How was emscripten_set_wheel_callback_on_thread generated in wasm file

We had met a bug about _emscripten_set_wheel_callback_on_thread in web app based on wasm. Relative post: https://www.weiy.city/2025/02/uncaught-typeerror-cannot-read-properties-of-null-reading-onwheel-from-_emscripten_set_wheel_callback_on_thread/

Let’s explore how was the function generated in the final wasm file.

The function _emscripten_set_wheel_callback_on_thread was from emscripten_set_wheel_callback_on_thread in src/library_html5.js. Read js files and html5.h under emsdk to find the called logic.

addToLibrary(LibraryHtml5WebGL); // in src\library_addfunction.js

<---

// emscripten_webgl_init_context_attributes: The method is usually called when a thread starts and exits.
// Example：https://cocalc.com/github/emscripten-core/emscripten/blob/main/test/gl_in_mainthread_after_pthread.c

// Execute in calling thread without proxying needed.
emscripten_webgl_init_context_attributes: (attributes) => {   // in src\library_html5_webgl.js

<---

emscripten_webgl_do_create_context__deps: [  // in src\library_html5_webgl.js
$JSEvents:{
	//...
	emscripten_set_wheel_callback_on_thread:  // src\library_html5.js
}
]


addToLibrary: convert these JavaScript functions into WebAssembly, which is a bytecode instruction format similar to assembly language

Code in src\library_addfunction.js:

addToLibrary will convert these js functions to assembly format in the final wasm file. emscripten_set_wheel_callback_on_thread will be added to a thread when it starts and exits by emscripten_webgl_init_context_attributes.

If we changed vtkRenderWindow mannaully, the old window need to be handle by oldSDLWin->Finalize(); to do ReleaseGraphicsResources and make contextId correctly.

//如何正确设置ContextId？那我们就去调用 vtkSDL2OpenGLRenderWindow::Clean

void vtkSDL2OpenGLRenderWindow::Finalize() //vtkSDL2OpenGLRenderWindow 析构函数中被调用
{
  this->DestroyWindow();
}
<---
void vtkSDL2OpenGLRenderWindow::DestroyWindow()
{
  this->Clean();
  if (this->WindowId)
  {
    SDL_DestroyWindow(this->WindowId);
    this->WindowId = nullptr;
  }
}
<---
void vtkSDL2OpenGLRenderWindow::Clean()
{
  /* finish OpenGL rendering */
  if (this->OwnContext && this->ContextId)
  {
    this->MakeCurrent();
    this->CleanUpRenderers();
    SDL_GL_DeleteContext(this->ContextId);
  }
  this->ContextId = nullptr;
}

Then use vtkRenderWindowInteractor::ReInitialize to bind new render window and interactor and set context correctly.

vtkRenderWindowInteractor:
  void ReInitialize()
  {
    this->Initialized = 0;
    this->Enabled = 0;
    this->Initialize();
  }

<---

void vtkOpenGLRenderWindow::Start()

void vtkSDL2OpenGLRenderWindow::Initialize()

<---

SDL_GL_CreateContext