A Comprehensive Guide to Angular Directives

Introduction

Directives are powerful features in Angular, they allow you to extend or modify the behaviour of HTML elements. Angular Directives are markers that are added to the DOM elements, which informs Angular that the HTML elements to which they have been attached have a specific set of functionalities that need to be applied. In this article, we will cover everything you need to know about Angular Directives, including what they are, how they work, how to build them, and how to use them.

What are Angular Directives?

Directives are one of the most powerful features in Angular. They are used to extend HTML functionality, encapsulate logic and reuse code. Directives allow you to create reusable components for your application, making it easier to structure, manage and maintain it.

Simply put, Directives are a way to teach HTML new tricks and give it more expressive power. Angular Directives allow you to create your own custom HTML elements and attributes that help you implement complex UI features and behaviours.

Types of Angular Directives

There are three types of Angular Directives:

  • Components Directives

  • Structural Directives

  • Attribute Directives

Components Directives are used to create reusable components that can be used across your application. Think of them as building blocks for your application user interface.

Structural Directives are used to manipulate the layout of the DOM by adding or removing elements. Examples of Structural Directives include ngFor and ngIf.

Attribute Directives are used to modify the behaviour of an existing element. Examples of Attribute Directives include ngModel and ngStyle.

How to Build Angular Directives

Creating your own custom Directive is a simple process. You can create a Directive using the @Directive decorator, which accepts a metadata object as its parameter. The metadata object specifies the name of the directive, its selector, and a set of instructions that Angular should execute when it encounters the directive on the DOM.

Below is an example of how to create a custom Directive: 

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



@Directive({

  selector: '[appCustomDirective]'

})

export class CustomDirective {

  constructor() { }

  // Directive logic here

}

In the example above, we create a Directive called CustomDirective. The selector, ‘[appCustomDirective]’, specifies that this directive is applied to any HTML element with the ‘appCustomDirective’ attribute.

Once you have created your directive, you can start implementing its functionality. This can include manipulating the host element, adding event listeners or creating new child elements.

How to Use Angular Directives

Using Angular Directives is straightforward. You can use Directives by adding them to your HTML elements using their selector. Below is an example of how to use our custom Directive, ‘CustomDirective’: 

Custom Directive Example

In the example above, we apply the ‘CustomDirective’ to a paragraph element using the ‘appCustomDirective’ selector. When Angular encounters this directive, it will execute the instructions specified in the metadata object of the Directive.

FAQs

What are the benefits of using Angular Directives?

There are numerous benefits to using Angular Directives, including:

  • Increased reusability of code

  • Improved application performance

  • Better maintainability and scalability

Can I use multiple Directives on a single element?

Yes, you can apply multiple Directives to a single HTML element by using multiple Directive selectors separated by a comma.

e.g., 

Multiple Directive Example

Can I pass input data to a Directive?

Yes, you can pass input data to a Directive by specifying the input property in the Directive class and binding it to the HTML element that uses the Directive.

e.g., 

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



@Directive({

  selector: '[appCustomDirective]'

})

export class CustomDirective {

  @Input() color: string;

  constructor() { }

  // Directive logic here

}

In the example above, we create an input property for our Directive called ‘color’. We can then bind this input to our HTML element that uses the Directive using square brackets, as shown below. 

Custom Directive Example

Can I create my own structural Directives?

Yes, you can create your own structural Directives by using the * syntax in the Directive selector. The * signifies that the Directive can modify the DOM structure of the component.

e.g., 

@Directive({

  selector: '[appCustomDirective]'

})

export class CustomDirective {

  constructor(private templateRef: TemplateRef) { }

  

  @Input() set appCustomDirective(condition: boolean) {

    if(condition){

      this.viewContainer.createEmbeddedView(this.templateRef);

    }

    else{

      this.viewContainer.clear();

    }

  }

  

 constructor(private viewContainer: ViewContainerRef,

              private templateRef: TemplateRef) { }



}

In the example above, we create a structural Directive called ‘appCustomDirective’, which uses the * syntax in the selector.

Conclusion

Angular Directives are an essential feature of the Angular framework, allowing developers to extend and modify the behaviour of HTML elements easily. In this article, we have covered what Angular directives are, the types of directives, how to build a directive, and how to use a directive. We hope this article provides you with the knowledge and tools to start using Angular Directives in your application.

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