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

Learn how to use callbacks to communicate between your plug-in, browser, and Unity project.
Read time 3 minutesLast updated 10 days ago
Callbacks are an important part of web development because they let your Unity project communicate better with your JavaScript browser, JavaScript plug-in, 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). This signature tells the underlying Emscripten toolchain what data types the function returns and accepts as parameters. For a list of all supported type characters, refer to the dyncall library documentation.
  • 'callback'
    is the name of the callback function.
  • (1, 2)
    are the arguments 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 scripts 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