Angular 2 - Troubleshooting [(ngModel)] Not Refreshing When [Value] Changes

Question

Angular 2 - Troubleshooting [(ngModel)] Not Refreshing When [Value] Changes

In my code, I am successfully setting the value of an input by calculating two other ngModels. However, despite the input value updating, the ngModel itself remains unchanged. Take a look at the snippet below:

<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>

Initially, {{product.totalPrice}} displays the correct value. Manually changing the input updates the expression correctly as well. However, when the input is set programmatically based on two other inputs, the label's expression does not update. What could be causing this issue?

It's puzzling because although the input value updates visibly, the expression {{product.totalPrice}} does not reflect these changes. It seems like the input value updates due to changes in other fields, but these updates do not affect the ngModel bindings.

For reference, I am working with Ionic 2

angular typescript ionic2 ngmodel

Answer 1

Answer №1

It seems like I may be fashionably late to the gathering, but since none of the existing answers offer a correct solution, I've decided to provide one in case someone else stumbles upon this page.

When developing a custom component that utilizes ngModel, it is essential for the component to implement the ControlValueAccessor interface, which is outlined below:

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

To acknowledge a change made by the component, it must invoke the onChange method offered by the registerOnChange function.

For instance, if we define our onChange method as follows:

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

Whenever the component alters the value, we need to execute the following line:

this.onChange(this.value)

I trust this explanation proves helpful.

Edit

Upon revisiting my initial response to this query, it appears that there might have been confusion about a custom child component not updating the parent component. Reassessing the situation now suggests that either there was an issue with the ionic-input or the original poster was misusing it.

This updated answer provides a clearer approach on creating a component and ensuring it updates a property of its parent using NgModel.

Answer 2

It seems like I may be fashionably late to the gathering, but since none of the existing answers offer a correct solution, I've decided to provide one in case someone else stumbles upon this page.

When developing a custom component that utilizes ngModel, it is essential for the component to implement the ControlValueAccessor interface, which is outlined below:

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

To acknowledge a change made by the component, it must invoke the onChange method offered by the registerOnChange function.

For instance, if we define our onChange method as follows:

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

Whenever the component alters the value, we need to execute the following line:

this.onChange(this.value)

I trust this explanation proves helpful.

Edit

Upon revisiting my initial response to this query, it appears that there might have been confusion about a custom child component not updating the parent component. Reassessing the situation now suggests that either there was an issue with the ionic-input or the original poster was misusing it.

This updated answer provides a clearer approach on creating a component and ensuring it updates a property of its parent using NgModel.

Answer 3

Answer №2

Essentially, the use of [] signifies binding data, while () indicates emitting changes or triggering an event with those changes from a UI control like <ion-input>. In essence, [()"> does not denote two-way data binding; rather, it signifies:

Binding data using []
Raising input changes with ().

Take a look at this example to see various ways of binding data with input and how to trigger changes.

Answer 4

Essentially, the use of [] signifies binding data, while () indicates emitting changes or triggering an event with those changes from a UI control like <ion-input>. In essence, [()"> does not denote two-way data binding; rather, it signifies:

Binding data using []
Raising input changes with ().

Take a look at this example to see various ways of binding data with input and how to trigger changes.

Answer 5

Answer №3

You may want to consider utilizing the (ngModelChange) directive to actively monitor changes in value and update the product.totalPrice. The implementation would resemble the following:

<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>
        (ngModelChange)="product.totalPrice = $event"
</ion-item>

If this component is contained within a <form> element and ngForm is aliased as #someForm="ngForm", you can utilize a template reference variable and incorporate it into the label, like so:

<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 }"
        name="totalPrice"
        #totalPrice="ngModel"></ion-input>
</ion-item>

I hope this guidance proves beneficial.

Answer 6

You may want to consider utilizing the (ngModelChange) directive to actively monitor changes in value and update the product.totalPrice. The implementation would resemble the following:

<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>
        (ngModelChange)="product.totalPrice = $event"
</ion-item>

If this component is contained within a <form> element and ngForm is aliased as #someForm="ngForm", you can utilize a template reference variable and incorporate it into the label, like so:

<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 }"
        name="totalPrice"
        #totalPrice="ngModel"></ion-input>
</ion-item>

I hope this guidance proves beneficial.

Answer 7

Answer №4

Encountering a challenge, I discovered that the two-way binding was not updating the model when modifying the input in the field. It turned out that the property being bound to was actually read-only. To uncover this issue, I decided to separate the two-way binding as follows:

[ngModel]="data.name" (ngModelChange)="adjustData($event)"

Additionally, here is the method:

adjustData($event) { data.name = $event }

This change revealed the read-only exception that was causing the problem.

Answer 8

Encountering a challenge, I discovered that the two-way binding was not updating the model when modifying the input in the field. It turned out that the property being bound to was actually read-only. To uncover this issue, I decided to separate the two-way binding as follows:

[ngModel]="data.name" (ngModelChange)="adjustData($event)"

Additionally, here is the method:

adjustData($event) { data.name = $event }

This change revealed the read-only exception that was causing the problem.

Answer 9

Answer №5

@ViewChild('textInput') textInput: any; 

this.textInput.valueChanges.subscribe((value: any) => {
    this.onInput(value);
    }

Answer 10

@ViewChild('textInput') textInput: any; 

this.textInput.valueChanges.subscribe((value: any) => {
    this.onInput(value);
    }

Angular 2 - Troubleshooting [(ngModel)] Not Refreshing When [Value] Changes

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Similar questions

Imitate a targeted ngxs store selection using ngxs

Steps to disable TypeScript error messages for unused variables

Unable to set up Typescript: The package v1.2.3 cannot meet the peerDependency requirements of its siblings

The JSON property is showing as undefined despite the fact that the variable holding the data is filled with a JSON object

The function is triggered by the button depending on its origin

Angular 6 - Accessing grandparent methods in grandchild components

Tips for aligning content produced by *ngFor within a bootstrap grid

"Utilizing Angular's Reactive Forms to Set a Default Object in

Error in Angular FormArray: Unable to access the 'push' property because it is undefined

Create the correct structure for an AWS S3 bucket

How to toggle tooltip visibility dynamically using Angular 2

Error: Unable to access the 'DASH' property as it is undefined

Do other effects promptly process actions dispatched by one ngrx Effects effect?

Aurelia validation is failing to work properly when the form is already populated with data

How can I properly containerize an Express Gatsby application with Docker?

Verify the completeness of data types within an array in typescript

Upgrading from Angular v6 to v8: Troubleshooting the issue of "The loader "datatable.component.css" did not return a string error"

Is there a method to uncover the code that controls the display of a <div> element?

Transitions fail to appear correctly when the page first loads

How can I capture the logs from Sentry and send them to my own custom backend system?