed .NET assemblies created with tools like Visual Studio) and Native plug-ins (platform-specific native code libraries). More info
See in Glossary, and C/C++/C# plug-ins. You might want to call functions from your C/C++/C# or JavaScript code in your Unity C# code, and callbacks are an effective way to do this.

Use the makeDynCall function to make callbacks

Use makeDynCall to call C, C++, or C# callbacks from JavaScript code. For example, the following JavaScript code calls the function “callback”:

{{{ makeDynCall('vii', 'callback') }}}(1, 2)

'vii' is the signature of the parameters. For example, the callback function is a void type (v), and the (1,2) are both integers (ii).
'callback' is the name of the callback function.
(1, 2) are the arguements to pass into the function.

The following code is another example of code that does a callback with the function “onresponse” and passes in a float (f) and multiple integer type values:

{{{ makeDynCall('vfii', 'onresponse') }}}(2.8, arg, 0);

Use callbacks in your scripts

The following scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary show an example of how to use callback functions to communicate between JavaScript, C++, and C# code.

Javascript plug-in code

The following code is an example of a .jslib file that schedules a timeout and invokes a callback.

mergeInto(LibraryManager.library, {
    JsSetTimeout: function (message, timeout, callback) {
        // Create copy of message because it might be deleted before callback is run
        var stringMessage = UTF8ToString(message);
        var buffer = stringToNewUTF8(stringMessage);
        setTimeout(function () {
            {{{ makeDynCall('vi', 'callback') }}} (buffer);
            _free(buffer);
        }, timeout);
    }
});

C++ plug-in code

The following code is an example of a C++ file that: * calls the external JavaScript function (JsSetTimeout) * contains the callback function the JavaScript calls when a timeout happens.


// CallbackExample.cpp
#include <stdio.h>

typedef void (*OnTimeOutCallback)(char* message);

extern "C"
{
    void JsSetTimeout(char* message, int timeout, OnTimeOutCallback callback);
}

void CCallback(char* message)
{
    printf("C callback received \"%s\"\n", message);
}

extern "C" void SetCTimeout()
{
    JsSetTimeout("Hello World", 1000, CCallback);
}

Unity C# code

The following code is an example of C# Unity code that receives the messages from the C++ and JavaScript callbacks.

using AOT;
using System;
using System.Text;
using System.Runtime.InteropServices;
using UnityEngine;

public class CallbackExample : MonoBehaviour
{
    void Awake()
    {
        SetCSharpTimeout();
        SetCTimeout();
    }

    void SetCSharpTimeout()
    {
        JsSetTimeout("Hello World", 500, CSSharpCallback);
    }

    [DllImport("__Internal")]
    public static extern void SetCTimeout();    
    
    [DllImport("__Internal")]
    public static extern void JsSetTimeout(string message, int timeout, Action<string> action);

    [MonoPInvokeCallback(typeof(Action<string>))]
    public static void CSSharpCallback(string message)
    {
       Debug.Log($"C# callback received \"{message}\"");
    }
}

Additional resources

Compile a static library as a Unity plug-in

JavaScript interface in Unity Web builds