Angular Signals, a powerful feature introduced in the Angular framework, opens up new possibilities for granular tracking and optimizing rendering updates within an application. Signals act as wrappers around values, notifying interested consumers when changes occur. In this article, we will delve into the intricacies of Angular Signals, understanding what signals are, their types, and how they play a crucial role in reactive programming.
Understanding Signals:
A signal in Angular is essentially a wrapper around a value that can notify consumers when the underlying value changes. Signals can encapsulate a wide range of values, from simple primitives to complex data structures. The key feature of signals is their ability to be tracked by Angular, allowing the framework to optimize rendering updates based on changes in these signals.
Types of Signals:
Writable Signals:
- Writable signals offer a programmable interface for directly modifying their values.
- Created using the signal function with an initial value, they have a type of WritableSignal.
- Examples include setting a new value with .set() or updating the value using .update().
Computed Signals:
- Computed signals obtain their values by deriving from other signals.
- Created using the computed function and specifying a derivation function, they have a type of Signal.
- Lazily evaluated and memoized, computed signals only recalculated their values when needed.
const count: WritableSignal<number> = signal(0);
const doubleCount: Signal<number> = computed(() => count() * 2);
- Dependencies of computed signals are dynamic, tracking only the signals read during derivation.
Effects:
- Effects are functions that execute whenever the values of one or more signals undergo a change.
- Created using the effect function, they execute asynchronously during the change detection process.
- Effects track signal value reads and rerun when any of these values change.
effect(() => {
console.log(`The current count is: ${count()}`);
});
- Effects are valuable for scenarios like logging, keeping data in sync with window.localStorage, and custom DOM behaviors.
Practical Usage and Considerations:
Use Cases for Effects:
- Logging data changes for analytics or debugging.
- Synchronizing data with window.localStorage.
- Adding custom DOM behavior beyond template syntax.
- Performing custom rendering to a <canvas>, charting library, or other third-party UI library.
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When Not to Use Effects:
- Avoid using effects for state change propagation to prevent errors and unnecessary change detection cycles.
- By default, effects prohibit the modification of signals, but this restriction can be lifted if needed.
Injection Context:
- Effects require an injection context by default, often provided by calling them within a component, directive, or service constructor.
@Component({...})
export class ExampleComponent {
readonly count = signal(0);
constructor() {
effect(() => {
console.log(`The count is: ${this.count()}`);
});
}- Effects can be assigned to fields or created outside of constructors by passing an Injector to the effect function.
Destroying Effects:
Effects are automatically destroyed when their enclosing context is destroyed.
Effects return an EffectRef that can be used to destroy them manually with the .destroy() operation.
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Advanced Topics:
Signal Equality Functions:
When creating a signal, an optional equality function can be provided to check whether the new value is different from the previous one.
import _ from 'lodash';
const data = signal(['test'], { equal: _.isEqual });Reading Without Tracking Dependencies:
Rarely, you may want to execute code that reads signals without creating a dependency. The untracked function can be used for this purpose.
effect(() => {
console.log(`User set to ${currentUser()} and the counter is ${untracked(counter)}`);
});Effect Cleanup Functions:
Effects might start long-running operations that should be canceled if the effect is destroyed or runs again before the first operation finishes.
The onCleanup function can be used to register a callback invoked before the next run of the effect begins or when the effect is destroyed.
effect((onCleanup) => {
const user = currentUser();
const timer = setTimeout(() => {
console.log(`The user transitioned one second ago: ${user}`);
}, 1000);
onCleanup(() => {
clearTimeout(timer);
});
});Conclusion:
Angular Signals and reactive programming bring a new level of responsiveness and efficiency to Angular applications. By understanding the intricacies of signals, including writable and computed signals, as well as the use of effects, developers can optimize rendering updates and create more robust and maintainable applications. As Angular continues to evolve, signals offer a powerful mechanism for handling state changes and facilitating reactive programming paradigms.
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