udemy.angular/docs/directives.md

# Directives

## Attribute vs Structural directives

<img src="../img/attribute_structural_directives.png" alt="attribute_structural_directives" width="800"/>

## Create basic attribute directives

Namingconvention: `name.directive.ts`

Generate it with the CLI:
```bash
ng generate directive <name>

# short:
ng g d <name>
```

<img src="../img/basic_directive.png" alt="basic_directive" width="550"/>

In `app.modules.ts` has to be added under `declarations` and *import* the source path to the directive.

Use it in the template.html with just adding the name `appBasicHighlight` to the component.

## Using Renderer for attribute directives

<img src="../img/renderer.png" alt="renderer" width="700"/>

Renderer is a better approach to accessing the DOM. Why? Because Angular dont works only in the Browser, but also with worker, which maight in certain circumstances will get errors. 

## HostListener

With HostListener is a neat way to style the Element based on certain events triggered from the host (mouseover, mouseenter, mouseleave, etc).

<img src="../img/reactive_directive.png" alt="renderer" width="800"/>

## HostBinding

with HostBinding is the renderer no longer needed and is much simpler to change an elements backgroundcolor.
```ts
@Directive({
  selector: '[appBetterHighlight]'
})
export class BetterHighlightDirective implements OnInit {
  @HostBinding('style.backgroundColor') backgroundColor: string = 'transparent';

  ngOnInit() {
  }

  @HostListener('mouseenter') mouseover(eventData: Event) {
    this.backgroundColor = this.highlightColor;
  }

  @HostListener('mouseleave') mouseleave(eventData: Event) {
    this.backgroundColor = this.defaultColor;
  }
}
```

## Binding to directive properties

The hardcoded colors can be set from the outside using the `@Input` like so

```html
<p appBetterHighlight [defaultColor]="'yellow'" [highlightColor]="'blue'">bliblubla</p>
```

```ts
import {
  Directive,
  Renderer2,
  OnInit,
  ElementRef,
  HostListener,
  HostBinding,
  Input
} from '@angular/core';

@Directive({
  selector: '[appBetterHighlight]'
})
export class BetterHighlightDirective implements OnInit {
  @Input() defaultColor: string = 'transparent';
  @Input('appBetterHighlight') highlightColor: string = 'blue';
  @HostBinding('style.backgroundColor') backgroundColor: string;

  ngOnInit() {
    this.backgroundColor = this.defaultColor;
  }

  @HostListener('mouseenter') mouseover(eventData: Event) {
    this.backgroundColor = this.highlightColor;
  }

  @HostListener('mouseleave') mouseleave(eventData: Event) {
    this.backgroundColor = this.defaultColor;
  }

}
```

## Building a structural directive

```ts
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) { }

}
```

## ngSwitch

<img src="../img/ngSwitch.png" alt="ngSwitch" width="400"/>

ngSwitch can be used for replacing a lot of ngIf statements.