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Creating Angular Directives Part 4 - Dispatching Events

I'm updating my Learn With tutorial books that teach Angular Development for Angular 8. I decided to include a deep dive into Angular Directives. This series of posts is a snippet of what will be in the full series, and is focused on creating attribute directives.

Check out Part 1, with project setup and a starter directive and part 2 that introduces the renderer2, and part 3 that discusses input properties.

We have created a directive named reverse which will toggle CSS Between two separate states.

Dispatching Events

A directive can dispatch its own events, just like a component can. Let's add an event to the directive that will dispatch whenever the state changes. First, I'll add a new class:

export class CSSReversedEvent {
addedCSSClass: string;
removedCSSClass: string;

I put this class in the reverse.directive.ts file, instead of creating its own class file. It includes one value for the CSS Class that was removed, and one for the CSS Class that was added.

Now inside the ReverseDirective class, add the output event:

@Output() stateChange: EventEmitter<CSSReversedEvent> = new EventEmitter();

This uses the @Output() metadata to create the EventEmitter named stateChange. How look at the changeState() method:

changeState() {
const newState: CSSReversedEvent = new CSSReversedEvent();
if (this.reversedState) {
this.renderer.removeClass(this.el.nativeElement, this.regularCSSClass);
this.renderer.addClass(this.el.nativeElement, this.reversedCSSClass);
newState.addedCSSClass = this.reversedCSSClass;
newState.removedCSSClass = this.regularCSSClass;
} else {
this.renderer.removeClass(this.el.nativeElement, this.reversedCSSClass);
this.renderer.addClass(this.el.nativeElement, this.regularCSSClass);
newState.addedCSSClass = this.regularCSSClass;
newState.removedCSSClass = this.reversedCSSClass;

The first line in the method creates a new instance of the CSSReversedEvent class. The middle of the method will add the CSS Values onto this new object, depending on which state we are entering. The final name of the method will dispatch the event.

Open up the create-attribute-directive.html file and add in the stateChange event listener:

<button (click)="reverseState()"

Reverse State
The event listener will call the onStateChange() event inside the create-attribute-directive.component.ts file:

onStateChange(event) {

For simplicity I just outputted the event to the console.

Rerun the code and start clicking buttons:

During component initialization this event is dispatched twice. Once after the reversedState value is set, but before the regularCSSClass and reversedCSSClass are set. We see the event dispatching undefined for each class in this case. Then it is dispatched a second time after the initialization is complete. This is when the initial styles are applied to the button.

Click the button a few times to see the events continue to dispatch:

At this point in the series, you should realize that there is a lot you can do with directives.

What's Next?

For the last article in this series, I'm going to give a sample of applying the same directive to different elements.

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