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onSnapshot() function

Attaches a listener for QuerySnapshot events. You may either pass individual onNext and onError callbacks or pass a single observer object with next and error callbacks. The listener can be cancelled by calling the function that is returned when onSnapshot is called.

NOTE: Although an onCompletion callback can be provided, it will never be called because the snapshot stream is never-ending.

Signature:

export declare function onSnapshot<T>(query: Query<T>, options: SnapshotListenOptions, onNext: (snapshot: QuerySnapshot<T>) => void, onError?: (error: FirestoreError) => void, onCompletion?: () => void): Unsubscribe;

Parameters

Parameter	Type	Description
query	Query<T>	The query to listen to.
options	SnapshotListenOptions	Options controlling the listen behavior.
onNext	(snapshot: QuerySnapshot<T>) => void	A callback to be called every time a new QuerySnapshot is available.
onError	(error: FirestoreError) => void	A callback to be called if the listen fails or is cancelled. No further callbacks will occur.
onCompletion	() => void	Can be provided, but will not be called since streams are never ending.

Returns:

Unsubscribe

An unsubscribe function that can be called to cancel the snapshot listener.