I am learning about emscripten and trying to understand it better. As far as I understand the use-case it was mostly designed for is to port existing C/C++-code to a web client (browser) and calling C/C++ code from JavaScript.
But I am wondering whether it is possible to use C++ and Emscripten to web page (note: this is more out of curiosity - I know that there are not many good reasons to do that at the moment). I manage to call Javascript functions from C++ and pass arguments of types string, int, double etc to them. But what I am missing is: calling a Javascript function from C++ and passing a C or C++ function as a handle. So as a simple example: How would I write the following Javascript code ind pure C++?
var myfun = function() { /* do something meaningful here */ }
document.onload(myfun);
TL;DR;
I wrote a library : js-bind which accepts any number of arguments to do that easily :
using namespace std::placeholders;
using emscripten::val;
// First find the HTML object to attach the event to
auto clickme_btn = val::global("document").call<val>("getElementById", string("clickme_btn"));
// Bind the event handler for click
auto onclick = [](val event){ cout << "hello world ! " << endl; };
clickme_btn.set("onclick", js::bind(onclick, _1));
This library is some Macro metaprogramming based on the explanation below.
DETAILED ANSWER:
You have different possibilites, like emscripten ccall, but what is easier to use in my opinion is Embind.
For example take binding event handlers of an XMLHttpRequest from within C++.
To enable it you have to compile with : --bind -s NO_EXIT_RUNTIME=1
Emscripten : bind freestanding functions
One could achieve it easily with freestanding functions and a singleton, as show here :
#include <iostream>
#include <emscripten.h>
#include <emscripten/bind.h>
#include <emscripten/val.h>
namespace xhr {
inline emscripten::val& singleton() {
using emscripten::val;
static val instance = val::global("XMLHttpRequest").new_();
return instance;
}
void on_load(emscripten::val event) {
std::cout << "Successful Query " << std::endl;
std::cout << "response is : " << singleton()["responseText"].as<std::string>() << std::endl;
}
void on_error(emscripten::val event) {
std::cout << "Error on query " << std::endl;
}
void on_progress(emscripten::val event) {
std::cout << "Progress on query " << std::endl;
std::cout << event["lengthComputable"].as<bool>() << ": " << event["loaded"].as<unsigned int>() / event["total"].as<unsigned int>() << std::endl;
}
using namespace emscripten;
EMSCRIPTEN_BINDINGS(xhr) {
function("on_load", &on_load);
function("on_error", &on_error);
function("on_progress", &on_progress);
}
}
int main(int argc, char** argv) {
using emscripten::val;
xhr::singleton().call<val>("open", std::string("GET"), std::string("http://127.0.0.1:8080/CMakeCache.txt"), true);
// Here I set the callback to &on_load function registered via the EMSCRIPTEN_BINDINGS macro.
xhr::singleton().set("onload",val::module_property("on_load"));
xhr::singleton().set("onprogress",val::module_property("on_progress"));
xhr::singleton().set("onerror",val::module_property("on_error"));
xhr::singleton().call<val>("send");
return 0;
}
Emscripten : bind member functions
Typically in C++ we are used to std::bind callbacks. This can also be achieved, taking the example of xhr in a cleaner way :
#include <iostream>
#include <functional>
#include <memory>
#include <emscripten.h>
#include <emscripten/bind.h>
#include <emscripten/val.h>
class MiniXhr : public std::enable_shared_from_this<MiniXhr> {
using val = emscripten::val;
using url_t = std::string;
public:
void set_url(const url_t& url) { url_ = url; }
void GET();
/**
*
* The member function to be called from javascript.
*/
void on_readystate(val event) {
std::cout << "ready " << std::endl;
std::cout << "xxhr::on_readystate: "
<< xhr["readyState"].as<size_t>() << " - " << url_ << " :: "
<< xhr["status"].as<size_t>() << ": "
<< xhr["statusText"].as<std::string>() << std::endl;
}
private:
url_t url_;
val xhr = val::global("XMLHttpRequest").new_();
};
using emscripten::class_;
EMSCRIPTEN_BINDINGS(MiniXhr) {
/**
* Binding for the class.
