Implement proper deregistration and make work back to iOS 15

Author: kylehughes

Sources/PersistentKeyValueKit/Persistent Key-Value Store/Extensions/Foundation/NSUbiquitousKeyValueStore+PersistentKeyValueStore.swift

@@ -42,6 +42,15 @@ extension NSUbiquitousKeyValueStore: PersistentKeyValueStore {
 
     // MARK: Observing Keys
 
+    @inlinable
+    public func deregister<Key>(
+        _ observer: NSObject,
+        for key: Key,
+        context: UnsafeMutableRawPointer?
+    ) where Key : PersistentKeyProtocol {
+        // We use selector-based notification APIs that do not need to perform manual observation deregistration.
+    }
+    
     @inlinable
     public func register<Key>(
         observer target: NSObject,

Sources/PersistentKeyValueKit/Persistent Key-Value Store/Extensions/Foundation/UserDefaults+PersistentKeyValueStore.swift

@@ -33,6 +33,15 @@ extension UserDefaults: PersistentKeyValueStore {
 
     // MARK: Observing Keys
 
+    @inlinable
+    public func deregister<Key>(
+        _ observer: NSObject,
+        for key: Key,
+        context: UnsafeMutableRawPointer?
+    ) where Key : PersistentKeyProtocol {
+        removeObserver(observer, forKeyPath: key.id, context: context)
+    }
+    
     @inlinable
     public func register<Key>(
         observer target: NSObject,

Sources/PersistentKeyValueKit/Persistent Key-Value Store/Implementations/InMemoryPersistentKeyValueStore.swift

@@ -66,6 +66,15 @@ extension InMemoryPersistentKeyValueStore: PersistentKeyValueStore {
 
     // MARK: Observing Keys
 
+    @inlinable
+    public func deregister<Key>(
+        _ observer: NSObject,
+        for key: Key,
+        context: UnsafeMutableRawPointer?
+    ) where Key : PersistentKeyProtocol {
+        // NO-OP
+    }
+    
     public func register<Key>(
         observer target: NSObject,
         for key: Key,

Sources/PersistentKeyValueKit/Persistent Key-Value Store/PersistentKeyValueStore.swift

@@ -40,19 +40,34 @@ public protocol PersistentKeyValueStore {
 
     // MARK: Observing Keys
 
+    /// Removes an observer for the given key.
+    ///
+    /// - Important: This is a `NO-OP`, and not a concern, for `NSUbiquitousKeyValueStore`.
+    /// - Parameter observer: The object to remove as an observer.
+    /// - Parameter key: The key to stop observing.
+    /// - Parameter context: Arbitrary data that more specifically identifies the observer to be removed.
+    func deregister<Key>(
+        _ observer: NSObject,
+        for key: Key,
+        context: UnsafeMutableRawPointer?
+    ) where Key: PersistentKeyProtocol
+    
     /// Registers an observer for the given key.
     ///
+    /// For `UserDefaults`, neither the object receiving this message, nor observer, are retained. An object that calls
+    /// this method must also eventually call either the ``deregister(_:forKeyPath:context:)`` method to unregister the
+    /// observer when participating in KVO.
+    ///
     /// - Important: This is used to orchestrate the SwiftUI property wrapper. This is not intended to provide a general
     ///   way to observe changes to keys: its ergonomics are specifically designed for implementing
     ///   ``PersistentKeyObserver``, knowing that the implementation is either `UserDefaults` and
     ///   `NSUbiquitousKeyValueStore`.
-    ///
     /// - Parameter observer: The observer to register.
     /// - Parameter key: The key to observe.
-    /// - Parameter context: The context to use for `UserDefaults` key-value observation. This is a NO-OP for
+    /// - Parameter context: The context to use for `UserDefaults` key-value observation. This is a `NO-OP` for
     ///   `NSUbiquitousKeyValueStore`.
     /// - Parameter selector: The selector to use for `NSUbiquitousKeyValueStore` notification observation. This is a
-    ///   NO-OP for `UserDefaults`.
+    ///   `NO-OP` for `UserDefaults`.
     func register<Key>(
         observer: NSObject,
         for key: Key,

