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In an Angular directive, use input() to observe a value supplied by a parent, computed() to derive a value from it, and effect() only when you need to synchronize with an imperative, non-signal API. An input signal is read by calling it—such as this.disabled()—and the directive cannot write to it.
How do signals fit into a directive?
Signals are reactive values that you read by calling their getter. Angular tracks signal reads in reactive contexts, so it can respond when a value changes. A directive can use different signal APIs depending on who owns the value and what the directive needs to do with it.
| Need | API | Who owns the value? | Can the directive write to it? |
|---|---|---|---|
| Observe a value bound by the parent | input() |
The parent supplies it; the directive receives a read-only view. | No. Input signals are read-only to the receiving class. |
| Maintain local mutable state | signal() |
The directive. | Yes. Writable signals support update operations. |
| Derive a value from other signals | computed() |
The derivation follows its source signals. | No. A computed signal is read-only. |
| Maintain dependent state that must also be manually adjustable | linkedSignal() |
The directive manages the linked state in relation to its source. | Yes. Use it when the dependent value must remain writable. |
| Synchronize with an imperative, non-signal API | effect() |
The effect reacts to the signals it reads and performs the synchronization. | It is not a state-copying API; use a state API for that relationship. |
Angular documents signals and their reactive behavior in its signals overview. The signals essentials guide also introduces reading and updating signals.
How do I declare and read a directive input signal?
Declare an input with input() in a class property initializer, then call that property wherever you need its current value:
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import { Directive, computed, input } from '@angular/core';
@Directive({
selector: '[appTone]',
})
export class ToneDirective {
tone = input('neutral');
normalizedTone = computed(() => this.tone().trim().toLowerCase());
}Here, tone is supplied from outside the directive, and normalizedTone is derived from the current input. The input signal is a read-only view: do not try to set it from the directive. Angular’s input guide explains signal inputs and notes that inputs are recorded statically at compile time; they cannot be added or removed at runtime.
The InputSignal API reference labels that API “stable since v19.0.” This describes the stability status of InputSignal, not a claim that all Angular signal APIs share the same status. See the InputSignal API reference.
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Should I use computed() or effect() for a signal input?
Use computed() for a value derived from the input
If the directive needs a normalized, formatted, or otherwise derived value, express that relationship with computed(). Computed signals are lazy and memoized: Angular evaluates the derivation when its value is needed and reuses the result until a dependency changes. Dependencies are dynamic, so Angular tracks the signals read during the most recent derivation.
For example, the normalizedTone property above stays in sync with tone without manually copying the input into another signal.
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Use linkedSignal() for dependent state that remains writable
Sometimes a value starts from another signal but the directive also needs to adjust it independently. In that case, linkedSignal() models dependent state that remains manually writable. It is a better fit than an effect that copies a source value into a writable signal.
Reserve effect() for synchronization with non-signal systems
An effect is appropriate when a signal change must be reflected in something outside Angular’s signal state, such as storage, custom DOM behavior, or a third-party rendering library. The Angular effects guide says, “Effects should be the last API you reach for.” Read its guidance on effects for non-reactive APIs before using one to synchronize with an imperative API.
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Do not use an effect simply to copy one signal into another. Angular warns that this pattern can cause expression-changed errors, circular updates, or unnecessary change detection. Model the relationship with computed() or linkedSignal() instead.
How do you create and clean up an effect in a directive?
Create the effect in an injection context, such as the directive’s constructor. It runs at least once, tracks the signals it reads dynamically, and executes asynchronously during change detection.
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import { Directive, effect, input } from '@angular/core';
@Directive({
selector: '[appPersistedValue]',
})
export class PersistedValueDirective {
value = input('');
constructor() {
effect(() => {
const currentValue = this.value();
localStorage.setItem('app-value', currentValue);
});
}
}This example synchronizes a signal input with browser storage, an API that is not itself a signal. If an effect is created outside an injection context, Angular documents passing an injector when creating it.
If an effect starts ongoing work—such as a timer, subscription, or other process that must stop before the next run—register cleanup for that work. Effects owned by a directive are automatically cleaned up when the directive is destroyed, but cleanup for work started by an effect is still important so it does not continue unnecessarily between runs.
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