Attribute Directives 4.0
An attribute directive changes the appearance or behavior of a DOM element.
There are three kinds of directives in Angular:
- Components—directives with a template.
- Structural directives—change the DOM layout by adding and removing DOM elements.
- Attribute directives—change the appearance or behavior of an element, component, or another directive.
Components are the most common of the three directives. You saw a simple component in the Starter App.
There are two kinds of attribute directive:
- Class-based: A full-featured attribute directive, implemented using a class.
- Functional: A stateless attribute directive, implemented using a top-level function.
Create a class-based attribute directive
Creating a class-based attribute directive requires writing 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 myHighlight attribute directive to set an element’s background color when the user hovers over that element. You can apply it like this:
Write the directive code
Follow the setup instructions for creating a new local project
Create the following source file in the indicated folder:
@Directive() requires a CSS selector to identify
the HTML in the template that is associated with the directive.
The CSS selector for an attribute
is the attribute name in square brackets.
Here, the directive’s selector is
Angular locates all elements in the template that have an attribute named
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 of colliding with third-party directive names.
Make sure you do not prefix the
highlight directive name with
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.
@Directive() metadata comes the directive’s controller class,
HighlightDirective, which contains the logic for the directive.
Angular creates a new instance of the directive’s controller class for each matching element, injecting an HTML Element into the constructor.
Apply the attribute directive
To use the new
HighlightDirective, create a template that
applies the directive as an attribute to a paragraph (
In Angular terms, the
<p> element is the attribute host.
Put the template in its own
file that looks like this:
Now reference this template in the
Highlight directive to the
directives list. This way Angular
recognizes the directive when it encounters
myHighlight in the template.
Refresh the browser.
The app runs, and the
myHighlight directive highlights the paragraph text.
Your directive isn’t working?
Did you remember to set the
directives attribute of
It is easy to forget!
Open the console in the browser tools and look for an error like this:
EXCEPTION: Template parse errors: Can't bind to 'myHighlight' since it isn't a known property of 'p'.
Angular detects that you’re trying to bind to something
but it can’t find this directive.
You let Angular know by listing
HighlightDirective in the
To summarize, Angular found the
myHighlight attribute on the
It created an instance of the
HighlightDirective class and
injected a reference to the
<p> element into the directive’s constructor
which sets the
<p> element’s background style to yellow.
Respond to user-initiated events
myHighlight simply sets an element color.
The directive could be more dynamic.
It could detect when the user mouses into or out of the element
and respond by setting or clearing the highlight color.
Add two eventhandlers that respond when the mouse enters or leaves,
each adorned by the
@HostListener annotation lets you subscribe to events of the DOM
element that hosts an attribute directive, the
<p> in this case.
- You have to write the listeners correctly.
- The code must detach the listener when the directive is destroyed to avoid memory leaks.
- Talking to DOM API directly isn’t a best practice.
The handlers delegate to a helper method that sets the color on the DOM element,
which you declare and initialize in the constructor.
Here’s the updated directive in full:
Refresh the browser. Confirm that the background color appears when
the mouse hovers over the
p and disappears as it moves out.
Pass values into the directive with an @Input data binding
Currently the highlight color is hard-coded within the directive. That’s inflexible. In this section, you give the developer the power to set the highlight color while applying the directive.
Start by adding a
highlightColor property to the directive class like this:
Binding to an @Input property
@Input annotation. It adds metadata to the class that makes the directive’s
highlightColor property available for binding.
It’s called an input property because data flows from the binding expression into the directive. Without that input metadata, Angular rejects the binding; see below for more about that.
Try it by adding the following directive binding variations to the
color property to the
Let it control the highlight color with a property binding.
That’s good, but it would be nice to simultaneously apply the directive and set the color in the same attribute like this.
[myHighlight] attribute binding both applies the highlighting directive to the
and sets the directive’s highlight color with a property binding.
You’re re-using the directive’s attribute selector (
[myHighlight]) to do both jobs.
That’s a crisp, compact syntax.
