SharedValueObservation

public final class SharedValueObservation<Element>

Experimental

A shared value observation that shares a single underlying database observation for all subscriptions, and thus spares database resources.

For example:

let sharedObservation = ValueObservation
    .tracking { db in try Player.fetchAll(db) }
    .shared(in: dbQueue)

let cancellable = try sharedObservation.start(
    onError: { error in ... },
    onChange: { players: [Player] in
        print("Players have changed.")
    })

The sharing only applies if you start observing the database from the same SharedValueObservation instance:

// NOT shared
let cancellable1 = ValueObservation.tracking { db in ... }.shared(in: dbQueue).start(...)
let cancellable2 = ValueObservation.tracking { db in ... }.shared(in: dbQueue).start(...)

// Shared
let sharedObservation = ValueObservation.tracking { db in ... }.shared(in: dbQueue)
let cancellable1 = sharedObservation.start(...)
let cancellable2 = sharedObservation.start(...)
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  • Starts observing the database.

    The observation lasts until the returned cancellable is cancelled or deallocated.

    For example:

    let sharedObservation = ValueObservation
    .tracking { db in try Player.fetchAll(db) }
    .shared(in: dbQueue)

let cancellable = try sharedObservation.start(
    onError: { error in ... },
    onChange: { players: [Player] in
        print("fresh players: \(players)")
    })

    Declaration

    Swift

    public func start(
    onError: @escaping (Error) -> Void,
    onChange: @escaping (Element) -> Void)
-> DatabaseCancellable

    Parameters

    onError

    A closure that is provided eventual errors that happen during observation
    onChange

    A closure that is provided fresh values

    Return Value

    a DatabaseCancellable

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  • Creates a publisher which tracks changes in database values.

    For example:

    let publisher = ValueObservation
    .tracking { db in try Player.fetchAll(db) }
    .shared(in: dbQueue)
    .publisher()

let cancellable = publisher.sink(
    receiveCompletion: { completion in ... },
    receiveValue: { players: [Player] in
        print("fresh players: \(players)")
    })

    Declaration

    Swift

    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
public func publisher() -> DatabasePublishers.Value<Element>

    Return Value

    A Combine publisher

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Asynchronous Observation