Angular 2 - [(ngModel)] not updating after [value] changes

Question

I'm setting the the value of an input calculating two other ngModels, and that seems to be working fine. But if I inspect my ngModel, it doesn't change at all. Let me show you:

<ion-item>
    <ion-label>Total price: {{product.totalPrice}}</ion-label>
    <ion-input 
        type="number"
        [value]="product.quantity * product.price"
        [(ngModel)]="product.totalPrice" 
        [ngModelOptions]="{ standalone: true }"></ion-input>
</ion-item>

So here {{product.totalPrice}} shows the initial value, which is fine. If I change manually that input, the changes will be reflected on the expression, that is also fine. But that input will be readonly and it will be set by changing two other inputs. When I change them, I see the value on the input is updating just fine, but not the expression in the label. What's wrong there?

It's really weird because the value in the input GETS UPDATED, but not the expression {{product.totalPrice}}, I guess the value is updating because the other fields are, but those value changes never actually hit the ngModel

By the way, I'm using Ionic 2

Answer 1

Actually [] means bind data and () mean emit changes / or let say raise an event with these changes form this UI control <ion-input>. So [()] doesn't mean two way data binding. It means:

• bind data using []
• raise input changes ().

Check this example it shows many ways of binding data with input and how to raise changes.

Answer 2

So I know that I'm late to the party, but seeing as none of the other answers are correct, I thought I'd add the solution in case anyone else ends up on this page.

When creating a custom component that accepts ngModel, the component must implement the ControlValueAccessor interface. Which is detailed below

interface ControlValueAccessor {  
  writeValue(obj: any): void
  registerOnChange(fn: any): void
  registerOnTouched(fn: any): void
  setDisabledState(isDisabled: boolean): void
}

In order to register a change made by the component, the component must call the onChange method provided by the registerOnChange method.

For example, if we register our onChange method like so:

 onChange: any = () => { };
 registerOnChange(fn) {
   this.onChange = (obj) => fn(obj);
 }

When ever our component makes a change to the value, we must execute the following line

this.onChange(this.value)

Hopes this helps.

Edit

When I first answered this question, for some reason I was under the impression that a custom child component was not updating the parent component. Rereading it now, it seems that either there was an issue with the ionic-input or the OP was incorrectly using it.

This answer better suits how one would create a component and have it update a property of it's parent using NgModel