In-depth Guides
Directives

Attribute directives

Change the appearance or behavior of DOM elements and Angular components with attribute directives.

Building an attribute directive

This section walks you through creating a highlight directive that sets the background color of the host element to yellow.

  1. To create a directive, use the CLI command ng generate directive.

          
    ng generate directive highlight

    The CLI creates src/app/highlight.directive.ts, a corresponding test file src/app/highlight.directive.spec.ts.

    src/app/highlight.directive.ts

          
    import {Directive} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {}

    The @Directive() decorator's configuration property specifies the directive's CSS attribute selector, [appHighlight].

  2. Import ElementRef from @angular/core. ElementRef grants direct access to the host DOM element through its nativeElement property.

  3. Add ElementRef in the directive's constructor() to inject a reference to the host DOM element, the element to which you apply appHighlight.

  4. Add logic to the HighlightDirective class that sets the background to yellow.

    src/app/highlight.directive.ts

          
    import {Directive, ElementRef} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {    this.el.nativeElement.style.backgroundColor = 'yellow';  }}

HELPFUL: Directives do not support namespaces.

src/app/app.component.avoid.html (unsupported)

      
<p app:Highlight>This is invalid</p>

Applying an attribute directive

  1. To use the HighlightDirective, add a <p> element to the HTML template with the directive as an attribute.

    src/app/app.component.html

          
    <h1>My First Attribute Directive</h1><p appHighlight>Highlight me!</p><p appHighlight="yellow">Highlighted in yellow</p><p [appHighlight]="'orange'">Highlighted in orange</p><p [appHighlight]="color">Highlighted with parent component's color</p>

Angular creates an instance of the HighlightDirective class and injects a reference to the <p> element into the directive's constructor, which sets the <p> element's background style to yellow.

Handling user events

This section shows you how to detect when a user mouses into or out of the element and to respond by setting or clearing the highlight color.

  1. Import HostListener from '@angular/core'.

    src/app/highlight.directive.ts (imports)

          
    import {Directive, ElementRef, HostListener} from '@angular/core';import {Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @HostListener('mouseenter') onMouseEnter() {    this.highlight('yellow');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

  2. Add two event handlers that respond when the mouse enters or leaves, each with the @HostListener() decorator.

    src/app/highlight.directive.ts (mouse-methods)

          
    import {Directive, ElementRef, HostListener} from '@angular/core';import {Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @HostListener('mouseenter') onMouseEnter() {    this.highlight('yellow');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

Subscribe to events of the DOM element that hosts an attribute directive, the <p> in this case, with the @HostListener() decorator.

HELPFUL: The handlers delegate to a helper method, highlight(), that sets the color on the host DOM element, el.

The complete directive is as follows:

src/app/highlight.directive.ts

      
import {Directive, ElementRef, HostListener} from '@angular/core';import {Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @HostListener('mouseenter') onMouseEnter() {    this.highlight('yellow');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

The background color appears when the pointer hovers over the paragraph element and disappears as the pointer moves out.

Second Highlight

Passing values into an attribute directive

This section walks you through setting the highlight color while applying the HighlightDirective.

  1. In highlight.directive.ts, import Input from @angular/core.

    src/app/highlight.directive.ts (imports)

          
    import {Directive, ElementRef, HostListener, Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @Input() appHighlight = '';  @HostListener('mouseenter') onMouseEnter() {    this.highlight(this.appHighlight || 'red');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

  2. Add an appHighlight @Input() property.

    src/app/highlight.directive.ts

          
    import {Directive, ElementRef, HostListener, Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @Input() appHighlight = '';  @HostListener('mouseenter') onMouseEnter() {    this.highlight(this.appHighlight || 'red');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

    The @Input() decorator adds metadata to the class that makes the directive's appHighlight property available for binding.

