Validation

Last week’s post covered model driven validation in Angular 2 and this week I will be looking at template driven validation. Template driven validation provides a simplified way to add validation without the need to directly use a control group. This is a great way to get up and running for simple form cases, and maybe more complex ones as well. I haven’t hit a case yet that wouldn’t work with either validation style.

Building a Template Form with Validation

I started with a simple form that contains labels and inputs for entering a name and email address that are bound to corresponding values in the associated model.

<form>
   <div class="form-group">
     <label for="name">Name</label>
     <input type="text" class="form-control" [(ngModel)]="name">
   </div>
   <div class="form-group">
     <label for="name">Email</label>
     <input type="text" class="form-control" [(ngModel)]="email">
   </div>
</form>

The next example is the same form with validation added.

<form #contactForm="ngForm">
   <div class="form-group">
     <label for="name">Name</label>
     <input type="text" class="form-control" ngControl="name" #nameControl="ngForm" [(ngModel)]="name" required minlength="3">
     <div [hidden]="nameControl.valid || !nameControl.errors || !nameControl.errors.required">Name is required</div>
     <div [hidden]="nameControl.valid || !nameControl.errors || !nameControl.errors.minlength">Min Length</div>
   </div>
   <div class="form-group">
     <label for="name">Email</label>
     <input type="text" class="form-control" ngControl="email" #emailControl="ngForm" [(ngModel)]="email" required>
     <div [hidden]="emailControl.valid || !emailControl.errors || !emailControl.errors.required">Email is required</div>
   </div>
</form>

The first change you will notice is that the form now has a name assigned using #contactForm=”ngForm”. In this case the name is not used, but it could be used to disable a submit button if any of the inputs the form contained didn’t have valid values. I am sure there are a lot more use cases as well.

On the inputs that need validation a controls name is assigned via ngControl=”name” and a name is assigned using #nameControl=”ngForm”. I had some trouble withe assigning names to my inputs at first I keep getting the following.

EXCEPTION: Cannot assign to a reference or variable!

The cause of the issue was that I was trying to use #name for the control name which was already a property on the model. Once I changed to #nameControl all worked fine.

For the input for name above required minlength=”3″ tells the Angular form that the input is required and must be at least three characters in length.

Displaying Validation

For the name input you can see that it has two divs used to show specific messages based on which validation condition failed.

<div [hidden]="nameControl.valid || !nameControl.errors || !nameControl.errors.required">Name is required</div>
<div [hidden]="nameControl.valid || !nameControl.errors || !nameControl.errors.minlength">Min Length</div>

I am not sure why, but when using the template style validation I had to check !nameControl.errors in addition to the specific error like !nameControl.errors.required or I would get an exception when the page tried to load. If your control doesn’t have more than a single validation then nameControl.valid would be a sufficient condition for showing validation messages.

Custom Validators

I am going to use the same email validator from last week with a few changes so can be used from a template. The following is the full class.

import {Control, NG_VALIDATORS, Validator} from '@angular/common';
import {Directive, Provider, forwardRef} from '@angular/core';

const EMAIL_VALIDATOR = new Provider(NG_VALIDATORS, { useExisting: forwardRef(() => EmailValidator), multi: true });

@Directive({
    selector: '[emailValidator]',
    providers: [EMAIL_VALIDATOR]
})
export class EmailValidator implements Validator {
    validate(control: Control): {[key: string]: any} {
        const emailRegexp = /^[a-z0-9!#$%&'*+\/=?^_`{|}~.-]+@[a-z0-9]([a-z0-9-]*[a-z0-9])?(\.[a-z0-9]([a-z0-9-]*[a-z0-9])?)*$/i;
        if (control.value !== null && control.value !== "" && (control.value.length <= 5 || !emailRegexp.test(control.value))) {
            return { "email": true };
        }
        return null;
    }
}

The biggest difference is the @Directive decorator. The selector is the attribute that will be used to mark that an input needs to pass the specified validation. I am not 100% sure of the specifics of the provider, but the above works. This is another topic I need to explore more.

Using a Custom Validator

First in the model the validator must be imported.

import {EmailValidator} from './email-validator';

Then the validator needs to be added to the directives section of the @Component decorator.

@Component({
    selector: 'contact-detail',
    templateUrl: 'contact.detail.html',
    directives: [
        EmailValidator
    ],
    providers: [
        HTTP_PROVIDERS
    ]
})

To use in the view just add the emailValidator selector to the proper input.

<input type="text" class="form-control" ngControl="email" #emailControl="ngForm" [(ngModel)]="email" required emailValidator>

Finally add a div to display the error.

