A Look into Angular Directives

HTML doesn't support conditional attributes (if, if-else, switch, etc.), loop statements (loops), and other attributes. Angular Directives are used to extend the power of existing Html markup by adding new attributes or tags. They change the appearance or behavior of a DOM element.

In this article, I will show you the types of directives in Angular 6 and the basic examples for using these directives.

Types of Directives in Angular

• Components: The directive with a template specification and this type of directive is the most common directive type in Angular

• Attribute directives: The directive that changes the appearance or its behavior of an element, component, or other directives

• Structural directives: The directive that changes the DOM layout by adding or removing the DOM elements

• Custom Directive: Custom directive can also be created if any of the above directives does not solve our purpose for the requirement

Components

Typically, a Component is a directive with a template. @Component decorator extends the @Directive decorator with template-oriented features. A component class is declared using the @Component decorator. @Component decorator contains metadata about the component i.e. selector, templateURL, styleUrls, etc.

import {Component} from '@angular/core' 
 
@Component({ 
 selector: 'app-root', 
 templateUrl: './app.component.html', 
 styleUrls: ['./app.component.css'] 
}) 
 
export class AppComponent{}

Attribute Directive

Attribute directives are used to change the appearance or behavior of an existing DOM element. ngStyle and ngClass are the commonly used example for attribute directives.

This is used to configure multiple inline styles for html element.

This is used to set the class attribute of the html element to apply the style.

 import { Component } from '@angular/core'; 
 
 @Component({ 
 selector: 'Satya-App', 
 template: `<button style='color:green' [ngStyle]="MyStyle()">Satyaprakash Samantaray</button>`
 }) 
 export class AppComponent { 
 isBold: boolean = true; 
 fontSize: number = 30; 
 isItalic: boolean = true; 
 
 MyStyle() { 
 let mystyles = { 
 'font-weight': this.isBold ? 'bold' : 'normal', 
 'font-style': this.isItalic ? 'italic' : 'normal', 
 'font-size.px': this.fontSize 
 }; 
 
 return mystyles; 
 } 
 } 

Here I added a button control in the template part with a style sheet named MyStyle(). This MyStyle CSS method is defined as below.

 MyStyle() { 
 let mystyles = { 
 'font-weight': this.isBold ? 'bold' : 'normal', 
 'font-style': this.isItalic ? 'italic' : 'normal', 
 'font-size.px': this.fontSize 
 }; 
 
 return mystyles;

 import { Component } from '@angular/core'; 
 
 @Component({ 
 selector: 'Satya-App', 
 template: `<button class='colorClass' [ngClass]='applyClasses()'>Satyaprakash Samantaray</button>`, 
 styles: [`.boldClass{ 
 font-weight:bold; 
 font-size : 30px; 
 } 
 
 .italicsClass{ 
 font-style:italic; 
 } 
 
 .colorClass{ 
 color:Red; 
 }`] 
 }) 
 export class AppComponent { 
 applyBoldClass: boolean = true; 
 applyItalicsClass: boolean = true; 
 
 applyClasses() { 
 let classes = { 
 boldClass: this.applyBoldClass, 
 italicsClass: this.applyItalicsClass 
 }; 
 
 return classes; 
 } 
 } 

In this NgClass we are using the class attribute of the HTML element to apply styles.

 template: `<button class='colorClass' [ngClass]='applyClasses()'>Satyaprakash Samantaray</button>`,

Structural Directive

Structural directives are used to alter the layout by adding, removing, and replacing elements in DOM. Asterisk (*) is used when these directives are used with the HTML elements. *ngIf, *ngFor, *ngSwitch are some commonly used structural directive.

It is used to create or remove a part of the DOM tree depending on a condition.

It is used for iteration like showing a list of items using loops.

