add iOS

iOS-SwiftUI-Code/GRDB.swift-6.29.3/GRDB/ValueObservation/Reducers/ValueReducer.swift

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+/// Implementation details of `ValueReducer`.
+public protocol _ValueReducer {
+    /// The type of fetched database values
+    associatedtype Fetched
+    
+    /// The type of observed values
+    associatedtype Value
+    
+    /// Transforms a fetched value into an eventual observed value. Returns nil
+    /// when observer should not be notified.
+    ///
+    /// This method runs in some unspecified dispatch queue.
+    ///
+    /// ValueReducer semantics require that the first invocation of this
+    /// method returns a non-nil value:
+    ///
+    ///     let reducer = MyReducer()
+    ///     reducer._value(...) // MUST NOT be nil
+    ///     reducer._value(...) // MAY be nil
+    ///     reducer._value(...) // MAY be nil
+    mutating func _value(_ fetched: Fetched) throws -> Value?
+}
+
+/// `ValueReducer` supports ``ValueObservation``.
+///
+/// A `ValueReducer` fetches and transforms the database values
+/// observed by a ``ValueObservation``.
+///
+/// ## Topics
+///
+/// ### Support
+///
+/// - ``ValueReducers``
+public protocol ValueReducer: _ValueReducer {
+    /// Fetches database values upon changes in an observed database region.
+    ///
+    /// This method must does not depend on the state of the reducer.
+    func _fetch(_ db: Database) throws -> Fetched
+}
+
+/// A namespace for concrete types that adopt the ``ValueReducer`` protocol.
+public enum ValueReducers { }