Angular

Using Custom Async Validators in Angular Reactive Forms

Published Dec 14, 2021

Updated Feb 13, 2023

4 min read

This article was written over 18 months ago and may contain information that is out of date. Some content may be relevant but please refer to the relevant official documentation or available resources for the latest information.

What is a validator in Angular?

First, let's talk about Validators. What is a validator, and how we can use them to improve our forms.

Ocasionally, we want to validate some input in our fields before submission, which will be validated against an asynchronous source. For instance, you may want to check if a label or some data exists before submission. In Angular, you can do this using Async Validators.

Create a basic application

We are going to create a very minimalist form that does one thing: check if the username already exists. In this case, we are going to use a mix of async and sync validators to accomplish our task.

It's very common, in most registration forms, to require an unique username. Using an API call, we to capture if that username is already in use according to the database.

import { Component } from '@angular/core';
import { FormBuilder, Validators } from '@angular/forms';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent {
  constructor(private fb: FormBuilder) {}

  registrationForm = this.fb.group({
    name: [null, [Validators.minLength(3), Validators.required]],
    username: [null, [Validators.minLength(3), Validators.required]],
  });
}

Created a basic form that has a FormGroup called registrationForm, with 2 FormControl. Also, I added 2 sync validators for make the input required and also to check if the input has a minimum length, using Validators.required and Validators.minLength in each case.

<p>Using Custom Async Validators</p>

<div class="container" [formGroup]="registrationForm">
  <mat-form-field class="example-full-width">
    <mat-label>Name</mat-label>
    <input matInput placeholder="Ex. John Doe" formControlName="name" />
  </mat-form-field>

  <mat-form-field class="example-full-width">
    <mat-label>Username</mat-label>
    <input matInput placeholder="Ex. Batman" formControlName="username" />
  </mat-form-field>

  <div class="button-container">
    <button
      mat-raised-button
      color="primary"
      [disabled]="!registrationForm.valid"
    >
      Submit
    </button>
  </div>
</div>

I'm using @angular/material just to simplify our styles, and because it provides a clean way to display errors using the mat-error component. This component will be shown to the user if the required error is present. The button also checks if the FormGroup is valid, otherwise it will be disabled.

Async validators

Our current app works pretty well, and in most cases, will meet all of the requirements. But what if we want to make sure that the username is unique before allowing the user to submit their information?. Creating an Async Validator could be simple. Let's find how to create it, and add it to our current form.

import { Injectable } from '@angular/core';
import { of } from 'rxjs';
import { delay } from 'rxjs/operators';

@Injectable({
  providedIn: 'root',
})
export class UserService {
  private existingUsernames = ['Batman', 'Superman', 'Joker', 'Luthor'];

  checkIfUsernameExists(value: string) {
    return of(this.existingUsernames.some((a) => a === value)).pipe(
      delay(1000)
    );
  }
}

Our method to check if the username already exists is called checkIfUsernameExists. It returns an observable with a 5 seconds delay to simulate a very slow API call. Now, we can create our Async Validator to check if the username exists against that method.

import {
  AbstractControl,
  AsyncValidatorFn,
  ValidationErrors,
} from '@angular/forms';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
import { UserService } from './user.service';

export class UsernameValidator {
  static createValidator(userService: UserService): AsyncValidatorFn {
    return (control: AbstractControl): Observable<ValidationErrors> => {
      return userService
        .checkIfUsernameExists(control.value)
        .pipe(
          map((result: boolean) =>
            result ? { usernameAlreadyExists: true } : null
          )
        );
    };
  }
}

Our UsernameValidator class takes our UserService as an argument. This method returns a AsyncValidatorFn which receives the FormControl that is placed on, providing us access to the current value.

An AsyncValidatorFn must return either a promise or an Observable of type ValidationErrors.

We use the RxJS map operator to check the value emitted, and either return null if the user doesn't exist, or return a ValidationError with an error type of usernameAlreadyExists.

import { Component } from '@angular/core';
import { FormBuilder, Validators } from '@angular/forms';
import { UserService } from './user.service';
import { UsernameValidator } from './username-validator';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss'],
})
export class AppComponent {
  constructor(private fb: FormBuilder, private userService: UserService) {}

  registrationForm = this.fb.group({
    name: [null, [Validators.minLength(3), Validators.required]],
    username: [
      null,
      [Validators.minLength(3), Validators.required],
      [UsernameValidator.createValidator(this.userService)],
    ],
  });
}

We import our UsernameValidator and UserService into our component, and declare in the constructor component.

