angular-directives（Angular directives 日程管理）

Angular Directives

Version: Angular 21 (2025) Tags: Directives, Components, DOM, Custom Directives

References: Directives Guide • Attribute Directives • Structural Directives

API Changes

This section documents recent version-specific API changes.

• NEW: Directive composition API — Use hostDirectives to compose directives source
• NEW: Signal inputs in directives — Use input() for reactive directive properties
• NEW: Standalone directives — All directives can be standalone
• NEW: Control flow syntax — @if, @for, @switch replace *ngIf, *ngFor

Best Practices

• Create attribute directives for reusable behavior

import { Directive, ElementRef, Renderer2, Input } from '@angular/core';

@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {
  @Input() set appHighlight(color: string) {
    this.renderer.setStyle(this.el.nativeElement, 'background-color', color || 'yellow');
  }

  constructor(private el: ElementRef, private renderer: Renderer2) {}
}

• Use @Input and @Output for directive communication

@Directive({
  selector: '[appClickTracker]'
})
export class ClickTrackerDirective {
  @Input() trackName = 'default';
  @Output() clicked = new EventEmitter<string>();

  @HostListener('click')
  onClick() {
    this.clicked.emit(this.trackName);
  }
}

• Use TemplateRef for structural directives

import { Directive, Input, TemplateRef, ViewContainerRef } from '@angular/core';

@Directive({
  selector: '[appUnless]'
})
export class UnlessDirective {
  @Input() set appUnless(condition: boolean) {
    if (!condition) {
      this.vcRef.createEmbeddedView(this.templateRef);
    } else {
      this.vcRef.clear();
    }
  }

  constructor(
    private templateRef: TemplateRef<any>,
    private vcRef: ViewContainerRef
  ) {}
}

• Use ViewContainerRef for complex structural logic

@Directive({ selector: '[appDynamic]' })
export class DynamicDirective {
  constructor(
    private vcr: ViewContainerRef,
    private templateRef: TemplateRef<any>
  ) {
    this.vcr.createEmbeddedView(this.templateRef);
  }
}

• Use hostDirectives for composition

@Component({
  selector: 'app-card',
  standalone: true,
  imports: [HighlightDirective],
  hostDirectives: [
    {
      directive: HighlightDirective,
      inputs: ['appHighlight: highlight']
    }
  ],
  template: `<ng-content></ng-content>`
})
export class CardComponent {}

• Use standalone: true for modern directives

@Directive({
  selector: '[appStandalone]',
  standalone: true
})
export class StandaloneDirective {}

• Use signals in directives

@Directive({
  selector: '[appSignal]'
})
export class SignalDirective {
  value = input<string>('');

  ngOnInit() {
    console.log(this.value());
  }
}

• Use @HostBinding for property binding

@Directive({
  selector: '[appDisable]'
})
export class DisableDirective {
  @Input() set disabled(value: boolean) {
    this.hostBinding.nativeElement.disabled = value;
  }

  constructor(private hostBinding: ElementRef) {}
}

• Keep directives focused — One responsibility

// ✅ Good - focused directive
@Directive({ selector: '[appTooltip]' })

// ❌ Bad - too many responsibilities
@Directive({ selector: '[appTooltip][appTooltipMaxWidth][appTooltipTheme]' })

• Use descriptive selectors

// ✅ Good
@Directive({ selector: '[appUserCard]' })

// ❌ Bad
@Directive({ selector: '[appUc]' })