add swiftUI code

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

@@ -0,0 +1,45 @@
+/// A protocol indicating that an activity or action supports cancellation.
+///
+/// ## Topics
+///
+/// ### Supporting Types
+///
+/// - ``AnyDatabaseCancellable``
+public protocol DatabaseCancellable {
+    /// Cancel the activity.
+    func cancel()
+}
+
+/// A type-erasing cancellable object that executes a provided closure
+/// when canceled.
+///
+/// An `AnyDatabaseCancellable` instance automatically calls ``cancel()``
+///  when deinitialized.
+public class AnyDatabaseCancellable: DatabaseCancellable {
+    private var _cancel: (() -> Void)?
+    
+    /// Initializes the cancellable object with the given cancel-time closure.
+    public init(cancel: @escaping () -> Void) {
+        _cancel = cancel
+    }
+    
+    /// Creates a cancellable object that forwards cancellation to `base`.
+    public convenience init(_ base: some DatabaseCancellable) {
+        var cancellable = Optional.some(base)
+        self.init {
+            cancellable?.cancel()
+            cancellable = nil // Release memory
+        }
+    }
+    
+    deinit {
+        _cancel?()
+    }
+    
+    public func cancel() {
+        // Don't prevent multiple concurrent calls to _cancel, because it is
+        // pointless. But release memory!
+        _cancel?()
+        _cancel = nil
+    }
+}