It can be used as a switch case in OOPS. It is used to select one case among various cases. Angular will add only the selected option HTML markup into the DOM.

 
 import { Component } from '@angular/core'; 
 
 @Component({ 
 selector: 'Satya-App', 
 template: `<div style='color:blue' *ngIf="true">You can See Satyaprakash....</div>`
 }) 
 export class AppComponent { 
 
 } 

Here If ngif= true So, the text will be visible on the web page.

 
 template: `<div style='color:blue' *ngIf="true">You can See Satyaprakash....</div>`

 
 import { Component } from '@angular/core'; 
 
 @Component({ 
 selector: 'Satya-App', 
 templateUrl: './app.component.html', 
 }) 
 export class AppComponent { 
 employees: any[] = [ 
 { 
 code: '1001', name: 'Satya', gender: 'Male', 
 annualSalary: 5500, dateOfBirth: '25/6/1988' 
 }, 
 { 
 code: '1002', name: 'Mohit', gender: 'Male', 
 annualSalary: 5700.95, dateOfBirth: '9/6/1982' 
 }, 
 { 
 code: '1003', name: 'Rohit', gender: 'Male', 
 annualSalary: 5900, dateOfBirth: '12/8/1979' 
 }, 
 { 
 code: '1004', name: 'Satyaprakash Samantaray', gender: 'Female', 
 annualSalary: 6500.826, dateOfBirth: '14/10/1980' 
 }, 
 ]; 
 } 

I have created a class named employees and inserted values to its corresponding properties.

 
 employees: any[] = [ 
 { 
 code: '1001', name: 'Satya', gender: 'Male', 
 annualSalary: 5500, dateOfBirth: '25/6/1988' 
 },
 

 
 <!DOCTYPE html> 
 <html> 
 <head> 
 <title></title> 
 <meta charset="utf-8" /> 
 <style> 
 table { 
 font-family: arial, sans-serif; 
 border-collapse: collapse; 
 width: 100%; 
 } 
 
 td, th { 
 border: 1px solid #dddddd; 
 text-align: left; 
 padding: 8px; 
 } 
 
 tr:nth-child(even) { 
 background-color: #dddddd; 
 } 
 </style> 
 </head> 
 <body> 
 <table align="center" border="1" cellpadding="4" cellspacing="4"> 
 <thead> 
 <tr> 
 <th style="background-color: Yellow;color: blue">Code</th> 
 <th style="background-color: Yellow;color: blue">Name</th> 
 <th style="background-color: Yellow;color: blue">Gender</th> 
 <th style="background-color: Yellow;color: blue">Annual Salary</th> 
 <th style="background-color: Yellow;color: blue">Date of Birth</th> 
 </tr> 
 </thead> 
 <tbody> 
 <tr *ngFor='let employee of employees'> 
 <td>{{employee.code}}</td> 
 <td>{{employee.name}}</td> 
 <td>{{employee.gender}}</td> 
 <td>{{employee.annualSalary}}</td> 
 <td>{{employee.dateOfBirth}}</td> 
 </tr> 
 <tr *ngIf="!employees || employees.length==0"> 
 <td colspan="5"> 
 No employees to show in the page.... 
 </td> 
 </tr> 
 </tbody> 
 </table> 
 </body> 
 </html> 

Here NgFor is used to display a list of items using repetitive loops that is data of employees class.

 
 <tr *ngFor='let employee of employees'> 
 <td>{{employee.code}}</td> 
 <td>{{employee.name}}</td> 
 <td>{{employee.gender}}</td> 
 <td>{{employee.annualSalary}}</td> 
 <td>{{employee.dateOfBirth}}</td> 
 </tr> 

 import { Component } from '@angular/core'; 
 
 @Component({ 
 selector: 'Satya-App', 
 template: `<h2>{{title}}</h2> 
 <p *ngIf="showElement">Show Element</p> 
 <div [ngSwitch]="inpvalue"> 
 <p style='color:blue' *ngSwitchCase="1">You have selected M S Dhoni</p> 
 <p style='color:blue' *ngSwitchCase="2">You have selected Sachin Tendulkar</p> 
 <p style='color:blue' *ngSwitchCase="3">You have selected Satyaprakash Samantaray</p> 
 <p style='color:red' *ngSwitchDefault>Sorry! Invalid selection....</p> 
 </div>` 
 }) 
 export class AppComponent { 

 inpvalue: number = 3; 
 } 

You can display one element from among several possible elements, based on a switch condition. Angular puts only the selected element into the DOM. Here we are using ngswitchcase to apply switch case and ngswitchdefault is for invalid selection. Using ngswitch directive we can show output by passing the value of inpvalue property in AppComponent class.