You’ll have to rename the directive’s
highlightColor property to
myHighlight because that’s now the color property binding name.
This is disagreeable. The word,
myHighlight, is a terrible property name and it doesn’t convey the property’s intent.
Bind to an @Input alias
Fortunately you can name the directive property whatever you want and alias it for binding purposes.
Restore the original property name and specify the selector as the alias in the argument to
Inside the directive the property is known as
Outside the directive, where you bind to it, it’s known as
You get the best of both worlds: the property name you want and the binding syntax you want:
Now that you’re binding to
highlightColor, modify the
onMouseEnter() method to use it.
If someone neglects to bind to
highlightColor, highlight in red:
Here’s the latest version of the directive class.
Write a harness to try it
It may be difficult to imagine how this directive actually works.
In this section, you’ll turn
AppComponent into a harness that
lets you pick the highlight color with a radio button and bind your color choice to the directive.
app_component.html as follows:
AppComponent.color so that it has no initial value.
open_in_browser Refresh the browser. Here are the harness and directive in action.
Bind to a second property
This highlight directive has a single customizable property. In a real app, it may need more.
At the moment, the default color—the color that prevails until the user picks a highlight color—is hard-coded as “red”. Let the template developer set the default color.
Add a second input property to
Revise the directive’s
onMouseEnter so that it first tries to highlight with the
then with the
defaultColor, and falls back to “red” if both properties are undefined.
How do you bind to a second property when you’re already binding to the
myHighlight attribute name?
As with components, you can add as many directive property bindings as you need by stringing them along in the template.
The developer should be able to write the following template HTML to both bind to the
and fall back to “violet” as the default color.
Angular knows that the
defaultColor binding belongs to the
because you made it public with the
open_in_browser Refresh the browser. Here’s how the harness should work when you’re done coding.
Write a functional directive
A functional directive is a stateless directive that is rendered once. You create a functional directive by annotating a public, top-level function with @Directive().
Create the following functional attribute directive:
Like constructor parameters in class-based directives, function parameters define the functional directive’s dependencies.
When you write a functional directive, follow these rules:
- Make the function return type
- In the @Directive() annotation,
use only the
selectorparameter and, if necessary,
While functional directives are stateless, they can be impure
(making use of global state), as the
autoId directive shows.
Add the following lines at the end of the app component template:
Refresh the browser. The app reports headings ID
This page covered how to:
- Create a class-based attribute directive that modifies the behavior of an element.
- Apply an attribute directive to an element in a template.
- Respond to events that change a class-based directive’s behavior.
- Bind values to a class-based directive.
- Write a functional attribute directive.
The final source code follows:
You can also experience and download the
Appendix: Why add @Input?
In this demo, the
hightlightColor property is an input property of
HighlightDirective. You’ve seen it applied without an alias:
You’ve seen it with an alias:
Either way, the
@Input annotation tells Angular that this property is
public and available for binding by a parent component.
@Input, Angular refuses to bind to the property.
You’ve bound template HTML to component properties before and never used
The difference is a matter of trust.
Angular treats a component’s template as belonging to the component.
The component and its template trust each other implicitly.
Therefore, the component’s own template may bind to any property of that component,
with or without the
But a component or directive shouldn’t blindly trust other components and directives.
The properties of a component or directive are hidden from binding by default.
They are private from an Angular binding perspective.
When adorned with the
@Input annotation, the property becomes public from an Angular binding perspective.
Only then can it be bound by some other component or directive.
You can tell if
@Input is needed by the position of the property name in a binding.
When it appears in the template expression to the right of the equals (=), it belongs to the template’s component and does not require the
When it appears in square brackets ([ ]) to the left of the equals (=), the property belongs to some other component or directive; that property must be adorned with the
Now apply that reasoning to the following example:
colorproperty in the expression on the right belongs to the template’s component. The template and its component trust each other. The
colorproperty doesn’t require the
myHighlightproperty on the left refers to an aliased property of the
HighlightDirective, not a property of the template’s component. There are trust issues. Therefore, the directive property must carry the