How to Create Custom Directives in Angular 13 App

In general, there are 3 types of directives: structural, attribute and components.

If you wish to remove or add elements from the DOM, then you will depend on structural directives. Here is a couple of examples for structural directives: *ngSwitch, *ngFor and *ngIf.

If you wish to alter the behaviour or styling of an element, you go for attribute directives.

We are going to take a look at an example to create a custom directive in Angular, It will apply rounded corners to HTML elements. I’ll take the help of appRoundBlock directive.

In order to remove strict type warnings or errors make sure to set “strict”: false and "strictTemplates": false under compilerOptions and angularCompilerOptions properties in tsconfig.json file.

Angular 13 Custom Directives Example

We are going to import Renderer2, ElementRef and Directive from @angular/core. Then we will use the renderer to set the style of the element according to our need:

Run the following command to generate custom directive in Angular app.

ng generate directive roundBlock

round-block.directive.ts

import { Directive, ElementRef, Renderer2 } from '@angular/core';
@Directive({
  selector: '[appRoundBlock]'
})
export class RoundBlockDirective {
  constructor(renderer: Renderer2, elmRef: ElementRef) {
    renderer.setStyle(elmRef.nativeElement, 'border-radius', '100px');
  }
}

We have wrapped the selector in brackets: [appRoundBlock]. This is to turn it into a border-radius directive.

Now let’s check this out in our app module.

When we ran the command to create custom directives in an Angular app, that command automatically registered custom directive in app module file.

app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { AppComponent } from './app.component';
import { RoundBlockDirective } from './round-block.directive';
@NgModule({
  declarations: [
    AppComponent,
    RoundBlockDirective
  ],
  imports: [
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

How to Use Custom Directive in our Angular Template?

We make use of the directive present in the template. We can easily use attribute directive in the template as we have shown below:

app.component.html

<div class="demo-block" appRoundBlock></div>

Set Up Dynamic Values in Custom Directives in Angular

Well, our appRoundBlock directive isn’t that smart. With the help of a style binding, we could have easily provided values to border-radius. Therefore, we are going to improve the directive by making it possible for us to pass values through to the directive.

round-block.directive.ts

import { Directive, ElementRef, Input, Renderer2, OnInit } from '@angular/core';
@Directive({
  selector: '[appRoundBlock]'
})
export class RoundBlockDirective implements OnInit {
  @Input() appRoundBlock: string;
  constructor(
   private elmRef: ElementRef, 
   private renderer: Renderer2) 
  { }
  ngOnInit() {
    let roundVal = `${this.appRoundBlock}`;
    this.renderer.setStyle(this.elmRef.nativeElement, 'border-radius', roundVal);
  }
}

This is how we gain use it in html component:

app.component.html

<div class="demo-block" [appRoundBlock]="'30px'"></div>