block includes
  include ../_util-fns

:marked
  An **Attribute** directive changes the appearance or behavior of a DOM element.


:marked
  # Contents


  * [Directives overview](#directive-overview)
  * [Build a simple attribute directive](#write-directive)
  * [Apply the attribute directive to an element in a template](#apply-directive)
  * [Respond to user-initiated events](#respond-to-user)
  * [Pass values into the directive using data binding](#bindings)
  * [Bind to a second property](#second-property)

  Try the <live-example></live-example>.

.l-main-section
a#directive-overview
:marked
  ## Directives overview

  There are three kinds of directives in Angular:
  1. Components&mdash;directives with a template.
  1. Structural directives&mdash;change the DOM layout by adding and removing DOM elements.
  1. Attribute directives&mdash;change the appearance or behavior of an element.

  *Components* are the most common of the three directives.
  You saw a component for the first time in the [QuickStart](../quickstart.html) example.

  *Structural Directives* change the structure of the view. Two examples are [NgFor](template-syntax.html#ngFor) and [NgIf](template-syntax.html#ngIf)
  in the [Template Syntax](template-syntax.html) page.

  *Attribute directives* are used as attributes of elements. The built-in [NgStyle](template-syntax.html#ngStyle) directive in the [Template Syntax](template-syntax.html) page, for example,
  can change several element styles at the same time.

.l-main-section
a#write-directive
:marked
  ## Build a simple attribute directive
  An attribute directive minimally requires building a controller class annotated with
  `@Directive`, which specifies the selector that identifies
  the attribute.
  The controller class implements the desired directive behavior.

  This page demonstrates building a simple attribute
  directive to set an element's background color
  when the user hovers over that element.
.l-sub-section
  :marked
    Technically, a directive isn't necessary to simply set the background color. Style binding can set styles as follows:

  +makeExample('attribute-directives/ts/app/app.component.1.html','p-style-background')

  :marked
    Read more about [style binding](template-syntax.html#style-binding) on the [Template Syntax](template-syntax.html) page.

    For a simple example, though, this will demonstrate how attribute directives work.



:marked
  ### Write the directive code
  Follow the [setup](setup.html) instructions for creating a new project
  named <span ngio-ex>attribute-directives</span>.

:marked
  Create the following source file in the indicated folder with the following code:
+makeExample('app/highlight.directive.1.ts')

block highlight-directive-1
  :marked
    The `import` statement specifies symbols from the Angular `core`:
    1. `Directive` provides the functionality of the `@Directive` decorator.
    1. `ElementRef` [injects](dependency-injection.html) into the directive's constructor
    so the code can access the DOM element.
    1. `Input` allows data to flow from the binding expression into the directive.

    Next, the `@Directive` decorator function contains the directive metadata in a configuration object
    as an argument.

:marked
  `@Directive` requires a CSS selector to identify
  the HTML in the template that is associated with the directive.
  The [CSS selector for an attribute](https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors)
  is the attribute name in square brackets.
  Here, the directive's selector is `[myHighlight]`.
  Angular will locate all elements in the template that have an attribute named `myHighlight`.
.l-sub-section
  :marked
    ### Why not call it "highlight"?
    Though *highlight* is a more concise name than *myHighlight* and would work,
    a best practice is to prefix selector names to ensure
    they don't conflict with  standard HTML attributes.
    This also reduces the risk colliding with third-party directive names.

    Make sure you do **not** prefix the `highlight` directive name with **`ng`** because
    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.

p
  | After the #[code @Directive] metadata comes the directive's controller class, called #[code HighlightDirective], which contains the logic for the directive.
  +ifDocsFor('ts')
    |  Exporting #[code HighlightDirective] makes it accessible to other components.
:marked
  Angular creates a new instance of the directive's controller class for
  each matching element, injecting an Angular `ElementRef`
  into the constructor.
  `ElementRef` is a service that grants direct access to the DOM element
  through its `nativeElement` property.

.l-main-section
a#apply-directive
:marked
  ## Apply the attribute directive
  To use the new `HighlightDirective`, create a template that
  applies the directive as an attribute to a paragraph (`p`) element.
  In Angular terms, the `<p>` element will be the attribute **host**.
p
  | Put the template in its own
  code #[+adjExPath('app.component.html')]
  | file that looks like this:
+makeExample('attribute-directives/ts/app/app.component.1.html',null,'app/app.component.html')(format=".")
:marked
  Now reference this template in the `AppComponent`:
+makeExample('attribute-directives/ts/app/app.component.ts',null,'app/app.component.ts')
:marked
  Next, add an `import` statement to fetch the `Highlight` directive and
  add that class to the `declarations` NgModule metadata. This way Angular
  recognizes the directive when it encounters `myHighlight` in the template.
+makeExample('attribute-directives/ts/app/app.module.ts',null,'app/app.module.ts')
:marked
  Now when the app runs, the `myHighlight` directive highlights the paragraph text.

figure.image-display
  img(src="/resources/images/devguide/attribute-directives/first-highlight.png" alt="First Highlight")
.l-sub-section
  :marked
    ### Your directive isn't working?

