将C++函数传递给emscripten中的javascript函数

Question

我正在学习emscripten并试图更好地理解它.据我所知,用例主要用于将现有的C/C++代码移植到Web客户端(浏览器)并从JavaScript调用C/C++代码.

但我想知道是否有可能将C++和Emscripten用于网页(注意:这更多是出于好奇 - 我知道目前没有太多合理的理由).我设法从C++调用Javascript函数,并将类型为string,int,double等的参数传递给它们.但我缺少的是:从C++调用Javascript函数并将C或C++函数作为句柄传递.所以作为一个简单的例子:我如何编写以下Javascript代码ind纯C++？

var myfun = function() { /* do something meaningful here */ }
document.onload(myfun);

Answer 1

TL; 博士;

我写了一个库：js-bind，它接受任意数量的参数来轻松做到这一点：

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));

这个库是一些基于以下解释的宏元编程。

详细回答：

你有不同的可能性，比如 emscripten ccall，但在我看来更容易使用的是 Embind。

例如，从 C++ 中获取 XMLHttpRequest 的绑定事件处理程序。

要启用它，您必须编译： --bind -s NO_EXIT_RUNTIME=1

Emscripten：绑定独立函数

可以使用独立函数和单例轻松实现它，如下所示：

#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 : 绑定成员函数

通常在 C++ 中，我们习惯于 std::bind 回调。这也可以实现，以更简洁的方式以 xhr 为例：

#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;
}

Answer 2

我不确定 emscripten，但总而言之，我知道您需要知道的是如何将 C++ 函数作为句柄传递给另一个 C++ 函数。我希望我能帮忙。

JavaScript、PHP 和其他更灵活的语言允许传递函数对象。在 C 和 C++ 中，略有不同，您必须将函数指针作为参数传递给其他函数。在 C 中，这个名称是回调，而不是句柄。

例如：

/* 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. 

为参数创建自定义类型是一种常见的做法，因此您不需要使用像int (*numberSource)(void). 它会是这样的：

//Function pointer called CallbackType that takes a float
//and returns an int
typedef int (*NameYouWantForTheType)(void);  

所以 printNumber 函数将是这样的：

void printNumber(NameYouWantForTheType numberSource ) {
    printf("%d", numberSource());
}

所以，在你的情况下， 如果你想翻译这个 JS 代码

var myfun = function() { /* do something meaningful here */ }
document.onload(myfun);

到 C，并且您有一个名为“document”的 C 对象，该对象接收执行其他一些操作的函数，您的 C 代码将是：

void myfun (void) {
  /* do something meaningful here */
}

document.onload(&myfun);