angular/aio/content/guide/attribute-directives.md

# Attribute directives

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

<div class="alert is-helpful">

See the <live-example></live-example> for a working example containing the code snippets in this guide.

</div>

## 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`](cli/generate).

    <code-example format="shell" language="shell">

    ng generate directive highlight

    </code-example>

    The CLI creates `src/app/highlight.directive.ts`, a corresponding test file `src/app/highlight.directive.spec.ts`, and declares the directive class in the `AppModule`.

    The CLI generates the default `src/app/highlight.directive.ts` as follows:

    <code-example header="src/app/highlight.directive.ts" path="attribute-directives/src/app/highlight.directive.0.ts"></code-example>

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

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

1.  Add `ElementRef` in the directive's `constructor()` to [inject](guide/dependency-injection) a reference to the host DOM element, the element to which you apply `appHighlight`.

1.  Add logic to the `HighlightDirective` class that sets the background to yellow.

    <code-example header="src/app/highlight.directive.ts" path="attribute-directives/src/app/highlight.directive.1.ts"></code-example>

<div class="alert is-helpful">

Directives *do not* support namespaces.

<code-example header="src/app/app.component.avoid.html (unsupported)" path="attribute-directives/src/app/app.component.avoid.html" region="unsupported"></code-example>

</div>

<a id="apply-directive"></a>

## Applying an attribute directive

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

    <code-example header="src/app/app.component.html" path="attribute-directives/src/app/app.component.1.html" region="applied"></code-example>

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.

<a id="respond-to-user"></a>

## 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'.

    <code-example header="src/app/highlight.directive.ts (imports)" path="attribute-directives/src/app/highlight.directive.2.ts" region="imports"></code-example>

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

    <code-example header="src/app/highlight.directive.ts (mouse-methods)" path="attribute-directives/src/app/highlight.directive.2.ts" region="mouse-methods"></code-example>

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

<div class="alert is-helpful">

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

</div>

The complete directive is as follows:

<code-example header="src/app/highlight.directive.ts" path="attribute-directives/src/app/highlight.directive.2.ts"></code-example>

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

<div class="lightbox">

<img alt="Second Highlight" src="generated/images/guide/attribute-directives/highlight-directive-anim.gif">

</div>

<a id="bindings"></a>

## 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`.

    <code-example header="src/app/highlight.directive.ts (imports)" path="attribute-directives/src/app/highlight.directive.3.ts" region="imports"></code-example>

1.  Add an `appHighlight` `@Input()` property.

    <code-example header="src/app/highlight.directive.ts" path="attribute-directives/src/app/highlight.directive.3.ts" region="input"></code-example>

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

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

    <code-example header="src/app/app.component.ts (class)" path="attribute-directives/src/app/app.component.1.ts" region="class"></code-example>

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

    <code-example header="src/app/app.component.html (color)" path="attribute-directives/src/app/app.component.html" region="color"></code-example>

    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:

    <code-example header="src/app/app.component.html (v2)" path="attribute-directives/src/app/app.component.html" region="v2"></code-example>

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

    <code-example header="src/app/app.component.ts (class)" path="attribute-directives/src/app/app.component.ts" region="class"></code-example>

1.  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`.

    <code-example header="src/app/highlight.directive.ts (mouse-enter)" path="attribute-directives/src/app/highlight.directive.3.ts" region="mouse-enter"></code-example>

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

    <div class="lightbox">

    <img alt="Animated gif of the refactored highlight directive changing color according to the radio button the user selects" src="generated/images/guide/attribute-directives/highlight-directive-v2-anim.gif">

    </div>

<a id="second-property"></a>

## 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`.

    <code-example header="src/app/highlight.directive.ts (defaultColor)" path="attribute-directives/src/app/highlight.directive.ts" region="defaultColor"></code-example>

1.  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`.

    <code-example header="src/app/highlight.directive.ts (mouse-enter)" path="attribute-directives/src/app/highlight.directive.ts" region="mouse-enter"></code-example>

1.  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.

    <code-example header="src/app/app.component.html (defaultColor)" path="attribute-directives/src/app/app.component.html" region="defaultColor"></code-example>

    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.

<div class="lightbox">

<img alt="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." src="generated/images/guide/attribute-directives/highlight-directive-final-anim.gif">

</div>

<a id="ngNonBindable"></a>

## 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`.

<code-example header="src/app/app.component.html" linenums="false" path="attribute-directives/src/app/app.component.html" region="ngNonBindable"></code-example>

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 }}`.

<code-example header="src/app/app.component.html" linenums="false" path="attribute-directives/src/app/app.component.html" region="ngNonBindable-with-directive"></code-example>

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.

<!-- links -->

<!-- external links -->

<!-- end links -->

@reviewed 2022-02-28