zeus
380 lines
14 KiB
Swift
380 lines
14 KiB
Swift
import Foundation
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/// The extent of the shared subscription to a ``SharedValueObservation``.
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public enum SharedValueObservationExtent: Sendable {
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/// The ``SharedValueObservation`` starts a single database observation,
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/// which stops when the `SharedValueObservation` is deallocated and all
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/// subscriptions terminated.
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///
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/// This extent prevents the shared observation from recovering from
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/// database errors. To recover from database errors, you must create a new
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/// shared `SharedValueObservation` instance.
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case observationLifetime
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/// The ``SharedValueObservation`` stops database observation when the
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/// number of subscriptions drops down to zero. Database observation
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/// restarts on the next subscription.
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///
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/// Database errors can be recovered by resubscribing to the
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/// shared observation.
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case whileObserved
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}
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extension ValueObservation {
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/// Returns a shared value observation that spares database resources by
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/// sharing a single underlying ``ValueObservation`` subscription.
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///
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/// - note: [**🔥 EXPERIMENTAL**](https://github.com/groue/GRDB.swift/blob/master/README.md#what-are-experimental-features)
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///
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/// For example:
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///
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/// ```swift
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/// let observation = ValueObservation.tracking { db in
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/// try Player.fetchAll(db)
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/// }
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///
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/// let sharedObservation = observation.shared(in: dbQueue)
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///
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/// let cancellable = try sharedObservation.start { error in
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/// // handle error
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/// } onChange: { (players: [Player]) in
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/// print("Fresh players: \(players)")
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/// }
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/// ```
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///
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/// The underlying subscription is shared if and only if you start observing
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/// the database from the same `SharedValueObservation` instance:
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///
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/// ```swift
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/// // Shared
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/// let sharedObservation = ValueObservation.tracking { db in ... }.shared(in: dbQueue)
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/// let cancellable1 = sharedObservation.start(...)
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/// let cancellable2 = sharedObservation.start(...)
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///
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/// // NOT shared
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/// let cancellable1 = ValueObservation.tracking { db in ... }.shared(in: dbQueue).start(...)
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/// let cancellable2 = ValueObservation.tracking { db in ... }.shared(in: dbQueue).start(...)
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/// ```
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///
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/// A shared observation starts observing the database as soon as it is
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/// subscribed. You can choose if database observation should stop, or not,
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/// when its number of subscriptions drops down to zero, with the `extent`
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/// parameter:
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///
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/// ```swift
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/// // The default: stops observing the database when the number of
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/// // subscriptions drops down to zero, and restart database observation
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/// // on the next subscription.
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/// //
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/// // Database errors can be recovered by resubscribing to the
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/// // shared observation.
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/// let sharedObservation = ValueObservation
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/// .tracking { db in try Player.fetchAll(db) }
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/// .shared(in: dbQueue, extent: .whileObserved)
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///
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/// // Only stops observing the database when the shared observation
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/// // is deinitialized, and all subscriptions are cancelled.
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/// //
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/// // This extent prevents the shared observation from recovering
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/// // from database errors. To recover from database errors, create a new
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/// // shared SharedValueObservation instance.
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/// let sharedObservation = ValueObservation
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/// .tracking { db in try Player.fetchAll(db) }
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/// .shared(in: dbQueue, extent: .observationLifetime)
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/// ```
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///
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/// By default, fresh values are dispatched asynchronously on the
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/// main dispatch queue. You can change this behavior by providing a
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/// scheduler.
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///
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/// For example, the ``ValueObservationScheduler/immediate`` scheduler
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/// notifies all values on the main dispatch queue, and notifies the first
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/// one immediately when the
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/// ``SharedValueObservation/start(onError:onChange:)`` method is called.
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/// The `immediate` scheduling requires that the observation starts from the
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/// main thread (a fatal error is raised otherwise):
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///
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/// ```swift
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/// let observation = ValueObservation.tracking { db in
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/// try Player.fetchAll(db)
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/// }
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///
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/// let sharedObservation = observation.shared(
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/// in: dbQueue,
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/// scheduling: .immediate)
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///
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/// let cancellable = try sharedObservation.start { error in
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/// // handle error
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/// } onChange: { (players: [Player]) in
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/// print("Fresh players: \(players)")
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/// }
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/// // <- here "Fresh players" is already printed.