The UsernameValidator is added to our FormControl as the third parameter, calling the createValidator method, and passing a reference to the UserService.

<mat-form-field class="example-full-width">
  <mat-label>Username</mat-label>
  <input matInput placeholder="Ex. Batman" formControlName="username" />
  <mat-error
    *ngIf="registrationForm.get('username').hasError('usernameAlreadyExists')"
  >
    Username already <strong>exists</strong>
  </mat-error>
</mat-form-field>

We update our template to check for an additional error called usernameAlreadyExists, so if our UserService finds that the username already exists, it will provide a to the user based on her status.

The validation status of the control. There are four possible validation status values:

VALID: This control has passed all validation checks.

INVALID: This control has failed at least one validation check.

PENDING: This control is in the midst of conducting a validation check.

DISABLED: This control is exempt from validation checks

Let's see StackBlitz in action.

Conclusion

Creating beautiful forms for our web app might seem simple, but it's typically more complicated than one would expect. Sometimes, we also need to verify the information, because we don't want to blindly send data to the backend, and then reject it. This can lead to a poor experience for users and result in low adoption rates.

Creating an AsyncValidator can help us improve the user-experience, and also avoid sending data to the backend, resulting in a rejection.

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About the author(s)

Daian Scuarissi
@asdaian @SDaian

Overview of the New Signal APIs in Angular

Overview of the New Signal APIs in Angular Google's Minko Gechev and Jeremy Elbourn announced many exciting things at NG Conf 2024. Among them is the addition of several new signal-based APIs. They are already in developer preview, so we can play around with them. Let's dig into it, starting with signal-based inputs and the new matching outputs API. Signal Based Inputs Discussions about signal-based inputs have been taking place in the Angular community for some time now, and they are finally here. Until now, you used the @Input() decorator to define inputs. This is what you'd have to write in your component to declare an optional and a required input: ` With the new signal-based inputs, you can write much less boilerplate code. Here is how you can define the same inputs using the new syntax: ` It's not only less boilerplate, but because the values are signals, you can also use them directly in computed signals and effects. That, effectively, means you get to avoid computing combined values in ngOnChanges or using setters for your inputs to be able to compute derived values. In addition, input signals are read-only. The New Output I intentionally avoid calling them signal-based outputs because they are not. They still work the same way as the old outputs. The Angular team has just introduced a new output API that is more consistent with the latest inputs API and allows you to write less boilerplate, similar to the new input API. Here is how you would define an output until now: ` Here is how you can define the same output using the new syntax: ` The thing I like about the new output API is that sometimes it happens to me that I forget to instantiate the EventEmitter because I do this instead: ` You won't forget to instantiate the output with the new syntax because the output function does it for you. Signal Queries I am sure most readers know the @ViewChild, @ViewChildren, @ContentChild, and @ContentChildren decorators very well and have experienced the pain of triggering the infamous ExpressionChangedAfterItHasBeenCheckedError or having the values unavailable when needed. Here is a refresher on how you would use these decorators until now: ` With the new signal queries, similar to the new input API, the values are signals, and you can use them directly in computed signals and effects. You can define the same queries using the new syntax: ` Jeremy Elbourn mentioned that the new signal queries have better type inference and are more consistent with the new input and output APIs. He also showcased a brand new feature not available with the old queries. You can now define a query as required, and the Angular compiler will throw an error if the query has no result, guaranteeing that the value won't be undefined. Here is how you can define a required query: ` Model Inputs Jeremy and Minko announced the last new feature is the model inputs. The name is vague, but the feature is cool—it simplifies the definition of two-way bindings. Until now, to achieve two-way binding, you would have to define an input and an output following a given naming convention. @Input and @Output had to be defined with the same name (followed by "Change" in the case of the output). Then, you could use the template's [()] syntax. ` ` That way, you could keep the value in sync between the parent and the child component. With the new model inputs, you can define a two-way binding with a single line of code. Here is how you can define the same two-way binding using the new syntax: ` The html template stays the same: ` The model function returns a writable signal that can be updated directly. The value will be propagated back to any two-way bindings. Conclusion The new signal-based APIs are a great addition to Angular. They allow you to write less boilerplate code and make your components more reactive. The new APIs are already in developer preview, so you can start playing around with them today. I look forward to seeing how the community will adopt these new features and what other exciting things the Angular team has in store for us, such as zoneless apps by default....