For valid selection >>

For Invalid selection >>

You can check code compilation status in the command prompt during a change in code using visual studio.

Difference between structural directive and attribute directive

Structural directives are used for adding, removing, and replacing elements in DOM. Whereas attribute directives are used to change the appearance or behavior of an existing element.

Custom Attribute Directive

Custom attribute directives can be created by using @Directive decorator. Custom attribute directives can be used to modify the background color, text information of an HTML element, and that process is called a host element. To change UI look angular provides ElementRef class that can directly access DOM.

Let's create a folder under an app called directives. Under directives create a file called red.directive.ts as given below:

 
 import { Directive, ElementRef } from '@angular/core';
 
 @Directive({ 
 selector: '[ColorRed]' 
 })
 export class MyCustomDirective {
 constructor(elRef: ElementRef) {
 elRef.nativeElement.style.color = 'darkred';
 }
 } 

Make a directive named as myRed directive. We can use it with HTML elements such as <p> and <div>, the text color within that element will be red. The @Directive is used as an angular directive to retrieve directive configuration metadata. To use our custom directive in our angular project follow the below code to set up in the application module.

 
 import { BrowserModule } from '@angular/platform-browser';
 import { NgModule } from '@angular/core';
 
 import { AppComponent } from './app.component';
 import { MyRedDirective } from './directives/red.directive';
 
 @NgModule({
 declarations: [
 AppComponent,
 MyCustomDirective
 ],
 imports: [
 BrowserModule
 ],
 providers: [],
 bootstrap: [AppComponent]
 })
 export class AppModule { }

 
 import { Component } from '@angular/core';
 
 @Component({
 selector: 'Satya-App',
 templateUrl: './app.component.html'
 })
 export class AppComponent { 
 txtsize = '25px';
 colors = ['red', 'blue', 'orange'];
 myColor = '';
 }

 
 <!DOCTYPE html> 
 <html> 
 <head> 
 <title></title> 
 <meta charset="utf-8" /> 
 <style> 
 table { 
 font-family: arial, sans-serif; 
 border-collapse: collapse; 
 width: 100%; 
 } 
 
 td, th { 
 border: 1px solid #dddddd; 
 text-align: left; 
 padding: 8px; 
 } 
 
 tr:nth-child(even) { 
 background-color: #dddddd; 
 } 
 </style> 
 </head> 
 <body> 
 <p ColorRed>Custom Attribute Directive By Satyaprakash</p> 
 </body> 
 </html> 

we are ready to use our directive in the HTML template. Find the code snippet.

 
 <body> 
 <p ColorRed>Custom Attribute Directive By Satyaprakash</p> 
 </body>

 
 import { BrowserModule } from '@angular/platform-browser';
 import { NgModule } from '@angular/core';
 
 import { AppComponent } from './app.component';
 import { MyRedDirective } from './directives/red.directive';
 
 @NgModule({
 declarations: [
 AppComponent,
 MyCustomDirective
 ],
 imports: [
 BrowserModule
 ],
 providers: [],
 bootstrap: [AppComponent]
 })
 export class AppModule { }

Here I import custom attribute directive reference to configure the custom attribute that I have created.

 
 import { MyRedDirective } from './directives/red.directive';
 
 @NgModule({
 declarations: [
 AppComponent,
 MyCustomDirective
 ],
 

Summary

Once we say that the components are the building blocks of the Angular applications, but eventually, we are saying that the directives are the building blocks of Angular applications which is used to deal with DOM elements.

In this article, you learned about the various types of directives in Angular for example. I hope, you will find it useful when you will work with your Angular Application. Dot Net Tricks Angular 2 to 10 web API live interactive training aligned with real-time which is delivered by Google GDE and Microsoft MVP globally. The angular online training program helps you learn to build projects using web API ASP.NET  MVC5  and node.js.Thanks for reading.

| Read More: Angular Interview Question and Answer

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