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/// ```
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///
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/// Note that the `.immediate` scheduler requires that the observation is
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/// subscribed from the main thread. It raises a fatal error otherwise.
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///
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/// - parameter reader: A DatabaseReader.
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/// - parameter scheduler: A Scheduler. By default, fresh values are
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/// dispatched asynchronously on the main queue.
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/// - parameter extent: The extent of the shared database observation.
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/// - returns: A `SharedValueObservation`
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public func shared(
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in reader: some DatabaseReader,
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scheduling scheduler: some ValueObservationScheduler = .async(onQueue: .main),
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extent: SharedValueObservationExtent = .whileObserved)
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-> SharedValueObservation<Reducer.Value>
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where Reducer: ValueReducer
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{
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SharedValueObservation(scheduling: scheduler, extent: extent) { onError, onChange in
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self.start(in: reader, scheduling: scheduler, onError: onError, onChange: onChange)
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}
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}
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}
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/// A shared value observation spares database resources by sharing a single
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/// underlying ``ValueObservation`` subscription.
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///
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/// - note: [**🔥 EXPERIMENTAL**](https://github.com/groue/GRDB.swift/blob/master/README.md#what-are-experimental-features)
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///
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/// You build a `SharedValueObservation` with the ``ValueObservation`` method
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/// ``ValueObservation/shared(in:scheduling:extent:)``. For example:
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///
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/// ```swift
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/// let observation = ValueObservation.tracking { db in
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/// try Player.fetchAll(db)
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/// }
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///
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/// let sharedObservation = observation.shared(in: dbQueue)
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///
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/// let cancellable = try sharedObservation.start { error in
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/// // handle error
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/// } onChange: { (players: [Player]) in
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/// print("Fresh players: \(players)")
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/// }
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/// ```
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///
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/// The underlying subscription is shared if and only if you start observing
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/// the database from the same `SharedValueObservation` instance:
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///
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/// ```swift
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/// // Shared
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/// let sharedObservation = ValueObservation.tracking { db in ... }.shared(in: dbQueue)
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/// let cancellable1 = sharedObservation.start(...)
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/// let cancellable2 = sharedObservation.start(...)
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///
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/// // NOT shared
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/// let cancellable1 = ValueObservation.tracking { db in ... }.shared(in: dbQueue).start(...)
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/// let cancellable2 = ValueObservation.tracking { db in ... }.shared(in: dbQueue).start(...)
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/// ```
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public final class SharedValueObservation<Element> {
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private let scheduler: any ValueObservationScheduler
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private let extent: SharedValueObservationExtent
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private let startObservation: ValueObservationStart<Element>
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private let lock = NSRecursiveLock() // support synchronous observation events
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// protected by lock
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private var clients: [Client]
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private var isObserving = false
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private var cancellable: AnyDatabaseCancellable?
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private var lastResult: Result<Element, Error>?
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private final class Client {
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var onError: (Error) -> Void
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var onChange: (Element) -> Void
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init(onError: @escaping (Error) -> Void, onChange: @escaping (Element) -> Void) {
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self.onError = onError
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self.onChange = onChange
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}
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}
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fileprivate init(
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scheduling scheduler: some ValueObservationScheduler,
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extent: SharedValueObservationExtent,
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startObservation: @escaping ValueObservationStart<Element>)
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{
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self.scheduler = scheduler
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self.extent = extent
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self.startObservation = startObservation
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self.clients = []
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}
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/// Starts observing the database.
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///
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/// The observation lasts until the returned cancellable is cancelled
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/// or deallocated.
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///
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/// For example:
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///
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/// ```swift
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/// let sharedObservation = ValueObservation
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/// .tracking { db in try Player.fetchAll(db) }
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/// .shared(in: dbQueue)
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///
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/// let cancellable = try sharedObservation.start { error in
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/// // handle error
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/// } onChange: { (players: [Player]) in
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/// print("fresh players: \(players)")
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/// }
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/// ```
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///
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/// - parameter onError: The closure to execute when the observation fails.
