Building Custom Directives in Angular

Import Directive, ElementRef and Renderer2 from @angular/core, then use the renderer to set the element’s style to our desired box-shadow value:

import { Directive, ElementRef, Renderer2 } from '@angular/core';

Notice how our selector is wrapped in brackets: [appShadow], to properly make it an attribute directive.

Here’s how you would declare our new directive in the app root module:

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';

import { ShadowDirective } from './shadow.directive';

Now it’s as simple as using the attribute directive in our template like so:

<span appShadow>Alligator</span>

Our appShadow directive is a bit dumb, and we could have applied a shadow simply with a style binding instead. Let’s therefore make it better by allowing to pass values to our directive:

import { Directive, ElementRef, Input, Renderer2, OnInit } from '@angular/core';

@Directive({ selector: '[appShadow]' })
export class ShadowDirective implements OnInit {
@Input() appShadow: string;
@Input() appShadowX: string;
@Input() appShadowY: string;
@Input() appShadowBlur: string;

constructor(private elem: ElementRef, private renderer: Renderer2) { }

ngOnInit() {
  let shadowStr = `${ this.appShadowX } ${ this.appShadowY } ${ this.appShadowBlur } ${ this.appShadow }`;
  this.renderer.setStyle(this.elem.nativeElement, 'box-shadow', shadowStr);
}

We used inputs to pass data from our component template to the directive. Notice also how we’re using the OnInit lifecycle hook now instead of doing the work in the constructor. That’s because our inputs don’t have any value at construction time.

You can now have full control over the shadow:

<span [appShadow]="'hotpink'"
      [appShadowX]="'12px'"
      [appShadowY]="'6px'"
      [appShadowBlur]="'30px'">Alligator</span>

👉 We called our directive appShadow, and not shadow, because it’s a best practice to keep your custom directives scoped to your application.