ValueScheduling

All values are notified on the main queue.

If the observation starts on the main queue, an initial value is notified right upon subscription, synchronously:

// On main queue
let observation = Player.observationForAll()
let observer = try observation.start(in: dbQueue) { players: [Player] in
    print("fresh players: \(players)")
}
// <- here "fresh players" is already printed.

If the observation does not start on the main queue, an initial value is also notified on the main queue, but asynchronously:

// Not on the main queue: "fresh players" is eventually printed
// on the main queue.
let observation = Player.observationForAll()
let observer = try observation.start(in: dbQueue) { players: [Player] in
    print("fresh players: \(players)")
}

When the database changes, fresh values are asynchronously notified on the main queue:

// Eventually prints "fresh players" on the main queue
try dbQueue.write { db in
    try Player(...).insert(db)
}

All values are asychronously notified on the specified queue.

An initial value is fetched and notified if startImmediately is true.

Values are not all notified on the same dispatch queue.

If startImmediately is true, an initial value is notified right upon subscription, synchronously, on the dispatch queue which starts the observation.

// On any queue
var observation = Player.observationForAll()
observation.scheduling = .unsafe(startImmediately: true)
let observer = try observation.start(in: dbQueue) { players: [Player] in
    print("fresh players: \(players)")
}
// <- here "fresh players" is already printed.

When the database changes, other values are notified on unspecified queues.