Angular Control Value Accessor Building Custom Form Controls

Introduction

Angular's Control Value Accessor (CVA) is a powerful feature that allows developers to create custom form controls and seamlessly integrate them into Angular's reactive forms. In this technical blog, we will delve into the depths of Angular's Control Value Accessor, understanding its purpose and implementation. We will walk through a detailed example to demonstrate how to create and utilize custom form controls effectively.

What is Control Value Accessor ?

The Control Value Accessor acts as a bridge between Angular's reactive forms and custom form controls, facilitating communication between the form control and the underlying DOM element. It ensures synchronization of data and user interactions by providing four essential methods:

a. writeValue: This method is responsible for writing a value to the form control. It allows you to update the form control's value based on external changes.

b. registerOnChange: By implementing this method, you can register a callback function that is invoked whenever the value of the form control changes. It enables you to react to value updates and perform any necessary actions.

c. registerOnTouched: This method allows you to register a callback function that is triggered when the form control is touched, indicating user interaction. It is useful for handling touch-related events and implementing touch-specific behaviors.

d. setDisabledState: This method sets the disabled state of the form control. It provides a way to enable or disable the custom control based on the disabled state of the parent form.

How to Create a Custom Form Control?

To create a custom form control using Control Value Accessor, follow these steps:

Step 1: Implementing the ControlValueAccessor Interface

Create a new class that implements the ControlValueAccessor interface and its required methods: writeValue, registerOnChange, registerOnTouched, and setDisabledState. These methods allow you to define the behavior of your custom form control.

Step 2: Integrating with Angular Forms

To use the custom control within an Angular form, you need to register it as a form control. This can be achieved by using either the `ngModel` or `formControlName` directive, depending on the context of your application.

3. Building a Custom Form Control with Control Value Accessor:

Let's illustrate the process of creating a custom form control with a practical example: a rating input component. This component will display a set of stars, allowing the user to select a rating by clicking on a star.

•  Creating the Component

Create a new component, `RatingInputComponent`, which represents the custom form control. Implement the ControlValueAccessor interface by providing the necessary methods: writeValue, registerOnChange, registerOnTouched, and setDisabledState.

• Template and Styling

Design the template for the `RatingInputComponent`, incorporating the star elements and adding event listeners for user interactions. Utilize CSS to style the stars based on the selected rating and provide visual feedback.

•  Handling User Interactions

Implement event handlers within the `RatingInputComponent` to handle user interactions. For example, you can handle the click event on a star to update the selected rating. Utilize the `registerOnChange` method to notify the form control about value changes.

• Integration with Angular Forms

To use the `RatingInputComponent` in an Angular form, register it as a form control using either the `ngModel` or `formControlName` directives, depending on the form structure. This integration allows the custom form control to seamlessly interact with other form elements.

Here's an example of creating a custom form control using Angular's Control Value Accessor.

 Implementing the ControlValueAccessor Interface First, let's create a custom form control called CustomInputComponent. In this example, we'll create a simple input field that converts input text to uppercase.

In the above code, we implement the ControlValueAccessor interface and provide the necessary methods. The template contains an input field that is bound to the value property using ngModel. The (input) event triggers the onChange method, and the (blur) event triggers the onTouched method.

Now, let's use the CustomInputComponent in an Angular form. Suppose we have a form with a name input field.

in this example, we bind the name property of the form to the CustomInputComponent using [(ngModel)]. The form's name attribute remains the same for validation purposes.

By using the CustomInputComponent, any text entered into the input field will automatically be converted to uppercase.

That's it! You have successfully created a custom form control using Angular's Control Value Accessor. The custom control seamlessly integrates into the Angular form, providing enhanced functionality.

Conclusion:

Angular's Control Value Accessor provides developers with a robust mechanism for creating custom form controls and integrating them smoothly into reactive forms. By implementing the ControlValueAccessor interface, developers can exercise fine-grained control over the behavior and appearance of their form controls, resulting in intuitive and user-friendly forms.