Apr 3, 2024

3 mins

AngularJavaScript

Exploring Angular Forms: A New Alternative with Signals

Exploring Angular Forms: A New Alternative with Signals In the world of Angular, forms are essential for user interaction, whether you're crafting a simple login page or a more complex user profile interface. Angular traditionally offers two primary approaches: template-driven forms and reactive forms. In my previous series on Angular Reactive Forms, I explored how to harness reactive forms' power to manage complex logic, create dynamic forms, and build custom form controls. A new tool for managing reactivity - signals - has been introduced in version 16 of Angular and has been the focus of Angular maintainers ever since, becoming stable with version 17. Signals allow you to handle state changes declaratively, offering an exciting alternative that combines the simplicity of template-driven forms with the robust reactivity of reactive forms. This article will examine how signals can add reactivity to both simple and complex forms in Angular. Recap: Angular Forms Approaches Before diving into the topic of enhancing template-driven forms with signals, let’s quickly recap Angular's traditional forms approaches: 1. Template-Driven Forms: Defined directly in the HTML template using directives like ngModel, these forms are easy to set up and are ideal for simple forms. However, they may not provide the fine-grained control required for more complex scenarios. Here's a minimal example of a template-driven form: ` ` 2. Reactive Forms: Managed programmatically in the component class using Angular's FormGroup, FormControl, and FormArray classes; reactive forms offer granular control over form state and validation. This approach is well-suited for complex forms, as my previous articles on Angular Reactive Forms discussed. And here's a minimal example of a reactive form: ` ` Introducing Signals as a New Way to Handle Form Reactivity With the release of Angular 16, signals have emerged as a new way to manage reactivity. Signals provide a declarative approach to state management, making your code more predictable and easier to understand. When applied to forms, signals can enhance the simplicity of template-driven forms while offering the reactivity and control typically associated with reactive forms. Let’s explore how signals can be used in both simple and complex form scenarios. Example 1: A Simple Template-Driven Form with Signals Consider a basic login form. Typically, this would be implemented using template-driven forms like this: ` ` This approach works well for simple forms, but by introducing signals, we can keep the simplicity while adding reactive capabilities: ` ` In this example, the form fields are defined as signals, allowing for reactive updates whenever the form state changes. The formValue signal provides a computed value that reflects the current state of the form. This approach offers a more declarative way to manage form state and reactivity, combining the simplicity of template-driven forms with the power of signals. You may be tempted to define the form directly as an object inside a signal. While such an approach may seem more concise, typing into the individual fields does not dispatch reactivity updates, which is usually a deal breaker. Here’s an example StackBlitz with a component suffering from such an issue: Therefore, if you'd like to react to changes in the form fields, it's better to define each field as a separate signal. By defining each form field as a separate signal, you ensure that changes to individual fields trigger reactivity updates correctly. Example 2: A Complex Form with Signals You may see little benefit in using signals for simple forms like the login form above, but they truly shine when handling more complex forms. Let's explore a more intricate scenario - a user profile form that includes fields like firstName, lastName, email, phoneNumbers, and address. The phoneNumbers field is dynamic, allowing users to add or remove phone numbers as needed. Here's how this form might be defined using signals: ` > Notice that the phoneNumbers field is defined as a signal of an array of signals. This structure allows us to track changes to individual phone numbers and update the form state reactively. The addPhoneNumber and removePhoneNumber methods update the phoneNumbers signal array, triggering reactivity updates in the form. ` > In the template, we use the phoneNumbers signal array to dynamically render the phone number input fields. The addPhoneNumber and removePhoneNumber methods allow users to reactively add or remove phone numbers, updating the form state. Notice the usage of the track function, which is necessary to ensure that the ngFor directive tracks changes to the phoneNumbers array correctly. Here's a StackBlitz demo of the complex form example for you to play around with: Validating Forms with Signals Validation is critical to any form, ensuring that user input meets the required criteria before submission. With signals, validation can be handled in a reactive and declarative manner. In the complex form example above, we've implemented a computed signal called formValid, which checks whether all fields meet specific validation criteria. The validation logic can easily be customized to accommodate different rules, such as checking for valid email formats or ensuring that all required fields are filled out. Using signals for validation allows you to create more maintainable and testable code, as the validation rules are clearly defined and react automatically to changes in form fields. It can even be abstracted into a separate utility to make it reusable across different forms. In the complex form example, the formValid signal ensures that all required fields are filled and validates the email and phone numbers format. This approach to validation is a bit simple and needs to be better connected to the actual form fields. While it will work for many use cases, in some cases, you might want to wait until explicit "signal forms" support is added to Angular. Tim Deschryver started implementing some abstractions around signal forms, including validation and wrote an article about it. Let's see if something like this will be added to Angular in the future. Why Use Signals in Angular Forms? The adoption of signals in Angular provides a powerful new way to manage form state and reactivity. Signals offer a flexible, declarative approach that can simplify complex form handling by combining the strengths of template-driven forms and reactive forms. Here are some key benefits of using signals in Angular forms: 1. Declarative State Management: Signals allow you to define form fields and computed values declaratively, making your code more predictable and easier to understand. 2. Reactivity: Signals provide reactive updates to form fields, ensuring that changes to the form state trigger reactivity updates automatically. 3. Granular Control: Signals allow you to define form fields at a granular level, enabling fine-grained control over form state and validation. 4. Dynamic Forms: Signals can be used to create dynamic forms with fields that can be added or removed dynamically, providing a flexible way to handle complex form scenarios. 5. Simplicity: Signals can offer a simpler, more concise way to manage form states than traditional reactive forms, making building and maintaining complex forms easier. Conclusion In my previous articles, we explored the powerful features of Angular reactive forms, from dynamic form construction to custom form controls. With the introduction of signals, Angular developers have a new tool that merges the simplicity of template-driven forms with the reactivity of reactive forms. While many use cases warrant Reactive Forms, signals provide a fresh, powerful alternative for managing form state in Angular applications requiring a more straightforward, declarative approach. As Angular continues to evolve, experimenting with these new features will help you build more maintainable, performant applications. Happy coding!...

