adev/src/content/guide/directives/attribute-directives.md
Change the appearance or behavior of DOM elements and Angular components with attribute directives.
This section walks you through creating a highlight directive that sets the background color of the host element to yellow.
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.
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].
Import ElementRef from @angular/core.
ElementRef grants direct access to the host DOM element through its nativeElement property.
Add ElementRef in the directive's constructor() to inject a reference to the host DOM element, the element to which you apply appHighlight.
Add logic to the HighlightDirective class that sets the background to yellow.
IMPORTANT: Directives do not support namespaces.
<p app:Highlight>This is invalid</p>
To use the HighlightDirective, add a <p> element to the HTML template with the directive as an attribute.
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.
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.
Configure host event bindings using the host property in the @Directive() decorator.
Add two event handler methods, and map host element events to them via the host property.
Subscribe to events of the DOM element that hosts an attribute directive (the <p> in this case) by configuring event listeners on the directive's host property.
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:
<docs-code header="highlight.directive.ts" path="adev/src/content/examples/attribute-directives/src/app/highlight.directive.2.ts"/>The background color appears when the pointer hovers over the paragraph element and disappears as the pointer moves out.
This section walks you through setting the highlight color while applying the HighlightDirective.
In highlight.directive.ts, import input from @angular/core.
Add an appHighlight input property.
The input() function adds metadata to the class that makes the directive's appHighlight property available for binding.
In app.component.ts, add a color property to the AppComponent.
To simultaneously apply the directive and the color, use property binding with the appHighlight directive selector, setting it equal to color.
The [appHighlight] attribute binding performs two tasks:
<p> elementThis section guides you through adding radio buttons to bind your color choice to the appHighlight directive.
Add markup to app.component.html for choosing a color as follows:
Revise the AppComponent.color so that it has no initial value.
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.
<docs-code header="highlight.directive.ts (mouse-enter)" path="adev/src/content/examples/attribute-directives/src/app/highlight.directive.3.ts" region="mouse-enter"/>
Serve your application to verify that the user can choose the color with the radio buttons.
This section guides you through configuring your application so the developer can set the default color.
Add a second input() property to HighlightDirective called defaultColor.
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.
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 the value is a static string, not a dynamic expression.
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.
NgNonBindableTo 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.
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 }}.
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.