Directives Overview

• Directives are Components that have no view
• Directives are used to attach behaviour to DOM elements
• Since Directives are base Components, they can also use property binding for inputs and event binding for outputs
• There are two main Directive categories:
  • Attribute
    • Change the appearance or behavior of a DOM element (e.g. NgClass)
  • Structural
    • Change the DOM layout by adding or removing elements (e.g. NgIf)

Directives Overview

• Custom Directives can be built either as Attribute or Structural Directives
   
• Custom Directives, like Components are built using the @Directive decorator
  • Main properties:
    • selector - Same as Component selector
    • inputs/outputs - Same as Component selector

Directives Overview

• selector may be declared as one of the following:

  • element-name: select by element name.
  • .class: select by class name.
  • [attribute]: select by attribute name.
  • [attribute=value]: select by attribute name and value.
  • :not(sub_selector): select only if the element does not match the sub_selector.
  • selector1, selector2: select if either selector1 or selector2 matches.

Custom Attribute Directives

• To create custom Attribue Directive, we need to have reference to the Element we want to change.
• Angular 2 allows us to get Element reference using an ElementRef object

  • ElementRef represents a location in a View that has an injection, change-detection and render context associated with it. (e.g. Component or Directive)

  • An ElementRef is created for each element in the template that contains a Directive, Component or data-binding.

Custom Attribute Directive

• Using ElementRef
• Import 
• Inject in constructor
• When using ElementRef we can access the underlying native element, ElementRef.nativeElement
  • ​For Browser platform this will be the native DOM Element

Custom Attribute Directive

• Simple Hightlight Directive example:

import {Directive, ElementRef} from 'angular2/core';
@Directive({
    selector: '[myHighlight]'  // Notice the selector syntax for Attribute Direcitve
})
export class HighlightDirective {
    constructor(el: ElementRef) {
       el.nativeElement.style.backgroundColor = 'yellow';
    }
}

<div [highlight]>My content</div>

• Use in template:

Custom Attribute Directive

• It is very useful to create custom behviour on property or evet change of a DOM Element
   
• Host bindings allow us to bind the Directive controller (class) to element property or event changes.
• There are two ways to do it:  
  • host property of the @Directive decorator
  • @HostBinding (properties) , @HostListener (event) decorators

Custom Attribute Directive

• Examples:

@Directive({
  selector: '[sgSample]',
  host: {
    '(mouseenter)': 'onMouseEnter()',
    'attr.role': 'button'
  }
})
class SampleDirective {
  role = 'button';
  onMouseEnter() {...}
}

@Directive({
  selector: '[sgSample]'
})
class SampleDirective {
  @HostBinding('attr.role') role = 'button';
  @HostListener('mouseenter') onMouseEnter() {...}
}

Custom Attribute Directive

• Using host bindings we can create Highlight Directive when mouse enters an element

import {Directive, ElementRef, Input} from 'angular2/core';
@Directive({
  selector: '[myHighlight]',
  host: {
    '(mouseenter)': 'onMouseEnter()',
    '(mouseleave)': 'onMouseLeave()'
  }
})
export class HighlightDirective {
  constructor(private el: ElementRef) { }
  onMouseEnter() { this._highlight("yellow"); }
  onMouseLeave() { this._highlight(null); }
  private _highlight(color: string) {
    this.el.nativeElement.style.backgroundColor = color;
  }
}

Custom Attribute Directive

• The example can be extended to use input value for color instead of constant

export class HighlightDirective {
  @Input('myHighlight') highlightColor: string;
  private _defaultColor = 'red';
  constructor(private el: ElementRef) { }
  onMouseEnter() { this._highlight(this.highlightColor || this._defaultColor); }
  onMouseLeave() { this._highlight(null); }
  private _highlight(color:string) {
    this.el.nativeElement.style.backgroundColor = color;
  }
}

• Last thing missing...

• Use in template:

<div hightlight="ligthblue">My Content</div>
<div [highlight]="'lightgreen'">My Content</div> <!-- Same -->

Custom Attribute Directive

• As we know, Angular 2 can work with different platforms not only Browser.
• To change DOM elemet we use the ElementRef.nativeElement and directly access the DOM Element.
   
• A better way to do it is to use a Renderer
  • ​Renderer encapsulates the nativeElement implementation using Standard API
  • Rendering implementation can changed for different platforms
  • In Browser platform Renderer is implemented as DomRenderer

Custom Attribute Directive

• Using Renderer:
  • Import Renderer
  • Inject to constructor
  • Use instead of native implementation

Custom Attribute Directive

import {Directive, Input, ElementRef, Renderer} from 'angular2/core';

@Directive({
  selector: '[highlight]',
  host: {
    '(mouseenter)': 'onMouseEnter()',
    '(mouseleave)' : 'onMouseLeave()'
  }
})
export class HighlightDirective implements OnInit {

  @Input('highlight') color: string;
  constructor(private elementRef: ElementRef, private renderer: Renderer) {
  }

  onMouseEnter() {
    this.highlightBackground(this.color);
  }
  onMouseLeave() {
    this.highlightBackground(null);
  }

  private highlightBackground(color: string) {
    this.renderer.setElementStyle(this.elementRef.nativeElement, "backgroundColor", 
    color); 
  }
}