Angular Cookbook

Chapter 2: Understanding and Using Angular Directives

In this chapter, you'll learn about Angular directives in depth. You'll learn about attribute directives, with a really good real-world example of using a highlight directive. You'll also write your first structural directive and see how ViewContainer and TemplateRef services work together to add/remove elements from the Document Object Model (DOM), just as in the case of *ngIf, and you'll create some really cool attribute directives that do different tasks. Finally, you'll learn how to use multiple structural directives on the same HyperText Markup Language (HTML) element and how to enhance template type checking for your custom directives.

Here are the recipes we're going to cover in this chapter:

Using attribute directives to handle the appearance of elements
Creating a directive to calculate the read time for articles
Creating a basic directive that allows you to vertically scroll to an element
Writing your first custom structural directive
How to use *ngIf and *ngSwitch together
Enhancing template type checking for your custom directives

Using attribute directives to handle the appearance of elements

In this recipe, you'll work with an Angular attribute directive named highlight. With this directive, you'll be able to search words and phrases within a paragraph and highlight them on the go. The whole paragraph's container background will also be changed when we have a search in action.

Getting ready

The project we are going to work with resides in chapter02/start_here/ad-attribute-directive, inside the cloned repository:

Open the project in Visual Studio Code (VS Code).
Open the terminal, and run npm install to install the dependencies of the project.
Once done, run ng serve -o.
This should open the app in a new browser tab, and you should see something like this:

Figure 2.1 – ad-attribute-directives app running on http://localhost:4200

How to do it…

So far, the app has a search input box and a paragraph text. We need to be able to type a search query into the search box so that we can highlight the matching text in the paragraph. Here are the steps on how we achieve this:

We'll create a property named searchText in the app.component.ts file that we'll use as a model for the search-text input:
```
...
export class AppComponent {
  title = 'ad-attribute-directive';
  searchText = '';
}
```
Then, we use this searchText property in the app.component.html file with the search input as a ngModel, as follows:
```
…
<div class="content" role="main">
  ...
    <input [(ngModel)]="searchText" type="text"     class="form-control" placeholder="Search Text"     aria-label="Username" aria-describedby=    "basic-addon1">
  </div>
```
Important note
Notice that ngModel doesn't work without FormsModule, and so we've already imported FormsModule into our app.module.ts file.
Now, we'll create an attribute directive named highlight by using the following command inside our ad-attributes-directive project:
```
 ng g d directives/highlight
```
The preceding command generated a directive that has a selector called appHighlight. See the How it works… section for why that happens. Now that we have the directive in place, we'll create two inputs for the directive to be passed from AppComponent (from app.component.html)—one for the search text and another for the highlight color. The code should look like this in the highlight.directive.ts file:
```
 import { Directive, Input } from '@angular/core';
@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {
  @Input() highlightText = '';
  @Input() highlightColor = 'yellow';
  constructor() { }
}
```
Since we have the inputs in place now, let's use the appHighlight directive in app.component.html and pass the searchText model from there to the appHighlight directive:
```
<div class="content" role="main">
  ...
  <p class="text-content" appHighlight   [highlightText]="searchText">
    ...
  </p>
</div>
```

We'll listen to the input changes now for the searchText input, using ngOnChanges. Please see the Using ngOnChanges to intercept input property changes recipe in Chapter 1, Winning Components Communication, for how to listen to input changes. For now, we'll only do a console.log when the input changes:

import { Directive, Input, SimpleChanges, OnChanges } from '@angular/core';
@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective implements OnChanges {
  ...
  ngOnChanges(changes: SimpleChanges) {
    if (changes.highlightText.firstChange) {
      return;
    }
    const { currentValue } = changes.highlightText;
    console.log(currentValue);
  }
}

Now, we'll write some logic for what to do when we actually have something to search for. For this, we'll first import the ElementRef service so that we can get access to the template element on which our directive is applied. Here's how we'll do this:

import { Directive, Input, SimpleChanges, OnChanges, ElementRef } from '@angular/core';
@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective implements OnChanges {
  @Input() highlightText = '';
  @Input() highlightColor = 'yellow';
  constructor(private el: ElementRef) { }
  ...
}

Now, we'll replace every matching text in our el element with a custom <span> tag with some hardcoded styles. Update your ngOnChanges code in highlight.directive.ts as follows, and see the result:

ngOnChanges(changes: SimpleChanges) {
    if (changes.highlightText.firstChange) {
      return;
    }
    const { currentValue } = changes.highlightText;
    if (currentValue) {
      const regExp = new RegExp(`(${currentValue})`,       'gi')
      this.el.nativeElement.innerHTML =       this.el.nativeElement.innerHTML.replace       (regExp, `<span style="background-color:       ${this.highlightColor}">\$1</span>`)
    }
 }

Tip

You'll notice that if you type a word, it will still just show only one letter highlighted. That's because whenever we replace the innerHTML property, we end up changing the original text. Let's fix that in the next step.

