In Emscripten C++ / wbasm how does one get an "on page closed" event - html

Question

I have C+ program that compiles to web assebmbly using the emscripten system. I would like to clean up some things, flush files, etc etc. when he page running the program is closed.
in main there is:
emscripten_set_main_loop_arg(onMainLoopTick, arg, 0, 1);
Currently when the page closes the "process" is simply exited and does not continue after the "loop simulator". I figure I need to get an event from the page that will block the main thread until the C++ code process it and cleans up it's mess.
What event should I forward to C++ and how should I use it ?

Answer 1

The first things to know is that there is no native library nor APIs for WebAssembly (I mean..yet, as of MVP. There are native features like threads coming as post-MVP feature). What is means that all system libraries in C++ are implemented by importing emulated JavaScript functions. So if you are looking for native features like detecting closing events, you should check if there is JS/HTML5 APIs that do the similar things.
To see how it works, open generated .wast file and search for import instructions and generated JS files. Also, you may want to search on Emscripten repo directly to check if there is JS/HTML5 bindings available on C++ side, as their documentation is quite large and hard to look through.
Sticking to the point, the HTML5 events that are fired when closing are beforeunload and unload. I would prefer using beforeunload event. Emscripten provides em_beforeunload_callback callback function type and emscripten_set_beforeunload_callback to register in html5.h bindings.
Otherwise, you use them directly. For example:
In C++:
void EMSCRIPTEN_KEEPALIVE clean_stuff() {
// Clean up the mess...
// You should use EMSCRIPTEN_KEEPALIVE or
// add it to EXPORTED_FUNCTIONS in emcc compilation options
// to make it callable in JS side.
}
In JS:
window.addEventListener("beforeunload", function (event) {
// Exported functions are prefixed by an underscore
Module._clean_stuff();
});