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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. Read more about creating them
in step three of [QuickStart](../quickstart.html#root-component).
*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
Create a new project folder (`attribute-directives`) and follow the steps in [QuickStart](../quickstart.html).
include ../_quickstart_repo
: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.
1. `Renderer` allows the code to change the DOM element's style.
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` and `Renderer`
into the constructor.
`ElementRef` is a service that grants direct access to the DOM element
through its `nativeElement` property and `Renderer` allows the code to set the element style.
.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.
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