ghostfolio/.agents/skills/angular-developer/references/outputs.md

Outputs (Custom Events)

Outputs allow a child component to emit custom events that a parent component can listen to. Angular recommends using the new output() function for modern applications.

Function-based outputs

Declare outputs using the output() function. This returns an OutputEmitterRef.

import {Component, output} from '@angular/core';

@Component({
  selector: 'custom-slider',
  template: `<button (click)="changeValue(50)">Set to 50</button>`,
})
export class CustomSlider {
  // Output without event data
  panelClosed = output<void>();

  // Output with event data (number)
  valueChanged = output<number>();

  changeValue(newValue: number) {
    this.valueChanged.emit(newValue);
  }
}

Usage in Template

Bind to the output event using parentheses (). If the event emits data, access it using the special $event variable.

<custom-slider (panelClosed)="savePanelState()" (valueChanged)="logValue($event)" />

Configuration Options

The output function accepts a config object to specify an alias.

@Component({...})
export class CustomSlider {
  // The event is named 'valueChanged' in the template,
  // but accessed as 'changed' in the component class.
  changed = output<number>({ alias: 'valueChanged' });
}

Programmatic Subscription

When creating components dynamically, you can subscribe to outputs programmatically:

const componentRef = viewContainerRef.createComponent(CustomSlider);

const subscription = componentRef.instance.valueChanged.subscribe((val) => {
  console.log('Value changed:', val);
});

// Clean up manually if needed (Angular cleans up destroyed components automatically)
subscription.unsubscribe();

Decorator-based Outputs (@Output)

The legacy API uses the @Output() decorator with an EventEmitter. It remains supported but is not recommended for new code.

import { Component, Output, EventEmitter } from '@angular/core';

@Component({...})
export class LegacyExample {
  @Output() valueChanged = new EventEmitter<number>();

  // With alias
  @Output('customEventName') changed = new EventEmitter<void>();
}

Best Practices

• Prefer output(): Use the function-based output() instead of @Output() and EventEmitter.
• Naming: Use camelCase for output names. Avoid prefixing with on (e.g., use valueChanged instead of onValueChanged).
• No DOM Bubbling: Angular custom events do not bubble up the DOM tree like native events.
• Avoid Collisions: Do not choose names that collide with native DOM events (like click or submit).