Building Custom Directives in Angular

Building directives in Angular 2+ is not much different than building components. After all, components are just directives with a view attached. In fact, there are three kinds of directives in Angular: components, attribute directives and structural directives.

Structural directives add or remove elements from the DOM. NgIf, ngFor and ngSwitch are examples of built-in structural directives. Attribute directives are used to change the styling or behavior of elements.

Let’s learn how to create a custom attribute directive with an example with a appShadow directive.

Defining the directive class

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

Directive: shadow.directive.ts

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

@Directive({ selector: '[appShadow]' })
export class ShadowDirective {
    constructor(elem: ElementRef, renderer: Renderer) {
       renderer.setElementStyle(elem.nativeElement, 'box-shadow', '2px 2px 12px #58A362');
    }
}

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

Declaring it in our app module

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

App Module: app.module.ts

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

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

@NgModule({
  imports: [ BrowserModule ],
  declarations: [
    AppComponent,
    ShadowDirective
  ],
  bootstrap: [ AppComponent ]
})
export class AppModule { }

Using the directive in our template

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

App Module: app.component.html

<span appShadow>Alligator</span>

Improving our directive

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:

Directive: shadow.directive.ts

import { Directive, ElementRef, Input, Renderer, 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: Renderer) { }

    ngOnInit() {
      let shadowStr = `${ this.appShadowX } ${ this.appShadowY } ${ this.appShadowBlur } ${ this.appShadow }`;
      this.renderer.setElementStyle(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:

App Module: app.component.html

<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.

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