Angular 16 has introduced a feature called “Signals” which allows defining reactive values and expressing dependencies between them.
What are Angular Signals?
A Signal according to Angular 16 documentation is:
A signal is a wrapper around a value that can notify interested consumers when that value changes. Signals can contain any value, from simple primitives to complex data structures.
A signal's value is always read through a getter function, which allows Angular to track where the signal is used.
Signals may be either writable or read-only.
The angular signal () function creates
a signal. This function takes two input parameters.
1. Initial value
2. Optional Equality
function.
Whenever there is a change or update in signal value, it propgates the change to effect () such that you can run custom code inside the effect () function.
Using the equality function, you can write your own
business logic to determine whether change should propagate to effect () or
not.
Example 1:
Using Signals to update a total price:
For the e-commerce application where users can add items to their shopping cart. To display the ‘Total Price’ of the items and update it every time new items are added or removed from the shopping cart. The angular signal will help us to update the ‘Total Price’ of the items accordingly.
Here’s how you can use Signals to achieve this:
@Component({
selector: 'my-cart',
template: `
<ul>
<li *ngFor="let item of
items">
{{item.name}} - ${{item.price}}
<button
(click)="removeItem(item)">Remove</button>
</li>
</ul>
Total Price: ${{totalPrice()}}
`,
})
export class
CartComponent {
items = [{ name: 'Product1, price: 5 }, { name: 'Product2', price: 8 }, { name: 'Product3', price: 10 }, ];
// Define a signal for the list of items
itemList = signal(this.items);
// Define a computed value for the total
price
totalPrice = computed(() => {
return this.itemList().reduce((acc, curr)
=> acc + curr.price, 0);
});
removeItem(item) {
// Update the itemList signal by removing
the selected item
this.itemList.set(this.itemList().filter((i) => i !== item));
}
}
Example 2:
Using Signals to show/hide a loading
spinner:
We have a component that fetches data from an API and displays it in a table. We want to load the spinner while the data is being fetched. The angular signal is helping us to show/Hide the loading spinner at the time API Request/Response process.
Here’s how you can use Signals to achieve this:
@Component({
selector: 'my-table',
template: `
<div *ngIf="isLoading()">
Loading...
</div>
<table
*ngIf="!isLoading()">
<tbody>...</tbody>
</table>
`,
})
export class
TableComponent {
// Define a signal for the loading state
(showing the spinner)
loading = signal(true);
ngOnInit() {
// Fetch the data from the API
fetchData().subscribe((data) => {
// Update the loading signal to hide the
spinner
this.loading.set(false);
// Render the data in the table
...
});
}
isLoading() {
return this.loading();
}
}
Example 3:
Using Signals for reactive programming
in applications:
This example incorporates multiple signals, computed signals, and effects to demonstrate their usage in a more complex scenario.
import {
signal, computed, effect } from 'signal';
// Writable
signals
const
firstName = signal(Anil);
const
lastName = signal(Singh);
const age =
signal(36);
// Computed
signal to calculate the full name
const
fullName = computed(() => `${firstName()} ${lastName()}`);
// Computed
signal to determine if the person is an adult or not
const
isAdult = computed(() => age() >= 18);
// Effect to
log the full name and adult status whenever any of the signals change
effect(()
=> {
console.log(`Full Name: ${fullName()}`);
console.log(`Is Adult: ${isAdult()}`);
});
// Update
the values of the writable signals
firstName.set('Aradhya');
lastName.set(Singh);
age.set(10);