Detecting changes in Angular is essential for reactivity. Watcheffect hook in Angular allows you to monitor specific values or properties and react to changes. This article explains the syntax, usage, and performance implications of Watcheffect. It a
The Watcheffect
in Angular is a hook that allows you to monitor changes in specific values or object properties. Its basic syntax is as follows:
<code class="typescript">@Watcheffect(propertyName or parameterName) effectCallback(changes: ObservableValue<any>) {}</code>
To detect changes in a reactive object using Watcheffect
, you can use the ngOnChanges
lifecycle hook. This hook provides an ngOnChanges
property that contains an object with key-value pairs representing the previous and current values of the changed properties. For instance:
<code class="typescript">@Component(...) class MyComponent { @Watcheffect("myProperty") ngOnChanges(changes: SimpleChanges) { if (changes["myProperty"]) { // Do something when the property changes } } }</code>
Watcheffect
uses zone.js to intercept property accesses and track changes. While it's an effective way to monitor changes, excessive use can lead to performance issues, especially with large objects or frequent property accesses. Consider using @Input
and @Output
properties or reactivity helpers like BehaviorSubject
or ReplaySubject
for better performance in most cases.
Watcheffect
also allows you to create custom detectors for specific values or expressions. This can be useful when you need more granular control over what triggers a change detection. To create a custom detector, use the @DetectionStrategy
property in the @Watcheffect
decorator:
<code class="typescript">@Watcheffect('myExpression', { detectionStrategy: customDetection }) effectCallback(changes: ObservableValue<any>) {}</code>
In the example above, customDetection
is a function that takes the input value and returns a tuple of values representing the previous and current states.
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