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/// - parameter onChange: The closure to execute on receipt of a
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/// fresh value.
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/// - returns: A DatabaseCancellable that can stop the observation.
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public func start(
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onError: @escaping (Error) -> Void,
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onChange: @escaping (Element) -> Void)
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-> AnyDatabaseCancellable
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{
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synchronized {
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// Support for reentrancy: a shared immediate observation is
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// started from the first value notification of that same shared
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// immediate observation. Yeah, users are nasty.
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// In this case, self.cancellable is still nil, because we are
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// still waiting for the upstream ValueObservation to start.
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// But we must not start another one.
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let needsStart = !isObserving
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// State change
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let client = Client(onError: onError, onChange: onChange)
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clients.append(client)
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isObserving = true
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// Side effect
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if needsStart {
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// Self retains the cancellable, so don't have the cancellable retain self.
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cancellable = startObservation(
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// onError
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{ [weak self] error in
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self?.handleError(error)
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},
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// onChange
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{ [weak self] element in
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self?.handleChange(element)
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})
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} else if let result = lastResult {
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// Notify last result as an initial value
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scheduler.scheduleInitial {
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switch result {
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case let .failure(error):
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onError(error)
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case let .success(value):
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onChange(value)
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}
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}
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}
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return AnyDatabaseCancellable {
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// Retain shared observation (self) until client cancels
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self.handleCancel(client)
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}
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}
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}
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#if canImport(Combine)
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/// Returns a publisher of observed values.
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///
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/// For example:
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///
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/// ```swift
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/// let observation = ValueObservation
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/// .tracking { db in try Player.fetchAll(db) }
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/// .shared(in: dbQueue)
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///
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/// let publisher = observation.publisher()
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///
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/// let cancellable = publisher.sink { completion in
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/// // handle completion
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/// } receiveValue: { (players: [Player]) in
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/// print("fresh players: \(players)")
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/// }
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/// ```
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@available(iOS 13, macOS 10.15, tvOS 13, watchOS 6, *)
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public func publisher() -> DatabasePublishers.Value<Element> {
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DatabasePublishers.Value { onError, onChange in
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self.start(onError: onError, onChange: onChange)
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}
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}
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#endif
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private func handleError(_ error: Error) {
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synchronized {
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let notifiedClients = clients
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// State change
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clients = []
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if extent == .whileObserved {
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isObserving = false
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cancellable = nil
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lastResult = nil
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} else {
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lastResult = .failure(error)
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}
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// Side effect
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for client in notifiedClients {
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client.onError(error)
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}
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}
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}
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private func handleChange(_ value: Element) {
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synchronized {
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// State change
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lastResult = .success(value)
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// Side effect
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for client in clients {
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client.onChange(value)
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}
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}
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}
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private func handleCancel(_ client: Client) {
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synchronized {
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// State change
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clients.removeFirst(where: { $0 === client })
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if clients.isEmpty && extent == .whileObserved {
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isObserving = false
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cancellable = nil
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lastResult = nil
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}
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}
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}
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private func synchronized<T>(_ execute: () throws -> T) rethrows -> T {
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lock.lock()
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defer { lock.unlock() }
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return try execute()
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}
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}
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extension SharedValueObservation {
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// MARK: - Asynchronous Observation
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/// Returns an asynchronous sequence of observed values.
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///
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/// - note: [**🔥 EXPERIMENTAL**](https://github.com/groue/GRDB.swift/blob/master/README.md#what-are-experimental-features)
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///
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/// For example:
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///
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/// ```swift
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/// let sharedObservation = ValueObservation
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/// .tracking { db in try Player.fetchAll(db) }
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/// .shared(in: dbQueue)
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///
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/// for try await players in sharedObservation.values() {
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/// print("Fresh players: \(players)")
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/// }
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/// ```
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@available(iOS 13, macOS 10.15, tvOS 13, watchOS 6, *)
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public func values(bufferingPolicy: AsyncValueObservation<Element>.BufferingPolicy = .unbounded)
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-> AsyncValueObservation<Element>
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{
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AsyncValueObservation(bufferingPolicy: bufferingPolicy) { onError, onChange in
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self.start(onError: onError, onChange: onChange)
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}
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}
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}
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