    Did you remember to add the directive to the the `declarations` attribute of `@NgModule`? It is easy to forget!

    Open the console in the browser tools and look for an error like this:
  code-example(format="nocode").
    EXCEPTION: Template parse errors:
      Can't bind to 'myHighlight' since it isn't a known property of 'p'.

  :marked
    Angular detects that you're trying to bind to *something* but it doesn't know what,
    so it looks to the `declarations` metadata array. By specifying `HighlightDirective`
    in the array, Angular knows to check the import statements and from there,
    to go to `highlight.directive.ts` to find out what `myHighlight` does.

:marked
  To summarize, Angular found the `myHighlight` attribute on the `<p>` element. It created
  an instance of the `HighlightDirective` class,
  injecting a reference to the element into the constructor
  where the `<p>` element's background style is set to yellow.

.l-main-section
a#respond-to-user
:marked
  ## Respond to user-initiated events

  Currently, `myHighlight` simply sets an element color.
  The directive should set the color when the user hovers over an element.

  This requires two things:
  1. detecting when the user hovers into and out of the element.
  2. responding to those actions by setting and clearing the highlight color.

  To do this, you can apply the `@HostListener` !{_decorator} to methods which are called when an event is raised.

+makeExample('attribute-directives/ts/app/highlight.directive.2.ts','host')(format=".")

.l-sub-section
  :marked
    The `@HostListener` !{_decorator} refers to the DOM element that hosts an attribute directive, the `<p>` in this case.

    It is possible to attach event listeners by manipulating the host DOM element directly, but
    there are at least three problems with such an approach:

    1. You have to write the listeners correctly.
    1. The code must *detach* the listener when the directive is destroyed to avoid memory leaks.
    1. Talking to DOM API directly isn't a best practice.

:marked
  Now implement the two mouse event handlers:
+makeExample('attribute-directives/ts/app/highlight.directive.2.ts','mouse-methods')(format=".")
:marked
  Notice that they delegate to a helper method that sets the color via a private local variable, `#{_priv}el`.
  Next, revise the constructor to capture the `ElementRef.nativeElement` in this variable.

+makeExample('attribute-directives/ts/app/highlight.directive.2.ts','ctor')(format=".")
:marked
  Here's the updated directive:
+makeExample('app/highlight.directive.2.ts')
:marked
  Run the app and confirm that the background color appears when the mouse hovers over the `p` and
  disappears as it moves out.
figure.image-display
  img(src="/resources/images/devguide/attribute-directives/highlight-directive-anim.gif" alt="Second Highlight")
.l-main-section
a#bindings
:marked
  ## Pass values into the directive using data binding

  Currently the highlight color is hard-coded within the directive. That's inflexible.
  A better practice is to set the color externally with a binding as follows:
+makeExample('attribute-directives/ts/app/app.component.html','pHost')
:marked
  You can extend the directive class with a bindable **input** `highlightColor` property and use it to highlight text.

  Here is the final version of the class:
+makeExcerpt('app/highlight.directive.ts', 'class')
a#input
:marked
  The new `highlightColor` property is called an *input* property because data flows from the binding expression into the directive.
  Notice the `@Input()` #{_decorator} applied to the property.
+makeExcerpt('app/highlight.directive.ts', 'color')
:marked
  `@Input` adds metadata to the class that makes the `highlightColor` property available for
  property binding under the `myHighlight` alias.
  Without this input metadata Angular rejects the binding.
  See the [appendix](#why-input) below for more information.
.l-sub-section
  :marked
    ### @Input(_alias_)
    Currently, the code **aliases** the `highlightColor` property with the attribute name by
    passing `myHighlight` into the `@Input` #{_decorator}:
  +makeExcerpt('app/highlight.directive.ts', 'color', '')
  :marked
    The code binds to the attribute name, `myHighlight`, but the
    the directive property name is `highlightColor`. That's a disconnect.

    You can resolve the discrepancy by renaming the property to `myHighlight` and define it as follows:

  +makeExcerpt('app/highlight.directive.ts', 'highlight', '')

:marked
  Now that you're getting the highlight color as an input, modify the `onMouseEnter()` method to use
  it instead of the hard-coded color name and define red as the default color.

