Angular Custom Form Controls - Complete Guide

Angular is a TypeScript-based open-source web application framework led by the Angular Team at Google and by a community of individuals and corporations
In this article, I will show you how to create a custom control in Angular, which we can use as reactive form control. Angular provides a lot of extensibility feature for extending the behaviour of the Angular framework.
To create custom control we need to implemented ControlValueAccessor interface which have following four method

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

For this article, I am going to create a custom spinner control.

Create an angular application by executing the following command

ng new app_name

Then create a component called SpinnerControl and paste the following code. All the code is self-explanatory.

import { Component, forwardRef, Input } from "@angular/core";
import { ControlValueAccessor, NG_VALUE_ACCESSOR } from "@angular/forms";

@Component({
  selector: "app-number",
  template: `
    <div class="container">
      <div class="left">
        <input
          class="number-input"
          [value]="value"
          type="number"
          [disabled]="disabled"
        />
      </div>
      <div class="right">
        <button (click)="onIncrement()">+</button>
        <button (click)="onDeincrement()">-</button>
      </div>
    </div>
  `,
  styles: [
    `
      .container {
        display: flex;
        padding: 0px;
      }
      .right {
        display: flex;
        flex-direction: column;
      }
      .number-input {
        height: 40px;
      }

      button {
        height: 20px;
        width: 20px;
        border: 0px;
        background-color: #aa33dd;
      }
    `
  ]
})
export class SpinnerControl implements ControlValueAccessor {
  @Input()
  public value: number;
  @Input()
  public disabled: boolean;

  onChanged: any = () => {};
  onTouched: any = () => {};

  writeValue(val): void {
    console.log(val);
    this.value = +val || 0;
  }

  registerOnChange(fn: any) {
    this.onChanged = fn;
  }

  registerOnTouched(fn: any) {
    this.onTouched = fn;
  }

  setDisabledState(isDisabled: boolean) {
    this.disabled = isDisabled;
  }

  numberChange($event) {
    this.onTouched();
    this.onChanged($event.target.value);
    this.setDisabledState(this.disabled);
  }

  onIncrement() {
    this.value = this.value + 1;
    this.onChanged(this.value);
  }

  onDeincrement() {
    this.value = this.value - 1;
    this.onChanged(this.value);
  }
}

Method Name Description
writeValue Write a new value to the element
registerOnChange Is called when the control values changes in the UI
registerOnTouched StartFragmentIs called by the forms API to update the form model on blur EndFragment
new Date (year, month, day, hours, minutes, seconds, milliseconds Based on Date and time

How to use?

Before using this custom reactive form control, you have to register this with angular providers array. See the below code for registration

providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: forwardRef(() => SpinnerControl),
      multi: true
    }
  ]

Final code looks like below.

import { Component, forwardRef, Input } from "@angular/core";
import { ControlValueAccessor, NG_VALUE_ACCESSOR } from "@angular/forms";

@Component({
  selector: "app-number",
  template: `
    <div class="container">
      <div class="left">
        <input
          class="number-input"
          [value]="value"
          type="number"
          [disabled]="disabled"
        />
      </div>
      <div class="right">
        <button (click)="onIncrement()">+</button>
        <button (click)="onDeincrement()">-</button>
      </div>
    </div>
  `,
  styles: [
    `
      .container {
        display: flex;
        padding: 0px;
      }
      .right {
        display: flex;
        flex-direction: column;
      }
      .number-input {
        height: 40px;
      }

      button {
        height: 20px;
        width: 20px;
        border: 0px;
        background-color: #aa33dd;
      }
    `
  ],
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: forwardRef(() => SpinnerControl),
      multi: true
    }
  ]
})
export class SpinnerControl implements ControlValueAccessor {
  @Input()
  public value: number;
  @Input()
  public disabled: boolean;

  onChanged: any = () => {};
  onTouched: any = () => {};

  writeValue(val): void {
    console.log(val);
    this.value = +val || 0;
  }

  registerOnChange(fn: any) {
    this.onChanged = fn;
  }

  registerOnTouched(fn: any) {
    this.onTouched = fn;
  }

  setDisabledState(isDisabled: boolean) {
    this.disabled = isDisabled;
  }

  numberChange($event) {
    this.onTouched();
    this.onChanged($event.target.value);
    this.setDisabledState(this.disabled);
  }

  onIncrement() {
    this.value = this.value + 1;
    this.onChanged(this.value);
  }

  onDeincrement() {
    this.value = this.value - 1;
    this.onChanged(this.value);
  }
}

What is forwardRef

From Angular’s API docs on forwardRef:

forwardRef is used when the token which we need to refer to for the purposes of DI is declared, but not yet defined. It is also used when the token which we use when creating a query is not yet defined.

In simple terms, Angular compiler uses the function to provide the provider type during runtime.

DEMO

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