Derive a C++ class in JavaScript

Override Message_ProgressIndicator virtual methods through a file-backed raw Embind registration.

OCCT progress callbacks require a derived Message_ProgressIndicator class. That needs Embind's allow_subclass, so it is a customBindings entry with scope: 'main' — raw Embind — rather than a generated binding.

1. Add one self-contained binding file

Create bindings/progress-indicator.cpp with the bridge class, wrapper, and registration in the same file:

#include <Message_ProgressIndicator.hxx>
#include <emscripten/bind.h>

using namespace emscripten;

struct Message_ProgressIndicator_JS : public Message_ProgressIndicator {
  using Message_ProgressIndicator::Show;
  using Message_ProgressIndicator::UserBreak;
  using Message_ProgressIndicator::Reset;
};

struct Message_ProgressIndicator_JSWrapper
    : public wrapper<Message_ProgressIndicator_JS> {
  EMSCRIPTEN_WRAPPER(Message_ProgressIndicator_JSWrapper);

  void Show(const Message_ProgressScope&, bool isForce) {
    call<void>("Show", GetPosition(), isForce);
  }
  bool UserBreak() { return call<bool>("UserBreak"); }
  void Reset() { call<void>("Reset"); }
};

EMSCRIPTEN_BINDINGS(progress_indicator_js) {
  class_<Message_ProgressIndicator_JS, base<Message_ProgressIndicator>>(
      "Message_ProgressIndicator_JS")
    .function("Show", &Message_ProgressIndicator_JS::Show, pure_virtual())
    .function("UserBreak", optional_override([](Message_ProgressIndicator_JS& self) {
      return self.Message_ProgressIndicator_JS::UserBreak();
    }))
    .function("Reset", optional_override([](Message_ProgressIndicator_JS& self) {
      self.Message_ProgressIndicator_JS::Reset();
    }))
    .allow_subclass<Message_ProgressIndicator_JSWrapper>(
      "Message_ProgressIndicator_JSWrapper");
}

The repository's executable Docker fixture contains the complete production registration, including inherited methods needed by its runtime test.

2. Declare the file

libcascade.config.ts

import { defineBuild } from '@libcascade/toolchain';

export default defineBuild({
  name: 'progress-callback',
  bindings: [
    'BRepAlgoAPI_Fuse',
    'BRepPrimAPI_MakeBox',
    'Message_ProgressIndicator',
    'Message_ProgressRange',
    'Message_ProgressScope',
    'Message_ProgressIndicator_JS',
    'gp_Pnt',
  ],
  customBindings: [
    {
      file: 'bindings/progress-indicator.cpp',
      symbols: ['Message_ProgressIndicator_JS'],
      scope: 'main',
    },
  ],
  variants: [{ name: 'single' }],
});

The registration name goes in symbols, which is what admits it to bindings without it being an OcctSymbol. Build with npx libcascade build — see the Quickstart. Raw Embind files skip the bindgen, so add a project-local TypeScript declaration for Message_ProgressIndicator_JS if the consumer is TypeScript.

3. Derive in JavaScript

import { createInstance } from './dist/init.js';

const oc = await createInstance();
const Progress = oc.Message_ProgressIndicator_JS.extend('Progress', {
  Show(position) {
    console.log('progress', position);
  },
  UserBreak() {
    return false;
  },
});

using progress = new Progress();
// Pass progress.Start() to a long-running OCCT algorithm.

See Extend with C++ for the distinction between generated and raw bindings.

Derive a C++ class in JavaScript

1. Add one self-contained binding file

2. Declare the file

3. Derive in JavaScript

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