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@ -4,11 +4,8 @@ block includes
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:marked
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An **Attribute** directive changes the appearance or behavior of a DOM element.
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:marked
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# Contents
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* [Directives overview](#directive-overview)
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* [Build a simple attribute directive](#write-directive)
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* [Apply the attribute directive to an element in a template](#apply-directive)
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@ -18,8 +15,7 @@ block includes
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Try the <live-example title="Attribute Directive example"></live-example>.
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.l-main-section
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a#directive-overview
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.l-main-section#directive-overview
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:marked
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## Directives overview
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@ -37,13 +33,14 @@ a#directive-overview
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Learn about them in the [Structural Directives](structural-directives.html) guide.
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*Attribute directives* are used as attributes of elements.
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The built-in [NgStyle](template-syntax.html#ngStyle) directive in the [Template Syntax](template-syntax.html) guide, for example,
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The built-in [NgStyle](template-syntax.html#ngStyle) directive in the
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[Template Syntax](template-syntax.html) guide, for example,
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can change several element styles at the same time.
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.l-main-section
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a#write-directive
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.l-main-section#write-directive
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:marked
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## Build a simple attribute directive
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An attribute directive minimally requires building a controller class annotated with
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`@Directive`, which specifies the selector that identifies
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the attribute.
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@ -53,27 +50,28 @@ a#write-directive
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directive to set an element's background color
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when the user hovers over that element. You can apply it like this:
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+makeExample('attribute-directives/ts/src/app/app.component.1.html','applied')
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+makeExcerpt('src/app/app.component.1.html', 'applied', '')
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:marked
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### Write the directive code
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Follow the [setup](setup.html) instructions for creating a new local project
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named <span ngio-ex>attribute-directives</span>.
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:marked
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Create the following source file in `src/app` with the following code:
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Create the following source file in the indicated folder:
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+makeExample('src/app/highlight.directive.1.ts')
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block highlight-directive-1
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:marked
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The `import` statement specifies symbols from the Angular `core`:
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1. `Directive` provides the functionality of the `@Directive` decorator.
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1. `ElementRef` [injects](dependency-injection.html) into the directive's constructor
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so the code can access the DOM element.
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1. `Input` allows data to flow from the binding expression into the directive.
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:marked
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The `import` statement specifies symbols from the Angular `core`:
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Next, the `@Directive` decorator function contains the directive metadata in a configuration object
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as an argument.
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1. `Directive` provides the functionality of the `@Directive` decorator.
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1. `ElementRef` [injects](dependency-injection.html) into the directive's constructor
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so the code can access the DOM element.
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1. `Input` allows data to flow from the binding expression into the directive.
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Next, the `@Directive` decorator function contains the directive metadata in a configuration object
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as an argument.
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:marked
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`@Directive` requires a CSS selector to identify
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@ -82,59 +80,70 @@ block highlight-directive-1
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is the attribute name in square brackets.
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Here, the directive's selector is `[myHighlight]`.
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Angular locates all elements in the template that have an attribute named `myHighlight`.
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.l-sub-section
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:marked
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### Why not call it "highlight"?
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Though *highlight* is a more concise name than *myHighlight* and would work,
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a best practice is to prefix selector names to ensure
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they don't conflict with standard HTML attributes.
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they don't conflict with standard HTML attributes.
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This also reduces the risk of colliding with third-party directive names.
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Make sure you do **not** prefix the `highlight` directive name with **`ng`** because
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that prefix is reserved for Angular and using it could cause bugs that are difficult to diagnose. For a simple demo, the short prefix, `my`, helps distinguish your custom directive.
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that prefix is reserved for Angular and using it could cause bugs that are difficult to diagnose.
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For a simple demo, the short prefix, `my`, helps distinguish your custom directive.
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p
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| After the #[code @Directive] metadata comes the directive's controller class, called #[code HighlightDirective], which contains the logic for the directive.
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+ifDocsFor('ts')
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| Exporting #[code HighlightDirective] makes it accessible to other components.
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:marked
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After the `@Directive` metadata comes the directive's controller class,
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called `HighlightDirective`, which contains the logic for the directive.
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<span if-docs="ts">Exporting `HighlightDirective` makes it accessible to other components.</span>
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Angular creates a new instance of the directive's controller class for
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each matching element, injecting an Angular `ElementRef`
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into the constructor.
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`ElementRef` is a service that grants direct access to the DOM element
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through its `nativeElement` property.