+makeExcerpt('attribute-directives/ts/app/highlight.directive.ts', 'mouse-enter', '')
:marked
  To let users pick the highlight color and bind their choice to the directive,
  update `app.component.html` as follows:

+makeExcerpt('attribute-directives/ts/app/app.component.html', 'v2', '')

.l-sub-section
  :marked
    ### Where is the templated *color* property?

    You may notice that the radio button click handlers in the template set a `color` property
    and the code is binding that `color` to the directive.
    However, you never defined a color property for the host `AppComponent`.
    Yet this code works. Where is the template `color` value going?

    Browser debugging reveals that Angular dynamically added a `color` property
    to the runtime instance of the `AppComponent`.

    This is *convenient* behavior but it is also *implicit* behavior that could be confusing.
    For clarity, consider adding the `color` property to the `AppComponent`.



:marked
  Here is the second version of the directive in action.
figure.image-display
  img(src="/resources/images/devguide/attribute-directives/highlight-directive-v2-anim.gif" alt="Highlight v.2")

.l-main-section
a#second-property
:marked
  ## Bind to a second property
  This example directive only has a single customizable property. A real app often needs more.

  Let's allow the template developer to set the default color&mdash;the color that prevails until the user picks a highlight color.
  To do this, first add a second **input** property to `HighlightDirective` called `defaultColor`:
+makeExample('attribute-directives/ts/app/highlight.directive.ts', 'defaultColor')(format=".")
:marked
  The `defaultColor` property has a setter that overrides the hard-coded default color, "red".
  You don't need a getter.

  How do you bind to it? The app is already using `myHighlight` attribute name as a binding target.

  Remember that a *component is a directive, too*.
  You can add as many component property bindings as you need by stringing them along in the template
  as in this example that sets the `a`, `b`, `c` properties to the string literals 'a', 'b', and 'c'.
code-example(format="." ).
  &lt;my-component [a]="'a'" [b]="'b'" [c]="'c'">&lt;my-component>
:marked
  The same holds true for an attribute directive.
+makeExample('attribute-directives/ts/app/app.component.html', 'defaultColor')(format=".")
:marked
  Here the code is binding the user's color choice to the `myHighlight` attribute as before.
  It is *also* binding the literal string, 'violet', to the `defaultColor`.


  Here is the final version of the directive in action.
figure.image-display
  img(src="/resources/images/devguide/attribute-directives/highlight-directive-final-anim.gif" alt="Final Highlight")

.l-main-section
:marked
  ## Summary
  This page covered how to:
  - [Build a simple **attribute directive** to attach behavior to an HTML element](#write-directive).
  - [Use that directive in a template](#apply-directive).
  - [Respond to **events** to change behavior based on an event](#respond-to-user).
  - [Use **binding** to pass values to the attribute directive](#bindings).

  The final source:

+makeTabs(
  `attribute-directives/ts/app/app.component.ts,
   attribute-directives/ts/app/app.component.html,
   attribute-directives/ts/app/highlight.directive.ts,
   attribute-directives/ts/app/app.module.ts,
   attribute-directives/ts/app/main.ts,
   attribute-directives/ts/index.html
  `,
  ',,full',
  `app.component.ts,
   app.component.html,
   highlight.directive.ts,
   app.module.ts,
   main.ts,
   index.html
  `)


a#why-input
.l-main-section
:marked
  ### Appendix: Input properties

  In this demo, the `highlightColor` property is an ***input*** property of
  `HighlightDirective`.

  You've seen properties in bindings before but never had to declare them as anything. Why now?

  Angular makes a subtle but important distinction between binding **sources** and **targets**.

  In all previous bindings, the directive or component property was a binding ***source***.
  A property is a *source* if it appears in the template expression to the ***right*** of the equals (=).

  A property is a *target* when it appears in **square brackets** ([ ]) to the **left** of the equals (=)
  as it is does when binding to the `myHighlight` property of the `HighlightDirective`.
+makeExample('attribute-directives/ts/app/app.component.html','pHost')(format=".")
:marked
  The 'color' in `[myHighlight]="color"` is a binding ***source***.
  A source property doesn't require a declaration.

  The 'myHighlight' in `[myHighlight]="color"` *is* a binding ***target***.
  You must declare it as an *input* property or
  Angular rejects the binding with a clear error.

  Angular treats a *target* property differently for a good reason.
  A component or directive in target position needs protection.

  Imagine that `HighlightDirective` did truly wonderous things in a
  popular open source project.

  Surprisingly, some people &mdash; perhaps naively &mdash;
  start binding to *every* property of the directive.
  Not just the one or two properties you expected them to target. *Every* property.
  That could really mess up your directive in ways you didn't anticipate and have no desire to support.

  The ***input*** declaration ensures that consumers of your directive can only bind to
  the properties of the public API but nothing else.