Oct 4, 2024

5 mins

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What's New in React 18?

Finally, we have a new version of React. Officially, React 18 is now ready to use! What new features were introduced in this version? Let’s find out in detail. In my opinion, the latest feature that revolutionized the React Ecosystem was React Hooks, introduced in React 16.8 back in 2019. Since then, we have seen many versions being released. But without any major changes, what will happen in React 18? Let’s dig into the details. New Features Concurrent React According to the official website of React, this is the major addition in React 18. With this new feature released, the devs will benefit from being able to prepare multiple versions of your UI at the same time. So, React will delegate to the dev to identify which changes are important to re-render the component. ` ` Automatic Batching With this new exciting feature, React will combine multiple setState() calls into a single re-render to improve performance. Before this, if we had multiple setState() inside of a setTimeout(), React will re-render for every state update which sometimes causes performance issues if we have larger components. You may already be familiar with this. This feature was available in React 17, but now it comes out-of-the-box. ` ` Transitions This new feature lets the dev split the state update into two categories: urgent and low priority. For some actions, like selecting in a dropdown or updating a input, you want the action to respond immediately, so now we had the possibility to distinguish between an urgent update and a non-urgent one. Updates wrapped in startTransition are handled as non-urgent, and will be interrupted if more urgent updates like clicks or keypresses come in. ` New Suspense Features Finally, we will have Suspense in the server. In the apps that are server-rendered, we will have only one Suspense API. Another cool thing is if we use Suspense in the server, we will start progressively streaming HTML. This enables selective hydration in the client. On paper, it looks awesome, but what does that mean? If you are waiting for some data on the server, React can stream HTML for the fallback, and let the user see the rest of the page. When the data is ready, the user will able to see the content HTML. This means that a single slow data source on the server will no longer hold the entire page back. New Hooks When we started this post, we mentioned that a few years ago, Hooks were the latest feature release in React. Now, React 18 will bring new Hooks: - useId : we can use this new hook to create a new identifier in the server and in the client. ` - useTransition: to create a new state update that has lower priority. ` - useDeferredValue: to update some deferred values in a controlled way (perfect for inputs, selects, etc). ` - useSyncExternalStore - useInsertionEffect IE11 not longer supported IE11 is no longer supported since React 18. If your app needs to continue bringing support to IE11, you will need to stay at React 17.x. Conclusion The most recent release from React includes some cool features like Automatic Batching or Concurrent Mode, which should improve the performance of React in the latest release. I'm really excited to see how these new features will perform in a real-life app, but they look more than promising. As we see, React 18 introduced out-of-the-box improvements, and new features that look incredible. It has cleared the way for new possibilities in React.js app development. If you want be aware of the upcoming or most recents changes, you can check out React JS Meetups....