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.l-main-section
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a#apply-directive
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.l-main-section#apply-directive
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:marked
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## Apply the attribute directive
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To use the new `HighlightDirective`, create a template that
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applies the directive as an attribute to a paragraph (`<p>`) element.
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In Angular terms, the `<p>` element is the attribute **host**.
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p
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| Put the template in its own
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code #[+adjExPath('app.component.html')]
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| file that looks like this:
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+makeExample('attribute-directives/ts/src/app/app.component.1.html',null,'src/app/app.component.html')(format=".")
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Put the template in its own <span ngio-ex>app.component.html</span>
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file that looks like this:
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+makeExample('src/app/app.component.1.html')
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:marked
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Now reference this template in the `AppComponent`:
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+makeExample('attribute-directives/ts/src/app/app.component.ts',null,'src/app/app.component.ts')
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+makeExample('src/app/app.component.ts')
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:marked
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Next, add an `import` statement to fetch the `Highlight` directive and
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add that class to the `declarations` NgModule metadata. This way Angular
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recognizes the directive when it encounters `myHighlight` in the template.
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+makeExample('attribute-directives/ts/src/app/app.module.ts',null,'src/app/app.module.ts')
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+makeExample('src/app/app.module.ts')
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:marked
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Now when the app runs, the `myHighlight` directive highlights the paragraph text.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/first-highlight.png" alt="First Highlight")
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.l-sub-section
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:marked
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### Your directive isn't working?
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Did you remember to add the directive to the `declarations` attribute of `@NgModule`? It is easy to forget!
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Did you remember to add the directive to the `declarations` attribute of `@NgModule`?
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It is easy to forget!
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Open the console in the browser tools and look for an error like this:
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code-example(format="nocode").
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EXCEPTION: Template parse errors:
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Can't bind to 'myHighlight' since it isn't a known property of 'p'.
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@ -151,8 +160,7 @@ figure.image-display
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injected a reference to the `<p>` element into the directive's constructor
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which sets the `<p>` element's background style to yellow.
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.l-main-section
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a#respond-to-user
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.l-main-section#respond-to-user
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:marked
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## Respond to user-initiated events
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@ -163,14 +171,18 @@ a#respond-to-user
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Begin by adding `HostListener` to the list of imported symbols;
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add the `Input` symbol as well because you'll need it soon.
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+makeExample('attribute-directives/ts/src/app/highlight.directive.ts','imports')(format=".")
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+makeExcerpt('src/app/highlight.directive.ts', 'imports', '')
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:marked
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Then add two eventhandlers that respond when the mouse enters or leaves, each adorned by the `HostListener` !{_decorator}.
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+makeExample('attribute-directives/ts/src/app/highlight.directive.2.ts','mouse-methods')(format=".")
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Then add two eventhandlers that respond when the mouse enters or leaves,
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each adorned by the `HostListener` !{_decorator}.
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+makeExcerpt('src/app/highlight.directive.2.ts','mouse-methods', '')
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:marked
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The `@HostListener` !{_decorator} lets you subscribe to events of the DOM element that hosts an attribute directive, the `<p>` in this case.
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The `@HostListener` !{_decorator} lets you subscribe to events of the DOM
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element that hosts an attribute directive, the `<p>` in this case.
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.l-sub-section
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:marked
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@ -185,17 +197,21 @@ a#respond-to-user
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The handlers delegate to a helper method that sets the color on the DOM element, `#{_priv}el`,
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which you declare and initialize in the constructor.
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+makeExample('attribute-directives/ts/src/app/highlight.directive.2.ts','ctor')(format=".")
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+makeExcerpt('src/app/highlight.directive.2.ts (constructor)', 'ctor')
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:marked
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Here's the updated directive in full:
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+makeExample('src/app/highlight.directive.2.ts')
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:marked
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Run the app and confirm that the background color appears when the mouse hovers over the `p` and
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disappears as it moves out.
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Run the app and confirm that the background color appears when
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the mouse hovers over the `p` and disappears as it moves out.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/highlight-directive-anim.gif" alt="Second Highlight")
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.l-main-section
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a#bindings
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.l-main-section#bindings
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:marked
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## Pass values into the directive with an _@Input_ data binding
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@ -203,7 +219,8 @@ a#bindings
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In this section, you give the developer the power to set the highlight color while applying the directive.
