Creating a custom form field control group using Angular Material

In this guide, we will learn that it is possible to create custom form field controls group that can be used inside <mat-form-field>.

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Creating a custom form field control group using Angular Material

Introduction

In one of my last tutorial, we learned how to create a completely reusable forms Angular.

In this guide we'll learn how to create a custom form for inputting address information and hook it up to work with <mat-form-field>.

How to build custom form group using Angular Material

Basic Address Form Component

In order to learn how to build custom form field controls, let's start with a simple form component that we want to work inside the form field. An address form that segments the parts of the address into their own form field inputs.

I created this form quickly using Angular Material schematic, simply run below command to create one:

ng generate @angular/material:address-form shared/address-form

It will generate template and class files like below:


<form [formGroup]="addressForm" novalidate (ngSubmit)="onSubmit()">
  <mat-card class="shipping-card">
    <mat-card-header>
      <mat-card-title>Shipping Information</mat-card-title>
    </mat-card-header>
    <mat-card-content>
      <div class="row">
        <div class="col">
          <mat-form-field>
            <input matInput placeholder="Company" formControlName="company">
          </mat-form-field>
        </div>
      </div>
      <div class="row">
        <div class="col">
          <mat-form-field>
            <input matInput placeholder="First name" formControlName="firstName">
            <mat-error *ngIf="addressForm.controls['firstName'].hasError('required')">
              First name is <strong>required</strong>
            </mat-error>
          </mat-form-field>
        </div>
        <div class="col">
          <mat-form-field>
            <input matInput placeholder="Last name" formControlName="lastName">
            <mat-error *ngIf="addressForm.controls['lastName'].hasError('required')">
              Last name is <strong>required</strong>
            </mat-error>
          </mat-form-field>
        </div>
      </div>
      <div class="row">
        <div class="col">
          <mat-form-field>
            <textarea matInput placeholder="Address" formControlName="address"></textarea>
            <mat-error *ngIf="addressForm.controls['address'].hasError('required')">
              Address is <strong>required</strong>
            </mat-error>
          </mat-form-field>
        </div>
      </div>
      <!-- Other form fields -->
    </mat-card-content>
    <mat-card-actions>
      <button mat-raised-button color="primary" type="submit">Submit</button>
    </mat-card-actions>
  </mat-card>
</form>

@Component({
  selector: 'app-address-form',
  templateUrl: './address-form.component.html',
  styleUrls: ['./address-form.component.css']
})
export class AddressFormComponent {
  private fb = inject(FormBuilder);
  addressForm = this.fb.group({
    company: null,
    firstName: [null, Validators.required],
    lastName: [null, Validators.required],
    address: [null, Validators.required],
    address2: null,
    city: [null, Validators.required],
    state: [null, Validators.required],
    postalCode: [null, Validators.compose([
      Validators.required, Validators.minLength(5), Validators.maxLength(5)])
    ],
    shipping: ['free', Validators.required]
  });

  hasUnitNumber = false;

  // rest of auto generated code
}

Now, in above code please make a few changes as listed below:

  1. Remove usage of <mat-card> so that parent or consumer component has more control over UI.

  2. Remove submit button, becuase this form is not going be used as a stand-alone form, rather this will be always used insider some other form.

  3. Change all the fields' appearance to outline

  4. Create a class called AddressForm and use it to create typed forms

So, now after the 1st, 2nd and 3rd change, the template will look like below:


<form
  novalidate
  [formGroup]="addressForm"
  class="address-form-container"
>
  <div class="row">
    <div class="col">
      <mat-form-field appearance="outline">
        <input
          matInput
          placeholder="Company"
          formControlName="company"
        />
      </mat-form-field>
    </div>
  </div>
  <div class="row">
    <div class="col">
      <mat-form-field appearance="outline">
        <input
          matInput
          placeholder="First name"
          formControlName="firstName"
        />
        <mat-error
          *ngIf="addressForm.controls['firstName'].hasError('required')"
        >
          First name is <strong>required</strong>
        </mat-error>
      </mat-form-field>
    </div>
    <div class="col">
      <mat-form-field appearance="outline">
        <input
          matInput
          placeholder="Last name"
          formControlName="lastName"
        />
        <mat-error
          *ngIf="addressForm.controls['lastName'].hasError('required')"
        >
          Last name is <strong>required</strong>
        </mat-error>
      </mat-form-field>
    </div>
  </div>
  <div class="row">
    <div class="col">
      <mat-form-field appearance="outline">
        <textarea
          matInput
          placeholder="Address"
          formControlName="address"
        ></textarea>
        <mat-error *ngIf="addressForm.controls['address'].hasError('required')">
          Address is <strong>required</strong>
        </mat-error>
      </mat-form-field>
    </div>
  </div>
  <!-- Other address fields will come here and submit button removed -->
</form>