To keep the original text intact, let's create a property name of originalHTML and assign an initial value to it on the first change. We'll also use the originalHTML property while replacing the values:

...
export class HighlightDirective implements OnChanges {
  @Input() highlightText = '';
  @Input() highlightColor = 'yellow';
  originalHTML = '';
  constructor(private el: ElementRef) { }
  ngOnChanges(changes: SimpleChanges) {
    if (changes.highlightText.firstChange) {
      this.originalHTML = this.el.nativeElement.      innerHTML;
      return;
    }
    const { currentValue } = changes.highlightText;
    if (currentValue) {
      const regExp = new RegExp(`(${currentValue})`,       'gi')
      this.el.nativeElement.innerHTML =       this.originalHTML.replace(regExp, `<span       style="background-color: ${this.      highlightColor}">\$1</span>`)
    }
  }
}

Now, we'll write some logic to reset everything back to the originalHTML property when we remove our search query (when the search text is empty). In order to do so, let's add an else condition, as follows:

...
export class HighlightDirective implements OnChanges {
  ...
  ngOnChanges(changes: SimpleChanges) {
   ...
    if (currentValue) {
      const regExp = new RegExp(`(${currentValue})`,       'gi')
      this.el.nativeElement.innerHTML = this.      originalHTML.replace(regExp, `<span       style="background-color: ${this.      highlightColor}">\$1</span>`)
    } else {
      this.el.nativeElement.innerHTML =       this.originalHTML;
    }
  }
}

How it works…

We create an attribute directive that takes the highlightText and highlightColor inputs and then listens to the input changes for the highlightText input using the SimpleChanges application programming interface (API) and the ngOnChanges life cycle hook.

First, we make sure to save the original content of the target element by getting the attached element using the ElementRef service, using the .nativeElement.innerHTML on the element, and then saving it to originalHTML property of the directive. Then, whenever the input changes, we replace the text with an additional HTML element (a <span> element) and add the background color to this span element. We then replace the innerHTML property of the target element with this modified version of the content. That's all the magic!

Creating a basic directive that allows you to vertically scroll to an element

In this recipe, you'll create a directive to allow the user to scroll to a particular element on the page, on click.

Getting ready

The project for this recipe resides in chapter02/start_here/ng-scroll-to-directive:

Open the project in VS Code.
Open the terminal, and run npm install to install the dependencies of the project.
Once done, run ng serve -o.
This should open the app in a new browser tab, and you should see something like this:

Figure 2.3 – ng-scroll-to-directive app running on http://localhost:4200

How to do it…

First off, we'll create a scroll-to directive so that we can enhance our application with smooth scrolls to different sections. We'll do this using the following command in the project:
```
ng g directive directives/scroll-to
```
Now, we need to make the directive capable of accepting an @Input() that'll contain the Cascading Style Sheets (CSS) Query Selector for our target section that we'll scroll to upon the element's click event. Let's add the input as follows to our scroll-to.directive.ts file:
```
import { Directive, Input } from '@angular/core';
@Directive({
  selector: '[appScrollTo]'
})
export class ScrollToDirective {
  @Input() target = '';
  constructor() { }
}
```

Now, we'll apply the appScrollTo directive to the links in the app.component.html file along with the respective targets so that we can implement the scroll logic in the next steps. The code should look like this:

...
<div class="content" role="main">
  <div class="page-links">
    <h4 class="page-links__heading">
      Links
    </h4>
    <a class="page-links__link" appScrollTo     target="#resources">Resources</a>
    <a class="page-links__link" appScrollTo     target="#nextSteps">Next Steps</a>
    <a class="page-links__link" appScrollTo     target="#moreContent">More Content</a>
    <a class="page-links__link" appScrollTo     target="#furtherContent">Further Content</a>
    <a class="page-links__link" appScrollTo     target="#moreToRead">More To Read</a>
  </div>
  ...
  <div class="to-top-button">
    <a appScrollTo target="#toolbar" class=    "material-icons">
      keyboard_arrow_up
    </a>
  </div>
</div>

Now, we'll implement the HostListener() decorator to bind the click event to the element the directive is attached to. We'll just log the target input when we click the links. Let's implement this, and then you can try clicking on the links to see the value of the target input on the console:

import { Directive, Input, HostListener } from '@angular/core';
@Directive({
  selector: '[appScrollTo]'
})
export class ScrollToDirective {
  @Input() target = '';
  @HostListener('click')
  onClick() {
    console.log(this.target);
  }
  ...
}