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Start by adding a `highlightColor` property to the directive class like this:
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+makeExample('attribute-directives/ts/src/app/highlight.directive.2.ts','color', 'src/app/highlight.directive.ts')
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+makeExcerpt('src/app/highlight.directive.2.ts (highlightColor)','color')
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a#input
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:marked
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@ -215,18 +232,23 @@ a#input
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Without that input metadata, Angular rejects the binding; see [below](#why-input "Why add @Input?") for more about that.
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Try it by adding the following directive binding variations to the `AppComponent` template:
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+makeExample('attribute-directives/ts/src/app/app.component.1.html','color-1', 'src/app/app.component.html')(format='.')
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+makeExcerpt('src/app/app.component.1.html (excerpt)', 'color-1')
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:marked
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Add a `color` property to the `AppComponent`.
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+makeExample('attribute-directives/ts/src/app/app.component.1.ts','class', 'src/app/app.component.ts (class)')(format='.')
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+makeExcerpt('src/app/app.component.1.ts (class)')
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:marked
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Let it control the highlight color with a property binding.
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+makeExample('attribute-directives/ts/src/app/app.component.1.html','color-2', 'src/app/app.component.html')
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+makeExcerpt('src/app/app.component.1.html (excerpt)', 'color-2')
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:marked
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That's good, but it would be nice to _simultaneously_ apply the directive and set the color _in the same attribute_ like this.
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+makeExample('attribute-directives/ts/src/app/app.component.html','color')
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+makeExcerpt('src/app/app.component.html', 'color', '')
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:marked
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The `[myHighlight]` attribute binding both applies the highlighting directive to the `<p>` element
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@ -235,7 +257,9 @@ a#input
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That's a crisp, compact syntax.
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You'll have to rename the directive's `highlightColor` property to `myHighlight` because that's now the color property binding name.
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+makeExample('attribute-directives/ts/src/app/highlight.directive.2.ts','color-2', 'src/app/highlight.directive.ts (renamed to match directive selector)')
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+makeExcerpt('src/app/highlight.directive.2.ts (renamed to match directive selector)', 'color-2')
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:marked
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This is disagreeable. The word, `myHighlight`, is a terrible property name and it doesn't convey the property's intent.
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@ -247,47 +271,53 @@ a#input-alias
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Restore the original property name and specify the selector as the alias in the argument to `@Input`.
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+makeExcerpt('src/app/highlight.directive.ts', 'color', 'src/app/highlight.directive.ts (color property with alias')
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+makeExcerpt('src/app/highlight.directive.ts (color property with alias)', 'color')
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:marked
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_Inside_ the directive the property is known as `highlightColor`.
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_Outside_ the directive, where you bind to it, it's known as `myHighlight`.
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You get the best of both worlds: the property name you want and the binding syntax you want:
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+makeExample('attribute-directives/ts/src/app/app.component.html','color')
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+makeExcerpt('src/app/app.component.html', 'color', '')
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:marked
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Now that you're binding to `highlightColor`, modify the `onMouseEnter()` method to use it.
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If someone neglects to bind to `highlightColor`, highlight in "red" by default.
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If someone neglects to bind to `highlightColor`, highlight in red:
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+makeExcerpt('src/app/highlight.directive.3.ts (mouse enter)', 'mouse-enter')
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+makeExample('attribute-directives/ts/src/app/highlight.directive.3.ts', 'mouse-enter', 'src/app/highlight.directive.ts (mouse enter)')(format='.')
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:marked
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Here's the latest version of the directive class.
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+makeExcerpt('src/app/highlight.directive.3.ts')
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:marked
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## Write a harness to try it
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:marked
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It may be difficult to imagine how this directive actually works.
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In this section, you'll turn `AppComponent` into a harness that
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lets you pick the highlight color with a radio button and bind your color choice to the directive.
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Update `app.component.html` as follows:
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Update <span ngio-ex>app.component.html</span> as follows:
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+makeExcerpt('attribute-directives/ts/src/app/app.component.html', 'v2', '')
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+makeExcerpt('src/app/app.component.html', 'v2', '')
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:marked
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Revise the `AppComponent.color` so that it has no initial value.
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+makeExcerpt('attribute-directives/ts/src/app/app.component.ts', 'class', '')
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+makeExcerpt('src/app/app.component.ts', 'class', '')
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:marked
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Here is the harness and directive in action.
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Here are the harness and directive in action.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/highlight-directive-v2-anim.gif" alt="Highlight v.2")
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.l-main-section
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a#second-property
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.l-main-section#second-property
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:marked
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## Bind to a second property
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This highlight directive has a single customizable property. In a real app, it may need more.
