Step 2: Derive a Custom Progress Indicator Class

In this step, we will derive a specialization from Message_ProgressIndicator_JS and use it in JS.

Again following Emscripten's documentation, we create MyProgressIndicator as a specialization of Message_ProgressIndicator_JS (and therefore Message_ProgressIndicator) and implement our own logic for Show. We can then use it in certain OpenCascade API's like BRepAlgoAPI_Fuse in this example.

const MyProgressIndicator = oc.Message_ProgressIndicator_JS.extend("Message_ProgressIndicator_JS", {
  Show: function (theScope, isForce) {
    console.log("Show", this.GetPosition());
  },
});
const p = new MyProgressIndicator();
const box1 = new oc.BRepPrimAPI_MakeBox_3(new oc.gp_Pnt_3(0, 0, 0), 2, 1, 1);
const box2 = new oc.BRepPrimAPI_MakeBox_3(new oc.gp_Pnt_3(1, 0, 0), 2, 1, 1);
const myBody = new oc.BRepAlgoAPI_Fuse_3(box1.Shape(), box2.Shape(), p.Start_1());

Note, how you can call this.GetPosition() from that method's body, i.e. this gives you access to the classes properties, as you would expect. This code gives the following output:

Show 0
Show 0.035
...
Show 0.9955000000000034
Show 1