*/
class_<MiniXhr>("MiniXhr")
.smart_ptr<std::shared_ptr<MiniXhr>>("shared_ptr<MiniXhr>")
.function("on_readystate", &MiniXhr::on_readystate)
;
/**
* More generic binding to bind a functor with one argument (event handler get the event)
* Here std::function call operator from C++ is bound to function opcall() in JS.
*/
class_<std::function<void(emscripten::val)>>("VoidValFunctor")
.constructor<>()
.function("opcall", &std::function<void(emscripten::val)>::operator());
}
/**
*
* Finally the interesting part : binding the member function on_readystate to the readystatechange event of XMLHttpRequest.
*
*/
void MiniXhr::GET() {
/**
* Here this lambda could be put as function in a library, to do an JS(std::bind),
* it should just be overloaded for different argument count. (Im on it).
*/
auto jsbind = [](val& target, const char* property, auto bind_expression ) {
// Create an std::function from the bind expression
std::function<void(emscripten::val)> functor = bind_expression;
// We ensure the correct object will always be bound to the this of the function
auto functor_adapter = val(functor)["opcall"].call<val>("bind", val(functor));
// Finally we simply set the eventhandler
target.set(property, functor_adapter);
};
// Here we could bind as many member function as we want.
// jsbind(xhr, "onload", std::bind(&MiniXhr::on_load, shared_from_this(), std::placeholders::_1));
// jsbind(xhr, "onerror", std::bind(&MiniXhr::on_error, shared_from_this(), std::placeholders::_1));
// jsbind(xhr, "onprogress", std::bind(&MiniXhr::on_progress, shared_from_this(), std::placeholders::_1));
jsbind(xhr, "onreadystatechange", std::bind(&MiniXhr::on_readystate, shared_from_this(), std::placeholders::_1));
// Note that we bind with shared_from_this(), as the scope where the class was instantiated may be dead
// and only later our callback will come back.
xhr.call<val>("open", std::string("GET"), url_, true);
xhr.call<val>("send");
}
int main(int argc, char** argv) {
auto x = std::make_shared<MiniXhr>();
x->set_url("notfound.json");
x->GET();
return 0;
}
Here's something I used a long while back when tinkering w/ Emscripten in C code:
void myfun(void(*f)(void)) { (*f)() }
and then here would be the JavaScript:
var theparty = Runtime.addFunction(function() { print("Will there be confetti?") });
Module.ccall("myfun", "number", ["number"], [theparty]);
Runtime.removeFunction(theparty); // output => Will there be confetti?
I always remove a function that is no longer needed after its execution to preserve memory.
This is a simple and seamless way to make code bits work together. You can obviously modify this to do whatever you want other than printing out information. :P
I'm not sure about emscripten, but to sum up, I understand that what you need to know is how to pass a C++ function as a handle to another C++ function. I hope I can help with that.
JavaScript, PHP, and other more flexible languages, allow a function object to be passed through. In C and C++, it is slightly different, you have to pass function pointers as arguments to other functions. In C, the name for this is a Callback, rather than a handle.
For instance:
/* This function takes a single callback as a parameter. */
//here we say that the parameter, that we name numberSource, is a function that receives no parameters itself (void), and return an int
void printNumber(int (*numberSource)(void)) {
printf("%d", numberSource());
}
/* A possible callback */
int oneExampleFunction(void) {
return 100;
}
/* Another possible callback. */
int otherExampleFunction(void) {
return 200;
}
/* This is how we would call printNumber with three different callbacks. */
//with "&" we are referencing the memory address of the function,
//since thats what printNumber is expecting
printNumber(&oneExampleFunction);
printNumber(&otherExampleFunction);
printNumber(&rand); //where are using rand(), a system function, that works as well.
It is a common practice to create a custom type to the argument, so you don't need to use something as ugly as int (*numberSource)(void). It will be something like:
//Function pointer called CallbackType that takes a float
//and returns an int
typedef int (*NameYouWantForTheType)(void);
So the printNumber function would be like this:
void printNumber(NameYouWantForTheType numberSource ) {
printf("%d", numberSource());
}
So, in your case,
if you want to translate this JS code
var myfun = function() { /* do something meaningful here */ }
document.onload(myfun);
to C, and you have a C object called "document" that receives a function that performs some other actions, your C code will be:
void myfun (void) {
/* do something meaningful here */
}
document.onload(&myfun);
