> For the complete documentation index, see [llms.txt](https://guide-angular.wishtack.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://guide-angular.wishtack.io/angular/formulaires/reactive-forms/observation-des-changements.md).

# Observation des Changements

La propriété `valueChanges` est l'une des propriétés les plus importantes des "controls". Il s'agit d'un `Observable` permettant d'**adopter une approche réactive en détectant tous les changements apportés aux données du "control"** *(le `FormGroup` regroupant tous les "controls" est lui-même un "control")*.

A titre d'exemple, le code suivant permet de construire un `Observable` contenant la liste des résultats associés à la recherche de l'utilisateur.\
L'opérateur `debounceTime` permet d'attendre 100ms d'inactivité avant de produire une requête.\
Grâce à l'opérateur `switchMap`, chaque requête en cours est annulée avant d'en produire une nouvelle.

```typescript
this.bookList$ = this.bookForm.valueChanges
    .pipe(
        debounceTime(100),
        switchMap(value => this._bookRepository.search({keywords: value.title}))
    );
```

{% hint style="info" %}
Tant que l'`Observable` n'est pas consommé *(via un `subscribe`)*, aucun traitement n'est exécuté.
{% endhint %}


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