Since we have the click handler set up already, we can now implement the logic to scroll to a particular target. For that, we'll use the document.querySelector method, using the target variable's value to get the element, and then the Element.scrollIntoView() web API to scroll the target element. With this change, you should have the page being scrolled to the target element already when you click the corresponding link:
```
...
export class ScrollToDirective {
  @Input() target = '';
  @HostListener('click')
  onClick() {
    const targetElement = document.querySelector     (this.target);
    targetElement.scrollIntoView();
  }
  ...
}
```
All right—we got the scroll working. "But what's new, Ahsan? Isn't this exactly what we were already doing with the href implementation before?" Well, you're right. But, we're going to make the scroll super smoooooth. We'll pass scrollIntoViewOptions as an argument to the scrollIntoView method with the {behavior: "smooth"} value to use an animation during the scroll. The code should look like this:
```
...
export class ScrollToDirective {
  @Input() target = '';
  @HostListener('click')
  onClick() {
    const targetElement = document.querySelector     (this.target);
    targetElement.scrollIntoView({behavior: 'smooth'});
  }
  constructor() { }
}
```

How it works…

The essence of this recipe is the web API that we're using within an Angular directive, and that is Element.scrollIntoView(). We first attach our appScrollTo directive to the elements that should trigger scrolling upon clicking them. We also specify which element to scroll to by using the target input for each directive attached. Then, we implement the click handler inside the directive with the scrollIntoView() method to scroll to a particular target, and to use a smooth animation while scrolling, we pass the {behavior: 'smooth'} object as an argument to the scrollIntoView() method.

There's more…

scrollIntoView() method documentation (https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollIntoView)
Angular attribute directives documentation (https://angular.io/guide/testing-attribute-directives)

Writing your first custom structural directive

In this recipe, you'll write your first custom structural directive named *appIfNot that will do the opposite of what *ngIf does—that is, you'll provide a Boolean value to the directive, and it will show the content attached to the directive when the value is false, as opposed to how the *ngIf directive shows the content when the value provided is true.

Getting ready

The project for this recipe resides in chapter02/start_here/ng-if-not-directive:

Open the project in VS Code.
Open the terminal, and run npm install to install the dependencies of the project.
Once done, run ng serve -o.
This should open the app in a new browser tab, and you should see something like this:

Figure 2.4 – ng-if-not-directive app running on http://localhost:4200

How to do it…

First of all, we'll create a directive using the following command in the project root:
```
ng g directive directives/if-not
```

Now, instead of the *ngIf directive in the app.component.html file, we can use our *appIfNot directive. We'll also reverse the condition from visibility === VISIBILITY.Off to visibility === VISIBILITY.On, as follows:

...
<div class="content" role="main">
  ...
  <div class="page-section" id="resources"   *appIfNot="visibility === VISIBILITY.On">
    <!-- Resources -->
    <h2>Content to show when visibility is off</h2>
  </div>
</div>

Now that we have set the condition, we need to create an @Input inside the *appIfNot directive that accepts a Boolean value. We'll use a setter to intercept the value changes and will log the value on the console for now:

import { Directive, Input } from '@angular/core';
@Directive({
  selector: '[appIfNot]'
})
export class IfNotDirective {
  constructor() { }
  @Input() set appIfNot(value: boolean) {
    console.log(`appIfNot value is ${value}`);
  }
}

If you tap on the Visibility On and Visibility Off buttons now, you should see the values being changed and reflected on the console, as follows:
Figure 2.5 – Console logs displaying changes for the appIfNot directive values

Now, we're moving toward the actual implementation of showing and hiding the content based on the value being false and true respectively, and for that, we first need the TemplateRef service and the ViewContainerRef service injected into the constructor of if-not.directive.ts. Let's add these, as follows:

import { Directive, Input, TemplateRef, ViewContainerRef } from '@angular/core';
@Directive({
  selector: '[appIfNot]'
})
export class IfNotDirective {
  constructor(private templateRef: TemplateRef<any>,   private viewContainerRef: ViewContainerRef) { }
  @Input() set appIfNot(value: boolean) {
    console.log(`appIfNot value is ${value}`);
  }
}

Finally, we can add the logic to add/remove the content from the DOM based on the appIfNot input's value, as follows:

...
export class IfNotDirective {
  constructor(private templateRef: TemplateRef<any>,   private viewContainerRef: ViewContainerRef) { }
  @Input() set appIfNot(value: boolean) {
    if (value === false) {
      this.viewContainerRef.      createEmbeddedView(this.templateRef);
    } else {
      this.viewContainerRef.clear()
    }
  }
}

How it works…

Structural directives in Angular are special for multiple reasons. First, they allow you to manipulate DOM elements—that is, adding/removing/manipulating based on your needs. Moreover, they have this * prefix that binds to all the magic Angular does behind the scenes. As an example, *ngIf and *ngFor are both structural directives that behind the scenes work with the <ng-template> directive containing the content you bind the directive to and create the required variables/properties for you in the scope of ng-template. In the recipe, we do the same. We use the TemplateRef service to access the <ng-template> directive that Angular creates for us behind the scenes, containing the host element on which our appIfNot directive is applied. Then, based on the value provided to the directive as input, we decide whether to add the magical ng-template to the view or clear the ViewContainerRef service to remove anything inside it.

Angular Cookbook: Over 80 actionable recipes every Angular developer should know

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