Angular Dynamic host Property Usage

December 28, 2023

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Angular is a powerful framework. Most folks know of it as the component framework, but it's much more than that.

For example, did you know about Angular directives?

Directives allow you to bind to an element via an attribute and change the behavior of said element.

typescript
import { Component, Directive } from '@angular/core';
@Directive({
selector: '[doNothing]',
standalone: true,
})
class DoNothingDirective {}
@Component({
selector: 'app-root',
standalone: true,
imports: [DoNothingDirective],
template: `
<p doNothing>I am currently unchanged.</p>
`,
})
class AppComponent {}

Think of them as components without templates. They can use lifecycle methods:

typescript
@Directive({
selector: '[alertOnDestroy]',
standalone: true,
})
class AlertOnDestroyDirective implements OnDestroy {
ngOnDestroy() {
alert('Element was unrendered!');
}
}
@Component({
selector: 'app-root',
standalone: true,
imports: [AlertOnDestroyDirective, NgIf],
template: `
<p *ngIf="render" alertOnDestroy>Unmount me to see an alert!</p>
<button (click)="render = !render">Toggle</button>
`,
})
class AppComponent {
render = true;
}

Store state:

typescript
@Directive({
selector: '[listenForEvents]',
standalone: true,
})
class ListenForEventDirective implements OnInit {
count = 0;
ngOnInit() {
document.addEventListener('hello', () => {
alert(`You sent this many events: ${++this.count}`);
});
}
}
@Component({
selector: 'app-root',
standalone: true,
imports: [ListenForEventDirective],
template: `
<p listenForEvents>This paragraph tag listens for events!</p>
<button (click)="sendEvent()">Send event</button>
`,
})
class AppComponent {
sendEvent() {
const event = new CustomEvent('hello');
document.dispatchEvent(event);
}
}

Use the inject function:

typescript
import { Component, Directive, inject, OnInit } from '@angular/core';
import { DOCUMENT } from '@angular/common';
@Directive({
selector: '[listenForEvents]',
standalone: true,
})
class ListenForEventDirective implements OnInit {
count = 0;
doc = inject(DOCUMENT);
ngOnInit() {
this.doc.addEventListener('hello', () => {
alert(`You sent this many events: ${++this.count}`);
});
}
}

And do just about anything else a component can do without a template of its own.

Accessing a directives' element with ElementRef

Because a directive is attached to an element, a typical usage of a directive is to modify the element it's attached to using ElementRef and inject; like so:

typescript
const injectAndGetEl = () => {
const el = inject(ElementRef);
console.log(el.nativeElement);
return el;
};
@Directive({
selector: '[logEl]',
standalone: true,
})
class LogElDirective {
_el = injectAndGetEl();
}

While this doesn't do anything yet, it logs the element to the console.log method. Let's instead change this code to make the attached element have a red background and white text:

typescript
import { Component, Directive, ElementRef, inject } from '@angular/core';
const injectAndMakeRed = () => {
const el = inject(ElementRef);
el.nativeElement.style.backgroundColor = 'red';
el.nativeElement.style.color = 'white';
};
@Directive({
selector: '[red]',
standalone: true,
})
class RedDirective {
_el = injectAndMakeRed();
}
@Component({
selector: 'app-root',
standalone: true,
imports: [RedDirective],
template: `
<p red>This is red</p>
`,
})
class AppComponent {}

host property binding

While the inject method works, there's a better way to bind an element: the host property.

typescript
@Directive({
selector: '[red]',
standalone: true,
host: {
style: 'background-color: red; color: white;',
},
})
class RedDirective {}
@Component({
selector: 'app-root',
standalone: true,
imports: [RedDirective],
template: `
<p red>This is red</p>
`,
})
class AppComponent {}

Here, host refers to the element the directive is attached to. We can use it to then attach new attributes to the parent element like we did above.

Dynamic host property binding

host isn't just useful for static attribute bindings either, you can use it with attribute binding and event listening using the same [] and () syntax you're familiar with:

typescript
@Directive({
selector: '[red]',
standalone: true,
host: {
'[style]': `selected ? 'background-color: red; color: white;' : ''`,
'(click)': 'selected = !selected',
},
})
class RedDirective {
selected = false;
}
@Component({
selector: 'app-root',
standalone: true,
imports: [RedDirective],
template: `
<p red>This is red when I am selected</p>
`,
})
class AppComponent {}

Using host property with Components

Because components are just like directives but with a template, complete with a host element, we can use the same host directive on components as well as directives:

typescript
@Component({
selector: 'red-div',
standalone: true,
host: {
'[style]': `selected ? 'background-color: red; color: white;' : ''`,
'(click)': 'selected = !selected',
},
template: `
<span><ng-content/></span>
`,
})
class RedDirective {
selected = false;
}
@Component({
selector: 'app-root',
standalone: true,
imports: [RedDirective],
template: `
<red-div>This is red when I am selected</red-div>
`,
})
class AppComponent {}

This will output to something akin to the following Angular template:

html
<red-div
[style]="selected ? 'background-color: red; color: white;' : ''"
(click)="selected = !selected"
>
<span>This is red when I am selected</span>
</red-div>
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