Angular 7 | Directives

Directives in Angular 7 are Typescript class which is declared with decorator @Directive. These are the Document Object Model (DOM) instruction sets, which decide how logic implementation can be done.
Angular directives can be classified into three types: 
 

1. Component Directives: It forms the main class and is declared by @Component. It contains the details on component processing, instantiated and usage at run time.
   Example: It contains certain parameters some of them are shown in this example. 
    

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})

1. 
   There are three parameters discussed below: 
   
   • Selector: Tells the template tag which specifies the beginning and end of the component.
   • templateURL: Consists of the template used for the component.
   • styleUrls: It is of array type which consists of all the style format files used for the template.
2. Structural Directives: Structure directives manipulate the DOM elements. These directives have a * sign before the directive. For example, *ngIf and *ngFor.
   Example: Let's look the implementation of *ng-if-else and *ng-for. Using them, we classify weekdays and weekends. 
   Component file: 
    

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

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  Weekdays:Array =[
'Sunday', 'Monday', 'Tuesday', 
'Wednesday', 'Thursday', 'Friday', 'Saturday']
}

1. 
   Template file: 
    

<div *ngFor="let day of Weekdays">
<ng-container *ngIf =
    "(day == 'Saturday' || day == 'Sunday'); else elseTemplate">
  <h1>{{day}} is a weekend</h1>
</ng-container>
<ng-template #elseTemplate>
  <h1>{{day}} is not a weekend</h1>
</ng-template>
</div>

1. 
   Output: 
    

1. 
    
2. Attribute Directives: Attribute directives are used to change the look and behavior of the DOM element. It provides the facility to create our own directive. 
   Example: This example describes how to make our own directive.
   Write command as follows: 
    

ng g directive 

1. 
   Directive: 
    

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

@Directive({
  selector: '[appChanges]'
})
export class ChangesDirective {
  constructor(private eltRef: ElementRef) {

    // Changing the background to green 
    this.eltRef.nativeElement.style.backgroundColor = 'green';
    this.eltRef.nativeElement.style.color = 'white';
    changing the text color to white
  }
  
  ngOnInit() {
  }
}

1. 
   The Component File: 
    

import { Component, OnInit, Directive } from '@angular/core';
import { ChangesDirective } from '../changes.directive';

@Component({
  selector: 'app-derived-directive',
  templateUrl: './derived-directive.component.html',
  styleUrls: ['./derived-directive.component.css']
})

export class DerivedDirectiveComponent implements OnInit {

  isClicked:boolean=false;
  constructor() { }
  buttonClick(){

    // Change controlled by button press
    this.isClicked = true;
  }
  ngOnInit() {
  }
}

1. 
   The Template 
    

<button>Click Here</button>
 
    <div style="width: 220px;height: 50px">
        <h1>GeeksForGeeks</h1>
    </div>


    <div style="color: green;width: 300px;height: 50px">
        <h1> GeeksForGeeks</h1>
    </div>

1. 
   Output: 
   
   • Before clicking the Button: 
      

• After clicking the Button: