Aurelia Custom Elements data binding

I am using a custom-element with my Aurelia app. When I am binding data from my view-model with the custom-element, it is working fine. Even if I make some changes in the data in the custom-element control, the change is also reflected to the data in my view model, thanks to the two-way data binding.

However, if I make some changes in the data from the view model (using javascript), the data is not updated in the custom-element. I have replicated this problem for a simpler setting.

Simple View Model

export class Playground {

  public testObj: any;
  counter = 0;


  constructor() {
    this.testObj = {
        prop1: "This is prop1 value"           
        , collection2: [{ id: "item21" }]
    }        
  }

  updateProp1() {
    alert("before update: "+this.testObj.prop1);
    this.testObj.prop1 = "Sayan " + this.counter++;
    alert(this.testObj.prop1);
  }

  verifyChange() {
    alert(this.testObj.prop1);
  }
}

Simple View

<template>
<h1>
    Playground
</h1>

<div >         
    <div repeat.for="item of testObj.collection2">
        <div class="row">
            <div class="col-sm-4">
                <input type="text" class="form-control" placeholder="prop1"
                       value.bind="$parent.testObj['prop1']">
            </div>
        </div>
    </div>

    <button click.delegate="updateProp1()" class="btn btn-primary"> Update Prop1 </button>
    <button click.delegate="verifyChange()" class="btn btn-primary"> Verify Change </button>

</div>

</template>

Now whenever I click Verify Change after changing the value in textbox, the changed value comes in the alert. But, if I click the Update Prop1 button, the prop1 value gets updated, but the change doesn't reflect in the view. I am not sure exactly what I am missing.

Note: Instead of using $parent.testObj['prop1'], if $parent.testObj.prop1 is used, the databinding works as it should. However, my actual custom-element is of generic kind and the property name is not known before hand, hence it seems that I need to keep using $parent.testObj['prop1'] notation instead of dot notation: $parent.testObj.prop1.

At pointer/suggestion/feedback is appreciated.

Update: If anyone can point out the the difference between the dot notation and indexer notation w.r.t. aurelia data-binding (I have checked this already), that will be of great help.