How to listen for value changes from class property TypeScript - Angular

You can still check component's field members value change by KeyValueDiffers via DoCheck lifehook.

import { DoCheck, KeyValueDiffers, KeyValueDiffer } from '@angular/core';

differ: KeyValueDiffer<string, any>;
constructor(private differs: KeyValueDiffers) {
  this.differ = this.differs.find({}).create();
}

ngDoCheck() {
  const change = this.differ.diff(this);
  if (change) {
    change.forEachChangedItem(item => {
      console.log('item changed', item);
    });
  }
}

see demo.

I think the nicest solution to your issue is to use a decorator that replaces the original field with a property automatically, then on the setter you can create a SimpleChanges object similar to the one created by angular in order to use the same notification callback as for angular (alternatively you could create a different interface for these notifications, but the same principle applies)

import { OnChanges, SimpleChanges, DoCheck, SimpleChange } from '@angular/core';

function Watch() : PropertyDecorator & MethodDecorator{
    function isOnChanges(val: OnChanges): val is OnChanges{
        return !!(val as OnChanges).ngOnChanges
    }
    return (target : any, key: string | symbol, propDesc?: PropertyDescriptor) => {
        let privateKey = "_" + key.toString();
        let isNotFirstChangePrivateKey = "_" + key.toString() + 'IsNotFirstChange';
        propDesc = propDesc || {
            configurable: true,
            enumerable: true,
        };
        propDesc.get = propDesc.get || (function (this: any) { return this[privateKey] });

        const originalSetter = propDesc.set || (function (this: any, val: any) { this[privateKey] = val });

        propDesc.set = function (this: any, val: any) {
            let oldValue = this[key];
            if(val != oldValue) {
                originalSetter.call(this, val);
                let isNotFirstChange = this[isNotFirstChangePrivateKey];
                this[isNotFirstChangePrivateKey] = true;
                if(isOnChanges(this)) {
                    var changes: SimpleChanges = {
                        [key]: new SimpleChange(oldValue, val, !isNotFirstChange)
                    }
                    this.ngOnChanges(changes);
                }
            }
        }
        return propDesc;
    }
}

// Usage
export class MyClass implements OnChanges {


    //Properties what I want to track !
    @Watch()
    myProperty_1: boolean  =  true
    @Watch()
    myProperty_2 =  ['A', 'B', 'C'];
    @Watch()
    myProperty_3 = {};

    constructor() { }
    ngOnChanges(changes: SimpleChanges) {
        console.log(changes);
    }
}

var myInatsnce = new MyClass(); // outputs original field setting with firstChange == true
myInatsnce.myProperty_2 = ["F"]; // will be notified on subsequent changes with firstChange == false