add iOS

iOS-SwiftUI-Code/GRDB.swift-6.29.3/GRDB/ValueObservation/ValueObservationScheduler.swift

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+import Dispatch
+import Foundation
+
+/// A type that determines when `ValueObservation` notifies its fresh values.
+///
+/// ## Topics
+///
+/// ### Built-In Schedulers
+///
+/// - ``async(onQueue:)``
+/// - ``immediate``
+/// - ``AsyncValueObservationScheduler``
+/// - ``ImmediateValueObservationScheduler``
+public protocol ValueObservationScheduler {
+    /// Returns whether the initial value should be immediately notified.
+    ///
+    /// If the result is true, then this method was called on the main thread.
+    func immediateInitialValue() -> Bool
+    
+    func schedule(_ action: @escaping () -> Void)
+}
+
+extension ValueObservationScheduler {
+    func scheduleInitial(_ action: @escaping () -> Void) {
+        if immediateInitialValue() {
+            action()
+        } else {
+            schedule(action)
+        }
+    }
+}
+
+// MARK: - AsyncValueObservationScheduler
+
+/// A scheduler that asynchronously notifies fresh value of a `DispatchQueue`.
+public struct AsyncValueObservationScheduler: ValueObservationScheduler {
+    var queue: DispatchQueue
+    
+    public init(queue: DispatchQueue) {
+        self.queue = queue
+    }
+    
+    public func immediateInitialValue() -> Bool { false }
+    
+    public func schedule(_ action: @escaping () -> Void) {
+        queue.async(execute: action)
+    }
+}
+
+extension ValueObservationScheduler where Self == AsyncValueObservationScheduler {
+    /// A scheduler that asynchronously notifies fresh value of the
+    /// given `DispatchQueue`.
+    ///
+    /// For example:
+    ///
+    /// ```swift
+    /// let observation = ValueObservation.tracking { db in
+    ///     try Player.fetchAll(db)
+    /// }
+    ///
+    /// let cancellable = try observation.start(
+    ///     in: dbQueue,
+    ///     scheduling: .async(onQueue: .main),
+    ///     onError: { error in ... },
+    ///     onChange: { (players: [Player]) in
+    ///         print("fresh players: \(players)")
+    ///     })
+    /// ```
+    ///
+    /// - warning: Make sure you provide a serial queue, because a
+    ///   concurrent one such as `DispachQueue.global(qos: .default)` would
+    ///   mess with the ordering of fresh value notifications.
+    public static func async(onQueue queue: DispatchQueue) -> AsyncValueObservationScheduler {
+        AsyncValueObservationScheduler(queue: queue)
+    }
+}
+
+// MARK: - ImmediateValueObservationScheduler
+
+/// A scheduler that notifies all values on the main `DispatchQueue`. The
+/// first value is immediately notified when the `ValueObservation`
+/// is started.
+public struct ImmediateValueObservationScheduler: ValueObservationScheduler, Sendable {
+    public init() { }
+    
+    public func immediateInitialValue() -> Bool {
+        GRDBPrecondition(
+            Thread.isMainThread,
+            "ValueObservation must be started from the main thread.")
+        return true
+    }
+    
+    public func schedule(_ action: @escaping () -> Void) {
+        DispatchQueue.main.async(execute: action)
+    }
+}
+
+extension ValueObservationScheduler where Self == ImmediateValueObservationScheduler {
+    /// A scheduler that notifies all values on the main `DispatchQueue`. The
+    /// first value is immediately notified when the `ValueObservation`
+    /// is started.
+    ///
+    /// For example:
+    ///
+    /// ```swift
+    /// let observation = ValueObservation.tracking { db in
+    ///     try Player.fetchAll(db)
+    /// }
+    ///
+    /// let cancellable = try observation.start(
+    ///     in: dbQueue,
+    ///     scheduling: .immediate,
+    ///     onError: { error in ... },
+    ///     onChange: { (players: [Player]) in
+    ///         print("fresh players: \(players)")
+    ///     })
+    /// // <- here "fresh players" is already printed.
+    /// ```
+    ///
+    /// - important: this scheduler requires that the observation is started
+    ///  from the main queue. A fatal error is raised otherwise.
+    public static var immediate: ImmediateValueObservationScheduler {
+        ImmediateValueObservationScheduler()
+    }
+}