And after 4th change, the will have a new file called address.ts:


import { FormControl } from '@angular/forms';

export class Address {
  constructor(
    public company: string | null = null,
    public firstName: string = '',
    public lastName: string = '',
    public address: string = '',
    public address2: string | null,
    public city: string = '',
    public state: string = '',
    public postalCode: number = 0,
    public shipping: AddressShipping = 'free'
  ) {}
}

export type AddressForm = {
  company: FormControl<string | null>;
  firstName: FormControl<string>;
  lastName: FormControl<string>;
  address: FormControl<string>;
  address2: FormControl<string | null>;
  city: FormControl<string>;
  state: FormControl<string>;
  postalCode: FormControl<number>;
  shipping: FormControl<AddressShipping>;
};

export type AddressShipping = 'free' | 'priority' | 'nextday';

and the component's class file will look like below:


@Component({
  selector: 'app-address-form',
  templateUrl: './address-form.component.html',
  styleUrls: ['./address-form.component.css']
})
export class AddressFormComponent {
  private fb = inject(FormBuilder);

  readonly freeShipping: AddressShipping = 'free';

  addressForm = this.fb.group<AddressForm>({
    company: this.fb.control(null),
    firstName: this.fb.control('', { nonNullable: true, validators: [Validators.required]}),
    lastName: this.fb.control('', { nonNullable: true, validators: [Validators.required]}),
    address: this.fb.control('', { nonNullable: true, validators: [Validators.required]}),
    address2: this.fb.control(null),
    city: this.fb.control('', { nonNullable: true, validators: [Validators.required]}),
    state: this.fb.control('', { nonNullable: true, validators: [Validators.required]}),
    postalCode: this.fb.control(0, { nonNullable: true, validators: Validators.compose([
      Validators.required, Validators.minLength(5), Validators.maxLength(5)]) }
    ),
    shipping: this.fb.control(this.freeShipping, { nonNullable: true, validators: Validators.required})
  });

  // rest of auto generated code
}

Providing our component as a MatFormFieldControl

The first step is to provide our new component as an implementation of the MatFormFieldControl interface that the <mat-form-field> knows how to work with. To do this, we will have our class implement MatFormFieldControl. Since this is a generic interface, we'll need to include a type parameter indicating the type of data our control will work with, in this case Address. We then add a provider to our component so that the form field will be able to inject it as a MatFormFieldControl.

@Component({
  ...
  providers: [
    { provide: MatFormFieldControl, useExisting: AddressFormComponent },
  ],
})
export class AddressFormComponent implements MatFormFieldControl<Address> {
  ...
}

This sets up our component, so it can work with <mat-form-field>, but now we need to implement the various methods and properties declared by the interface we just implemented. To learn more about the MatFormFieldControl interface, see the form field API documentation.

Implementing the methods and properties of MatFormFieldControl

value

This property allows someone to set or get the value of our control. Its type should be the same type we used for the type parameter when we implemented MatFormFieldControl. Since our component already has a value property, we don't need to do anything for this one.

stateChanges

Because the <mat-form-field> uses the OnPush change detection strategy, we need to let it know when something happens in the form field control that may require the form field to run change detection. We do this via the stateChanges property. So far the only thing the form field needs to know about is when the value changes. We'll need to emit on the stateChanges stream when that happens, and as we continue flushing out these properties we'll likely find more places we need to emit. We should also make sure to complete stateChanges when our component is destroyed.


stateChanges = new Subject<void>();

set value(address: Address | null) {
  address = address || new Address('', '', '', '', '', '', '', 0, this.freeShipping);
  this.addressForm.setValue(address);

  this.stateChanges.next();
}

ngOnDestroy() {
  this.stateChanges.complete();
}

id

This property should return the ID of an element in the component's template that we want the <mat-form-field> to associate all of its labels and hints with. In this case, we'll use the host element and just generate a unique ID for it.

static nextId = 0;

@HostBinding() id = `address-form-${AddressFormComponent.nextId++}`;

placeholder

This property allows us to tell the <mat-form-field> what to use as a placeholder. In this example, we'll do the same thing as matInput and <mat-select> and allow the user to specify it via an @Input(). Since the value of the placeholder may change over time, we need to make sure to trigger change detection in the parent form field by emitting on the stateChanges stream when the placeholder changes.