  3. In app.component.ts, add a color property to the AppComponent.

    src/app/app.component.ts (class)

          
    import {Component} from '@angular/core';import {HighlightDirective} from './highlight.directive';@Component({  selector: 'app-root',  templateUrl: './app.component.1.html',  imports: [HighlightDirective],})export class AppComponent {  color = 'yellow';}

  4. To simultaneously apply the directive and the color, use property binding with the appHighlight directive selector, setting it equal to color.

    src/app/app.component.html (color)

          
    <h1>My First Attribute Directive</h1><h2>Pick a highlight color</h2><div>  <input type="radio" name="colors" (click)="color='lightgreen'">Green  <input type="radio" name="colors" (click)="color='yellow'">Yellow  <input type="radio" name="colors" (click)="color='cyan'">Cyan</div><p [appHighlight]="color">Highlight me!</p><p [appHighlight]="color" defaultColor="violet">  Highlight me too!</p><hr /><h2>Mouse over the following lines to see fixed highlights</h2><p [appHighlight]="'yellow'">Highlighted in yellow</p><p appHighlight="orange">Highlighted in orange</p><hr /><h2>ngNonBindable</h2><p>Use ngNonBindable to stop evaluation.</p><p ngNonBindable>This should not evaluate: {{ 1 + 1 }}</p><h3>ngNonBindable with a directive</h3><div ngNonBindable [appHighlight]="'yellow'">This should not evaluate: {{ 1 +1 }}, but will highlight yellow.</div>

    The [appHighlight] attribute binding performs two tasks:

    • Applies the highlighting directive to the <p> element
    • Sets the directive's highlight color with a property binding

Setting the value with user input

This section guides you through adding radio buttons to bind your color choice to the appHighlight directive.

  1. Add markup to app.component.html for choosing a color as follows:

    src/app/app.component.html (v2)

          
    <h1>My First Attribute Directive</h1><h2>Pick a highlight color</h2><div>  <input type="radio" name="colors" (click)="color='lightgreen'">Green  <input type="radio" name="colors" (click)="color='yellow'">Yellow  <input type="radio" name="colors" (click)="color='cyan'">Cyan</div><p [appHighlight]="color">Highlight me!</p><p [appHighlight]="color" defaultColor="violet">  Highlight me too!</p><hr /><h2>Mouse over the following lines to see fixed highlights</h2><p [appHighlight]="'yellow'">Highlighted in yellow</p><p appHighlight="orange">Highlighted in orange</p><hr /><h2>ngNonBindable</h2><p>Use ngNonBindable to stop evaluation.</p><p ngNonBindable>This should not evaluate: {{ 1 + 1 }}</p><h3>ngNonBindable with a directive</h3><div ngNonBindable [appHighlight]="'yellow'">This should not evaluate: {{ 1 +1 }}, but will highlight yellow.</div>

  2. Revise the AppComponent.color so that it has no initial value.

    src/app/app.component.ts (class)

          
    import {Component} from '@angular/core';import {HighlightDirective} from './highlight.directive';@Component({  selector: 'app-root',  templateUrl: './app.component.html',  imports: [HighlightDirective],})export class AppComponent {  color = '';}

  3. In highlight.directive.ts, revise onMouseEnter method so that it first tries to highlight with appHighlight and falls back to red if appHighlight is undefined.

    src/app/highlight.directive.ts (mouse-enter)

          
    import {Directive, ElementRef, HostListener, Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @Input() appHighlight = '';  @HostListener('mouseenter') onMouseEnter() {    this.highlight(this.appHighlight || 'red');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

  4. Serve your application to verify that the user can choose the color with the radio buttons.

    Animated gif of the refactored highlight directive changing color according to the radio button the user selects

Binding to a second property

This section guides you through configuring your application so the developer can set the default color.

  1. Add a second Input() property to HighlightDirective called defaultColor.

    src/app/highlight.directive.ts (defaultColor)

          
    import {Directive, ElementRef, HostListener, Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @Input() defaultColor = '';  @Input() appHighlight = '';  @HostListener('mouseenter') onMouseEnter() {    this.highlight(this.appHighlight || this.defaultColor || 'red');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

  2. Revise the directive's onMouseEnter so that it first tries to highlight with the appHighlight, then with the defaultColor, and falls back to red if both properties are undefined.

    src/app/highlight.directive.ts (mouse-enter)

          
    import {Directive, ElementRef, HostListener, Input} from '@angular/core';@Directive({  selector: '[appHighlight]',})export class HighlightDirective {  constructor(private el: ElementRef) {}  @Input() defaultColor = '';  @Input() appHighlight = '';  @HostListener('mouseenter') onMouseEnter() {    this.highlight(this.appHighlight || this.defaultColor || 'red');  }  @HostListener('mouseleave') onMouseLeave() {    this.highlight('');  }  private highlight(color: string) {    this.el.nativeElement.style.backgroundColor = color;  }}