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Understanding the Difference Between `:focus` and `:focus-visible` in CSS

Understanding the Difference Between :focus and :focus-visible in CSS I have learned my fair share about the importance of keyboard accessibility, so I know that visual indication of the focused element is very important. But the well-known :focus pseudo-class is not always the best fit for this job. That's where :focus-visible comes in. Let's look at the differences between these two pseudo-classes and explore the best practices for using them effectively. What is the :focus Pseudo-Class? The :focus pseudo-class is a CSS selector that applies styles to any element that receives focus, regardless of how that focus was triggered. This includes focus events from keyboard navigation, mouse clicks, and touch interactions. Example Usage of :focus ` In this example, the button will display a blue outline whenever it is focused, whether the user clicks on it with a mouse, taps it on a touchscreen, or navigates to it using the keyboard. What is the :focus-visible Pseudo-Class? The :focus-visible pseudo-class is more specialized. It only applies styles to an element when the browser determines that the focus should be visible. This typically occurs when the user navigates via the keyboard or assistive technologies rather than through mouse or touch input. Example Usage of :focus-visible ` Here, the button will only show a blue outline when focused through keyboard navigation or another input method that usually requires visible focus indicators. Key Differences Between :focus and :focus-visible :focus - Behavior: Applies to any element that receives focus, regardless of the input method. - Use Cases: Ensures that all interactions with the element are visually indicated, whether by mouse, keyboard, or touch. :focus-visible - Behavior: Applies styles only when the focus should be visible, such as using a keyboard or assistive technology. - Use Cases: Ideal for scenarios where you want to provide focus indicators only to keyboard and assistive technology users while avoiding unnecessary outlines for mouse and touch users, typically required by design. Accessibility Implications :focus - Pros: - Guarantees that all users can see when an element is focused, which is critical for accessibility. - Cons: - Can lead to a suboptimal experience for mouse users, as focus styles may appear unnecessarily during mouse interactions. :focus-visible - Pros: - Enhances user experience by showing focus indicators only when necessary, thus keeping the interface clean for mouse and touch users. - Tailors the experience for keyboard and assistive technology users, providing them with clear visual cues. - Cons: - Additional considerations may be required to ensure that focus indicators are not accidentally omitted, especially in older browsers that do not support :focus-visible. - There may be cases where you want to show focus indicators for all users, regardless of input method. Best Practices for Using :focus and :focus-visible To achieve the best accessibility and user experience, combining both :focus and :focus-visible in your CSS is often a good idea. Combining :focus and :focus-visible ` Here is a Stackblitz example of what such styling could look like for you to try out and play with. Additional Tips - Test with Keyboard and Assistive Technology: Ensure that your web application is navigable using a keyboard (Tab, Shift + Tab, etc.) and that focus indicators are visible for those who rely on them. It's never a bad idea to include accessibility testing in your e2e testing suite. - Provide Clear Focus Indicators: Make sure that focus indicators are prominent and easy to see. A subtle or hard-to-spot focus indicator can severely impact accessibility for users who rely on keyboard navigation. Conclusion The :focus-visible pseudo-class offers a more refined way to manage focus indicators, improving accessibility and user experience, particularly for keyboard and assistive technology users. By understanding the differences between :focus and :focus-visible, and applying best practices in your CSS, you can create more accessible and user-friendly web applications. Remember, accessibility should never be an afterthought. By thoughtfully applying focus styles, you ensure that all users, regardless of how they interact with your site, can easily navigate and interact....

Nov 8, 2024

3 mins

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Using Custom Async Validators in Angular Reactive Forms

What is a validator in Angular?

Create a basic application

Async validators

Conclusion

Daian Scuarissi

You might also like

Overview of the New Signal APIs in Angular

Exploring Angular Forms: A New Alternative with Signals

What's New in React 18?

Understanding the Difference Between `:focus` and `:focus-visible` in CSS

Let's innovate together!

You might also like

Overview of the New Signal APIs in Angular

Exploring Angular Forms: A New Alternative with Signals

What's New in React 18?

Understanding the Difference Between `:focus` and `:focus-visible` in CSS