Lesson 4
Attribute Directives in Angular
Introduction

Welcome to the lesson on attribute directives in Angular! In this lesson, we'll explore how attribute directives can enhance the appearance and behavior of DOM elements in your Angular applications. Attribute directives are distinct from structural directives, as they modify existing elements rather than adding or removing them. Understanding this distinction is crucial as we dive into the world of Angular directives. Let's get started! 🌟

Exploring Built-in Attribute Directives

Angular provides several built-in attribute directives that allow you to dynamically alter the styles and classes of elements. Two commonly used directives are ngClass and ngStyle. These directives enable you to bind dynamic values to an element's class and style properties, respectively.

TypeScript
1<div [ngClass]="{'active': isActive, 'disabled': !isActive}">
2  This div's class changes based on the isActive property.
3</div>
4
5<div [ngStyle]="{'color': textColor, 'font-size': fontSize + 'px'}">
6  This div's style changes dynamically.
7</div>

In the examples above, ngClass is used to conditionally apply the 'active' or 'disabled' class based on the isActive property. Similarly, ngStyle dynamically sets the text color and font size based on the textColor and fontSize properties. These directives provide a powerful way to control the presentation of your elements based on component data.

Creating a Custom Attribute Directive

Now, let's create a custom attribute directive to understand how directives work under the hood. We'll build a directive that changes the background color of an element when hovered over.

TypeScript
1import { Directive, ElementRef, Renderer2, HostListener } from '@angular/core';
2
3@Directive({
4  selector: '[appHoverHighlight]'
5})
6export class HoverHighlightDirective {
7  constructor(private el: ElementRef, private renderer: Renderer2) {}
8
9  @HostListener('mouseenter') onMouseEnter() {
10    this.renderer.setStyle(this.el.nativeElement, 'backgroundColor', 'yellow');
11  }
12
13  @HostListener('mouseleave') onMouseLeave() {
14    this.renderer.setStyle(this.el.nativeElement, 'backgroundColor', 'white');
15  }
16}

In this directive, we use @Directive to define a new directive with the selector appHoverHighlight. The Renderer2 service is used to safely manipulate the DOM, setting the background color to yellow on mouse enter and reverting it to white on mouse leave. This example demonstrates how custom attribute directives can enhance user interaction.

Implementing the DisableIfNotSubscribed Directive

Another example of a practical use case for attribute directives is the DisableIfNotSubscribed directive. This directive will disable buttons based on a bool value whether the user has a paid subscription.

TypeScript
1import { Directive, ElementRef, Input, Renderer2, OnChanges, SimpleChanges } from '@angular/core';
2
3@Directive({ selector: '[disableIfNotSubscribed]' })
4export class DisableIfNotSubscribedDirective implements OnChanges {
5  @Input() disableIfNotSubscribed: boolean;
6
7  constructor(private el: ElementRef, private renderer: Renderer2) {}
8
9  ngOnChanges(changes: SimpleChanges) {
10    if (changes['disableIfNotSubscribed']) {
11      if (!this.disableIfNotSubscribed) {
12        this.renderer.setAttribute(this.el.nativeElement, 'disabled', 'true');
13        this.renderer.setStyle(this.el.nativeElement, 'opacity', '0.5');
14      } else {
15        this.renderer.removeAttribute(this.el.nativeElement, 'disabled');
16        this.renderer.setStyle(this.el.nativeElement, 'opacity', '1');
17      }
18    }
19  }
20}

In this directive, we use @Input to bind the disableIfNotSubscribed property, which determines whether the element should be disabled. The ngOnChanges lifecycle method checks for changes to this property and updates the element's attributes and styles accordingly. This directive is a practical example of how attribute directives can control element behavior based on application logic.

Best Practices and Lifecycle Methods

When working with attribute directives, it's important to follow best practices to ensure compatibility and security. One key practice is using the Renderer2 service for DOM manipulation, as it abstracts away direct DOM access, making your code more secure and compatible across different platforms.

The ngOnChanges lifecycle method is crucial for attribute directives, as it allows you to respond to changes in input properties. By implementing this method, you can ensure that your directive updates the DOM whenever the bound data changes, providing a dynamic and responsive user experience.

Conclusion and Next Steps

In this lesson, we've explored the world of attribute directives in Angular. We started by understanding built-in directives like ngClass and ngStyle, then moved on to creating a custom directive to enhance user interaction. We also implemented the DisableIfNotSubscribed directive, demonstrating a real-world application of attribute directives. As you move on to the practice exercises, you'll have the opportunity to reinforce these concepts and experiment with attribute directives in your own projects. Keep up the great work, and get ready for more advanced Angular topics in the upcoming lessons! 🚀

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