  3. To bind to the AppComponent.color and fall back to "violet" as the default color, add the following HTML. In this case, the defaultColor binding doesn't use square brackets, [], because it is static.

    src/app/app.component.html (defaultColor)

          
    <h1>My First Attribute Directive</h1><h2>Pick a highlight color</h2><div>  <input type="radio" name="colors" (click)="color='lightgreen'">Green  <input type="radio" name="colors" (click)="color='yellow'">Yellow  <input type="radio" name="colors" (click)="color='cyan'">Cyan</div><p [appHighlight]="color">Highlight me!</p><p [appHighlight]="color" defaultColor="violet">  Highlight me too!</p><hr /><h2>Mouse over the following lines to see fixed highlights</h2><p [appHighlight]="'yellow'">Highlighted in yellow</p><p appHighlight="orange">Highlighted in orange</p><hr /><h2>ngNonBindable</h2><p>Use ngNonBindable to stop evaluation.</p><p ngNonBindable>This should not evaluate: {{ 1 + 1 }}</p><h3>ngNonBindable with a directive</h3><div ngNonBindable [appHighlight]="'yellow'">This should not evaluate: {{ 1 +1 }}, but will highlight yellow.</div>

    As with components, you can add multiple directive property bindings to a host element.

The default color is red if there is no default color binding. When the user chooses a color the selected color becomes the active highlight color.

Animated gif of final highlight directive that shows red color with no binding and violet with the default color set. When user selects color, the selection takes precedence.

Deactivating Angular processing with NgNonBindable

To prevent expression evaluation in the browser, add ngNonBindable to the host element. ngNonBindable deactivates interpolation, directives, and binding in templates.

In the following example, the expression {{ 1 + 1 }} renders just as it does in your code editor, and does not display 2.

src/app/app.component.html

      
<h1>My First Attribute Directive</h1><h2>Pick a highlight color</h2><div>  <input type="radio" name="colors" (click)="color='lightgreen'">Green  <input type="radio" name="colors" (click)="color='yellow'">Yellow  <input type="radio" name="colors" (click)="color='cyan'">Cyan</div><p [appHighlight]="color">Highlight me!</p><p [appHighlight]="color" defaultColor="violet">  Highlight me too!</p><hr /><h2>Mouse over the following lines to see fixed highlights</h2><p [appHighlight]="'yellow'">Highlighted in yellow</p><p appHighlight="orange">Highlighted in orange</p><hr /><h2>ngNonBindable</h2><p>Use ngNonBindable to stop evaluation.</p><p ngNonBindable>This should not evaluate: {{ 1 + 1 }}</p><h3>ngNonBindable with a directive</h3><div ngNonBindable [appHighlight]="'yellow'">This should not evaluate: {{ 1 +1 }}, but will highlight yellow.</div>

Applying ngNonBindable to an element stops binding for that element's child elements. However, ngNonBindable still lets directives work on the element where you apply ngNonBindable. In the following example, the appHighlight directive is still active but Angular does not evaluate the expression {{ 1 + 1 }}.

src/app/app.component.html

      
<h1>My First Attribute Directive</h1><h2>Pick a highlight color</h2><div>  <input type="radio" name="colors" (click)="color='lightgreen'">Green  <input type="radio" name="colors" (click)="color='yellow'">Yellow  <input type="radio" name="colors" (click)="color='cyan'">Cyan</div><p [appHighlight]="color">Highlight me!</p><p [appHighlight]="color" defaultColor="violet">  Highlight me too!</p><hr /><h2>Mouse over the following lines to see fixed highlights</h2><p [appHighlight]="'yellow'">Highlighted in yellow</p><p appHighlight="orange">Highlighted in orange</p><hr /><h2>ngNonBindable</h2><p>Use ngNonBindable to stop evaluation.</p><p ngNonBindable>This should not evaluate: {{ 1 + 1 }}</p><h3>ngNonBindable with a directive</h3><div ngNonBindable [appHighlight]="'yellow'">This should not evaluate: {{ 1 +1 }}, but will highlight yellow.</div>

If you apply ngNonBindable to a parent element, Angular disables interpolation and binding of any sort, such as property binding or event binding, for the element's children.