Note that as ours is a set of form-fields, and not individual form-field, we are not using placeholder anywhere in the template. But, I have given the code below in case you want to integrate it.

@Input()
get placeholder() {
  return this._placeholder;
}
set placeholder(plh) {
  this._placeholder = plh;
  this.stateChanges.next();
}
private _placeholder: string;

ngControl and ControlValueAccessor

This property allows the form field control to specify the @angular/forms control that is bound to this component.

Let's implement ControlValueAccessor so that our component can work with formControl and ngModel. As we are implementing ControlValueAccessor we will need to get a reference to the NgControl associated with our control and make it publicly available.

The easy way is to add it as a public property to your constructor and let dependency injection handle it:

constructor(
  ...,
  @Optional() @Self() public ngControl: NgControl,
  ...,
) { }

Now as our component implements ControlValueAccessor, to avoid cannot instantiate cyclic dependency error, instead of using the NG_VALUE_ACCESSOR provider, we will set the value accessor directly:

@Component({
  ...
})
export class AddressFormComponent implements MatFormFieldControl<Address>, ControlValueAccessor {
  constructor(
    ...,
    @Optional() @Self() public ngControl: NgControl,
    ...,
  ) {

    if (this.ngControl != null) {
      // Setting the value accessor directly (instead of using
      // the providers) to avoid running into a circular import.
      this.ngControl.valueAccessor = this;
    }
  }
}

For additional information about ControlValueAccessor see the API docs.

focused

This property indicates whether the form field control should be considered to be in a focused state. When it is in a focused state, the form field is displayed with a solid color underline. For the purposes of our component, we want to consider it focused if any of the part inputs are focused. We can use the focusin and focusout events to easily check this. We also need to remember to emit on the stateChanges when the focused stated changes stream so change detection can happen.

In addition to updating the focused state, we use the focusin and focusout methods to update the internal touched state of our component, which we'll use to determine the error state.

focused = false;

onFocusIn(event: FocusEvent) {
  if (!this.focused) {
    this.focused = true;
    this.stateChanges.next();
  }
}

onFocusOut(event: FocusEvent) {
  if (!this._elementRef.nativeElement.contains(event.relatedTarget as Element)) {
    this.touched = true;
    this.focused = false;
    this.onTouched();
    this.stateChanges.next();
  }
}

empty

This property indicates whether the form field control is empty. For our control, we'll consider it empty if all the fields are empty.

get empty() {
    const {
      value: {
        company,
        firstName,
        address2,
        address,
        city,
        lastName,
        postalCode,
        shipping,
        state,
      },
    } = this.addressForm;

    return (
      !company &&
      !firstName &&
      !address &&
      !address2 &&
      !city &&
      !lastName &&
      !postalCode &&
      !shipping &&
      !state
    );
  }

shouldLabelFloat

This property is used to indicate whether the label should be in the floating position. We'll use the same logic as matInput and float the placeholder when the input is focused or non-empty.

@HostBinding('class.floating')
get shouldLabelFloat() {
  return this.focused || !this.empty;
}
span {
  opacity: 0;
  transition: opacity 200ms;
}
:host.floating span {
  opacity: 1;
}

required

This property is used to indicate whether the input is required. <mat-form-field> uses this information to add a required indicator to the placeholder. Again, we'll want to make sure we run change detection if the required state changes.

@Input()
get required() {
  return this._required;
}
set required(req: BooleanInput) {
  this._required = coerceBooleanProperty(req);
  this.stateChanges.next();
}
private _required = false;

disabled

This property tells the form field when it should be in the disabled state. In addition to reporting the right state to the form field, we need to set the disabled state on the individual inputs that make up our component.

@Input()
  get disabled(): boolean {
    return this._disabled;
  }
  set disabled(value: BooleanInput) {
    this._disabled = coerceBooleanProperty(value);
    this._disabled ? this.addressForm.disable() : this.addressForm.enable();
    this.stateChanges.next();
  }
  private _disabled = false;

errorState

This property indicates whether the associated NgControl is in an error state. In this example, we show an error if the input is invalid and our component has been touched.

get errorState(): boolean {
    return this.addressForm.invalid && this.touched;
  }

controlType

This property allows us to specify a unique string for the type of control in form field. The <mat-form-field> will add a class based on this type that can be used to easily apply special styles to a <mat-form-field> that contains a specific type of control. In this example we'll use address-form as our control type which will result in the form field adding the class mat-form-field-type-address-form.

controlType = 'address-form';

setDescribedByIds(ids: string[])

This method is used by the <mat-form-field> to set element ids that should be used for thearia-describedby attribute of your control. The ids are controlled through the form field as hints or errors are conditionally displayed and should be reflected in the control's aria-describedby attribute for an improved accessibility experience.