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At the moment, the default color—the color that prevails until
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@ -295,30 +325,39 @@ a#second-property
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Let the template developer set the default color.
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Add a second **input** property to `HighlightDirective` called `defaultColor`:
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+makeExcerpt('attribute-directives/ts/src/app/highlight.directive.ts', 'defaultColor','src/app/highlight.directive.ts (defaultColor)')
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+makeExcerpt('src/app/highlight.directive.ts (defaultColor)')
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:marked
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Revise the directive's `onMouseEnter` so that it first tries to highlight with the `highlightColor`,
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then with the `defaultColor`, and falls back to "red" if both properties are undefined.
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+makeExample('attribute-directives/ts/src/app/highlight.directive.ts', 'mouse-enter')(format=".")
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+makeExcerpt('src/app/highlight.directive.ts', 'mouse-enter', '')
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:marked
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How do you bind to a second property when you're already binding to the `myHighlight` attribute name?
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As with components, you can add as many directive property bindings as you need by stringing them along in the template.
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The developer should be able to write the following template HTML to both bind to the `AppComponent.color`
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and fall back to "violet" as the default color.
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+makeExample('attribute-directives/ts/src/app/app.component.html', 'defaultColor')(format=".")
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+makeExcerpt('src/app/app.component.html', 'defaultColor', '')
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:marked
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Angular knows that the `defaultColor` binding belongs to the `HighlightDirective`
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because you made it _public_ with the `@Input` !{_decorator}.
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Here's how the harness should work when you're done coding.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/highlight-directive-final-anim.gif" alt="Final Highlight")
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.l-main-section
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:marked
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## Summary
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This page covered how to:
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- [Build an **attribute directive**](#write-directive) that modifies the behavior of an element.
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- [Apply the directive](#apply-directive) to an element in a template.
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- [Respond to **events**](#respond-to-user) that change the directive's behavior.
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@ -346,17 +385,19 @@ figure.image-display
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:marked
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You can also experience and download the <live-example title="Attribute Directive example"></live-example>.
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a#why-input
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.l-main-section
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.l-main-section#why-input
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:marked
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### Appendix: Why add _@Input_?
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In this demo, the `hightlightColor` property is an ***input*** property of
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the `HighlightDirective`. You've seen it applied without an alias:
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+makeExample('attribute-directives/ts/src/app/highlight.directive.2.ts','color')
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+makeExcerpt('src/app/highlight.directive.2.ts', 'color', '')
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:marked
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You've seen it with an alias:
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+makeExample('attribute-directives/ts/src/app/highlight.directive.ts','color')
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+makeExcerpt('src/app/highlight.directive.ts', 'color', '')
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:marked
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Either way, the `@Input` !{_decorator} tells Angular that this property is
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@ -381,19 +422,21 @@ a#why-input
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You can tell if `@Input` is needed by the position of the property name in a binding.
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* When it appears in the template expression to the ***right*** of the equals (=),
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it belongs to the template's component and does not require the `@Input` !{_decorator}.
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it belongs to the template's component and does not require the `@Input` !{_decorator}.
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* When it appears in **square brackets** ([ ]) to the **left** of the equals (=),
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the property belongs to some _other_ component or directive;
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that property must be adorned with the `@Input` !{_decorator}.
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the property belongs to some _other_ component or directive;
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that property must be adorned with the `@Input` !{_decorator}.
|
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Now apply that reasoning to the following example:
|
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|
|
+makeExample('attribute-directives/ts/src/app/app.component.html','color')(format=".")
|
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|
|
+makeExcerpt('src/app/app.component.html', 'color', '')
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:marked
|
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* The `color` property in the expression on the right belongs to the template's component.
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The template and its component trust each other.
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The `color` property doesn't require the `@Input` !{_decorator}.
|
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The template and its component trust each other.
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The `color` property doesn't require the `@Input` !{_decorator}.
|
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* The `myHighlight` property on the left refers to an _aliased_ property of the `MyHighlightDirective`,
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not a property of the template's component. There are trust issues.
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Therefore, the directive property must carry the `@Input` !{_decorator}.
|
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* The `myHighlight` property on the left refers to an _aliased_ property of the `HighlightDirective`,
|
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not a property of the template's component. There are trust issues.
|
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Therefore, the directive property must carry the `@Input` !{_decorator}.
|
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