import { observable } from 'aurelia-framework';

  export class Car {
    @observable color = {rgb: '', hex: ''};

    colorChanged(newValue, oldValue) {
      // this will fire whenever the 'color' property changes
    }
  }

import {bindable, bindingMode} from 'aurelia-framework';

  export class SecretMessageCustomElement {
    @bindable data;

    dataChanged () {
       // -------
    }
  }