Server side onchange event for custom class

Question

If I create a custom class in Google Apps Script and assign it to a variable, can I create a server side onchange event that will react when the values change? For example, something like:

    var Polygon = function(height, width) {
      this.height = height;
      this.width = width;
      this.save = function() { <code to draw the polygon here ...> };
    }
    Polygon.onchange = function() {
      currentPolygon = this.value;
      currentPolygon.draw();
    }
    var myPolygon = new Polygon(10, 12);
    myPolygon.height = 20; // triggers draw

Or, must it be included in a set function? For example:

    var Polygon = function(height, width) {
      var myHeight = height;
      var myWidth = width;
      this.height = function() { return myHeight; }
      this.width = function() { return myWidth; }
      this.draw = function() { <code to draw the polygon here ...> };
      this.changeHeight = function(value) {
        myHeight = value;
        this.draw();
      }
      this.changeWidth = function(value) {
        myWidth = value;
        this.draw();
      }
    }
    var myPolygon = new Polygon(10, 12);
    myPolygon.changeHeight(20);

Answer 1

There is no such handler. But you can use a proxy to intercept all set calls:

/*<ignore>*/console.config({maximize:true,timeStamps:false,autoScroll:false});/*</ignore>*/

const Polygon = function(height, width) {
  this.height = height;
  this.width = width;
  this.drawn = 0;
  this.draw = function() {
    this.drawn += 1;
  };
};
const PolygonOnchangeHandler = {
  set(target, prop, value) {
    Reflect.set(target, prop, value);//calls set
    Reflect.apply(target.draw, target, []);//calls draw
  },
};
const myPolygon = new Proxy(new Polygon(10, 12), PolygonOnchangeHandler);
myPolygon.height = 20; // trigges draw
console.log(myPolygon.drawn);//drawn once
myPolygon.width = 5;
console.log(myPolygon.drawn);//drawn twice

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