add swiftUI code

iOS-SwiftUI-Code/GRDB.swift-6.29.3/GRDB/Core/TransactionClock.swift

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+import Foundation
+
+/// A type that provides the moment of a transaction.
+///
+/// - note: [**🔥 EXPERIMENTAL**](https://github.com/groue/GRDB.swift/blob/master/README.md#what-are-experimental-features)
+///
+/// ## Topics
+///
+/// ### Built-in Clocks
+///
+/// - ``DefaultTransactionClock``
+/// - ``CustomTransactionClock``
+public protocol TransactionClock {
+    /// Returns the date of the current transaction.
+    ///
+    /// This function is called whenever a transaction starts - precisely
+    /// speaking, whenever the database connection leaves the auto-commit mode.
+    ///
+    /// It is also called when the ``Database/transactionDate`` property is
+    /// called, and the database connection is not in a transaction.
+    ///
+    /// Related SQLite documentation: <https://www.sqlite.org/c3ref/get_autocommit.html>
+    func now(_ db: Database) throws -> Date
+}
+
+extension TransactionClock where Self == DefaultTransactionClock {
+    /// Returns the default clock.
+    public static var `default`: Self { DefaultTransactionClock() }
+}
+
+extension TransactionClock where Self == CustomTransactionClock {
+    /// Returns a custom clock.
+    ///
+    /// The provided closure is called whenever a transaction starts - precisely
+    /// speaking, whenever the database connection leaves the auto-commit mode.
+    ///
+    /// It is also called when the ``Database/transactionDate`` property is
+    /// called, and the database connection is not in a transaction.
+    public static func custom(_ now: @escaping (Database) throws -> Date) -> Self {
+        CustomTransactionClock(now)
+    }
+}
+
+/// The default transaction clock.
+public struct DefaultTransactionClock: TransactionClock {
+    /// Returns the start date of the current transaction.
+    public func now(_ db: Database) throws -> Date {
+        // An opportunity to fetch transaction time from the database when
+        // SQLite supports the feature.
+        Date()
+    }
+}
+
+/// A custom transaction clock.
+public struct CustomTransactionClock: TransactionClock {
+    let _now: (Database) throws -> Date
+    
+    public init(_ now: @escaping (Database) throws -> Date) {
+        self._now = now
+    }
+    
+    public func now(_ db: Database) throws -> Date {
+        try _now(db)
+    }
+}