<div [hidden]="emailControl.valid || !emailControl.errors || !emailControl.errors.email">Email is invalid</div>

Wrapping Up

Using template driven validation you can quickly get a form validated without having to make changes outside of your template (unless you are using a custom validator). Angular 2 offers a lot of options when it comes to validation and where it should be put.

In this post a couple of weeks ago I covered Aurelia’s new validation plugin and I thought it would be good to see what Angular 2 has to offer validation wise. Angular’s offering is vastly different that Aurelia. Angular can use model driven or template driven validations. This post is going to look at the model driven style.

Forms

Angular forms is the framework’s way to handle groups of data entry that need support for data binding and validation among other things.

Building a Form with Validation

The first step to building a model driven form is import the classes that will be needed.

import {FormBuilder, ControlGroup, Validators} from '@angular/common';

Next add a variable for control group and setup the control group in the constructor of the class.

public contactForm: ControlGroup;

constructor(formBuilder: FormBuilder) {
    this.contactForm = formBuilder.group({
        name: ["", Validators.compose([Validators.required, Validators.minLength(3)])],
        email: ["", Validators.required]
    });
}

The above uses FormBuilder to define a name property that is required with a minimum length of three characters and an email property that is required. Later in the post I will add better email validation via customer validator.

Angular 2 only comes with required, minimum length, max length and pattern (match given regex) validators out of the box. Thankfully custom validators are pretty easy to implement.

Using a From with a View

Now that the form is setup the view needs to utilize it. The following is the code form the view.

<form class="form-horizontal" [ngFormModel]="contactForm">
  <div class="form-group">
      <label class="control-label">Name</label>
      <input type="text" ngControl="name" #name="ngForm" class="form-control">
      <div [hidden]="name.valid || name.pristine || !name.errors.required">Required</div>
      <div [hidden]="name.valid || name.pristine || !name.errors.minlength">Min Length</div>
  </div>
  <div class="form-group">
      <label class="control-label">Email</label>
      <input type="email" ngControl="email" #email="ngForm" class="form-control">
      <div [hidden]="email.valid || email.pristine || !email.errors.required">Required</div>
  </div>
</form>

First notice in the form tag [ngFormModel]=”contactForm” is used to bind the form to the control group crated in the model.

Instead of [(ngModel)]=”name” we use ngControl=”name” to bind the input box to the name control in the control group that was crated using the form builder above. #name=”ngForm” defines name as a variable that can be used later.

<div [hidden]=”name.valid || name.pristine || !name.errors.required”>Required</div>  is used to display validation errors to the user. This code is just saying to hide the div if name is valid or pristine (not changed by the user)  or if the validation error is not because the field is required. While this style works I much prefer Aurelia as it handles the extra work needed to automatically display validation errors.

Custom Validators

As I mention above I am going to show you a custom validator that can be used to better handle email validation. Most of this validation I found in a search but unfortunately I forgot to save the URL so I can’t link to the exact source. The following is the complete code for a custom email validator.

import {Control} from '@angular/common';

export class EmailValidator {
    static email(control: Control) {
        const emailRegexp = /^[a-z0-9!#$%&'*+\/=?^_`{|}~.-]+@[a-z0-9]([a-z0-9-]*[a-z0-9])?(\.[a-z0-9]([a-z0-9-]*[a-z0-9])?)*$/i;

        if (control.value !== "" && (control.value.length <= 5 || !emailRegexp.test(control.value))) {
            return { "email": true };
        }

        return null;
    }
}

Note that returning null means that validation passed.

Using a Custom Validator in a Model

First the custom validator needs to be imported.

import {EmailValidator} from './email.validator';

Now the validator can be used in the classes form builder.

this.contactForm = formBuilder.group({
    name: ["", Validators.compose([Validators.required, Validators.minLength(3)])],
    email: ["", Validators.compose([Validators.required, EmailValidator.email])]
});

Notices that the email control is now using Validators.compose which is used to apply multiple validations to a single control.

Using a Custom Validator in a View

Now the view just has to be changed to show the user that the reason for the failed validation is an invalid email address.

<div class="form-group">
    <label class="control-label">Email</label>
    <input type="email" ngControl="email" #email="ngForm" class="form-control">
    <div [hidden]="email.valid || email.pristine || !email.errors.required">Required</div>
    <div [hidden]="email.valid || email.pristine || !email.errors.email">Invalid address</div>
</div>

Note that the entry in errors will match the return value from the custom validator. I had issues trying to use the minimum length validator because I was trying to use minLength instead of minlength. If you are having trouble with validation message showing up that would be the first thing to check.

Wrapping Up

That covers the basics of model driven validation in Angular 2. Be on the look out for a post in the future that covers the same concepts but using a template driven approach. I also wanted to point out this post by Pascal Precht and this post by David Den Toom which helped me a lot in writing this post.