Sources/PersistentKeyValueKit/Persistent Key/PersistentKeyObserver.swift

@@ -57,6 +57,10 @@ public final class PersistentKeyObserver<Key>: ObservableObject where Key: Persi
             and: #selector(ObserverObject.didReceive(_:))
         )
     }
+    
+    deinit {
+        _store?.deregister(observerObject, for: key, context: &context)
+    }
 
     // MARK: Public Instance Interface
 
@@ -66,6 +70,8 @@ public final class PersistentKeyObserver<Key>: ObservableObject where Key: Persi
             _store
         }
         set {
+            _store?.deregister(observerObject, for: key, context: &context)
+            
             _store = newValue
 
             newValue?.register(

Tests/PersistentKeyValueKitTests/Tests/Persistent Key/AbstractPersistentKeyTests.swift

@@ -129,4 +129,17 @@ public class AbstractPersistentKeyTests<Key>: XCTestCase where Key: PersistentKe
 
         XCTAssertEqual(userDefaults.dictionaryRepresentation()[id] as? Key.Value, storedValue)
     }
+    
+    // MARK: Internal Instance Interface
+    
+    /// Skip the test if the environment is not iOS 16, or later, or equivalent.
+    ///
+    /// There is an issue with the implementation of tryCast across the Objective-C bridge when testing on
+    /// iOS 15 from Xcode 16. Additionally, marking test cases as available <= iOS 16 is not working in the
+    /// same environment.
+    func skipIfNotiOS16OrLaterOrEquivalent() throws {
+        guard #available(iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0, *) else {
+            throw XCTSkip(">= iOS 16 is required for this test.")
+        }
+    }
 }

Tests/PersistentKeyValueKitTests/Tests/Persistent Key/Implementations/PersistentDebugKeyTests.swift

@@ -30,7 +30,9 @@ final class PersistentDebugKeyTests: AbstractPersistentKeyTests<PersistentDebugK
     // MARK: Initialization Tests
 
     @MainActor
-    func test_init_otherRepresentation() {
+    func test_init_otherRepresentation() throws {
+        try skipIfNotiOS16OrLaterOrEquivalent()
+        
         let representation = ReferenceProxyPersistentKeyValueRepresentation<String, String>(
             serializing: \.self,
             deserializing: \.self
@@ -48,6 +50,8 @@ final class PersistentDebugKeyTests: AbstractPersistentKeyTests<PersistentDebugK
 
     @MainActor
     func test_init_otherRepresentation_optional() throws {
+        try skipIfNotiOS16OrLaterOrEquivalent()
+        
         let representation = ReferenceProxyPersistentKeyValueRepresentation<String, String>(
             serializing: \.self,
             deserializing: \.self

Tests/PersistentKeyValueKitTests/Tests/Persistent Key/Implementations/PersistentKeyTests.swift

@@ -30,7 +30,9 @@ final class PersistentKeyTests: AbstractPersistentKeyTests<PersistentKey<String>
     // MARK: Initialization Tests
 
     @MainActor
-    func test_init_otherRepresentation() {
+    func test_init_otherRepresentation() throws {
+        try skipIfNotiOS16OrLaterOrEquivalent()
+        
         let representation = ReferenceProxyPersistentKeyValueRepresentation<String, String>(
             serializing: \.self,
             deserializing: \.self
@@ -48,6 +50,8 @@ final class PersistentKeyTests: AbstractPersistentKeyTests<PersistentKey<String>
 
     @MainActor
     func test_init_otherRepresentation_optional() throws {
+        try skipIfNotiOS16OrLaterOrEquivalent()
+        
         let representation = ReferenceProxyPersistentKeyValueRepresentation<String, String>(
             serializing: \.self,
             deserializing: \.self