The setDescribedByIds method is invoked whenever the control's state changes. Custom controls need to implement this method and update the aria-describedby attribute based on the specified element ids. Below is an example that shows how this can be achieved.

Note that the method by default will not respect element ids that have been set manually on the control element through the aria-describedby attribute. To ensure that your control does not accidentally override existing element ids specified by consumers of your control, create an input called userAriaDescribedby like followed:

@Input('aria-describedby') userAriaDescribedBy: string;

The form field will then pick up the user specified aria-describedby ids and merge them with ids for hints or errors whenever setDescribedByIds is invoked.

setDescribedByIds(ids: string[]) {
    const controlElement = this._elementRef.nativeElement.querySelector(
      '.address-form-container'
    )!;
    controlElement.setAttribute('aria-describedby', ids.join(' '));
  }

onContainerClick(event: MouseEvent)

This method will be called when the form field is clicked on. It allows your component to hook in and handle that click however it wants. The method has one parameter, the MouseEvent for the click. In our case we'll just focus the first <input> if the user isn't about to click an <input> anyways.

@ViewChild('company')
  companyInput!: HTMLInputElement;
  @ViewChild('firstName')
  firstNameInput!: HTMLInputElement;
  @ViewChild('lastName')
  lastNameInput!: HTMLInputElement;
  @ViewChild('address')
  addressInput!: HTMLTextAreaElement;
  @ViewChild('address2')
  address2Input!: HTMLTextAreaElement;
  @ViewChild('city')
  cityInput!: HTMLInputElement;
  @ViewChild('state')
  stateInput!: MatSelect;
  @ViewChild('postalCode')
  postalCodeInput!: HTMLInputElement;
  @ViewChild('shipping')
  shippingInput!: MatRadioGroup;

onContainerClick() {
    if (this.addressForm.controls.company.valid) {
      this._focusMonitor.focusVia(this.companyInput, 'program');
    } else if (this.addressForm.controls.firstName.valid) {
      this._focusMonitor.focusVia(this.firstNameInput, 'program');
    } else if (this.addressForm.controls.lastName.valid) {
      this._focusMonitor.focusVia(this.lastNameInput, 'program');
    } else if (this.addressForm.controls.address.valid) {
      this._focusMonitor.focusVia(this.addressInput, 'program');
    } else if (this.addressForm.controls.address2.valid) {
      this._focusMonitor.focusVia(this.address2Input, 'program');
    } else if (this.addressForm.controls.city.valid) {
      this._focusMonitor.focusVia(this.cityInput, 'program');
    } else if (this.addressForm.controls.state.valid) {
      this._focusMonitor.focusVia(
        this.stateInput._elementRef.nativeElement,
        'program'
      );
    } else if (this.addressForm.controls.postalCode.valid) {
      this._focusMonitor.focusVia(this.postalCodeInput, 'program');
    } else {
      this.shippingInput._radios.first.focus(undefined, 'program');
    }
  }

Improving accessibility

One significant piece of information is missing for screen reader users. They won't be able to tell what this input group represents. To improve this, we should add a label for the group element using either aria-label or aria-labelledby.

It's recommended to link the group to the label that is displayed as part of the parent <mat-form-field>. This ensures that explicitly specified labels (using <mat-label>) are actually used for labelling the control.

In our concrete example, we add an attribute binding for aria-labelledby and bind it to the label element id provided by the parent <mat-form-field>.

export class AddressFormComponent implements ... {
  ...

  constructor(...
              @Optional() public parentFormField: MatFormField,) {
@Component({
  selector: 'app-address-form',
  template: `
    <form [formGroup]="addressForm"
         [attr.aria-labelledby]="parentFormField?.getLabelId()">

Trying it out

Now that we've fully implemented the interface, we're ready to try our component out! All we need to do is place it inside a <mat-form-field>, which is inside another form:

<div [formGroup]="form">
  <mat-card class="shipping-card">
    <mat-card-header>
      <mat-card-title>Shipping Information</mat-card-title>
    </mat-card-header>
    <mat-card-content>
      <mat-form-field  appearance="outline">
        <app-address-form formControlName="address" required></app-address-form>
      </mat-form-field>
    </mat-card-content>
  </mat-card>
</div>

Full code is available on github/reusable-material-form.

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