angular-directives

Angular Directives

Create custom directives for reusable DOM manipulation and behavior in Angular v20+.

Attribute Directives

Modify the appearance or behavior of an element:

import { Directive, input, effect, inject, ElementRef } from '@angular/core';

@Directive({
  selector: '[appHighlight]',
})
export class Highlight {
  private el = inject(ElementRef<HTMLElement>);

  // Input with alias matching selector
  color = input('yellow', { alias: 'appHighlight' });

  constructor() {
    effect(() => {
      this.el.nativeElement.style.backgroundColor = this.color();
    });
  }
}

// Usage: <p appHighlight="lightblue">Highlighted text</p>
// Usage: <p appHighlight>Default yellow highlight</p>

Using host Property

Prefer host over @HostBinding/@HostListener:

@Directive({
  selector: '[appTooltip]',
  host: {
    '(mouseenter)': 'show()',
    '(mouseleave)': 'hide()',
    '[attr.aria-describedby]': 'tooltipId',
  },
})
export class Tooltip {
  text = input.required<string>({ alias: 'appTooltip' });
  position = input<'top' | 'bottom' | 'left' | 'right'>('top');

  tooltipId = `tooltip-${crypto.randomUUID()}`;
  private tooltipEl: HTMLElement | null = null;
  private el = inject(ElementRef<HTMLElement>);

  show() {
    this.tooltipEl = document.createElement('div');
    this.tooltipEl.id = this.tooltipId;
    this.tooltipEl.className = `tooltip tooltip-${this.position()}`;
    this.tooltipEl.textContent = this.text();
    this.tooltipEl.setAttribute('role', 'tooltip');
    document.body.appendChild(this.tooltipEl);
    this.positionTooltip();
  }

  hide() {
    this.tooltipEl?.remove();
    this.tooltipEl = null;
  }

  private positionTooltip() {
    // Position logic based on this.position() and this.el
  }
}

// Usage: <button appTooltip="Click to save" position="bottom">Save</button>

Class and Style Manipulation

@Directive({
  selector: '[appButton]',
  host: {
    'class': 'btn',
    '[class.btn-primary]': 'variant() === "primary"',
    '[class.btn-secondary]': 'variant() === "secondary"',
    '[class.btn-sm]': 'size() === "small"',
    '[class.btn-lg]': 'size() === "large"',
    '[class.disabled]': 'disabled()',
    '[attr.disabled]': 'disabled() || null',
  },
})
export class Button {
  variant = input<'primary' | 'secondary'>('primary');
  size = input<'small' | 'medium' | 'large'>('medium');
  disabled = input(false, { transform: booleanAttribute });
}

// Usage: <button appButton variant="primary" size="large">Click</button>