ValueObservation

Creates a publisher which tracks changes in database values.

For example:

let observation = ValueObservation.tracking { db in
    try Player.fetchAll(db)
}
let cancellable = observation
    .publisher(in: dbQueue)
    .sink(
        receiveCompletion: { completion in ... },
        receiveValue: { players: [Player] in
            print("fresh players: \(players)")
        })

By default, fresh values are dispatched asynchronously on the main queue. You can change this behavior by by providing a scheduler.

For example, .immediate notifies all values on the main queue as well, and the first one is immediately notified when the publisher is subscribed:

let cancellable = observation
    .publisher(
        in: dbQueue,
        scheduling: .immediate) // <-
    .sink(
        receiveCompletion: { completion in ... },
        receiveValue: { players: [Player] in
            print("fresh players: \(players)")
        })
// <- here "fresh players" is already printed.

Note that the .immediate scheduler requires that the publisher is subscribed from the main thread. It raises a fatal error otherwise.