Swift Observation support

KMPObservableViewModelCore/ObservableProperties.swift

@@ -0,0 +1,62 @@
+//
+//  ObservableProperties.swift
+//  KMPObservableViewModelCore
+//
+//  Created by Rick Clephas on 25/10/2025.
+//
+
+import Observation
+import KMPObservableViewModelCoreObjC
+
+/// Helper object that maps any external `Property` to an `ObservableProperty`.
+@available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *)
+internal final class ObservableProperties {
+
+    private var properties: [ObjectIdentifier:ObservableProperty] = [:]
+
+    private func observableProperty(_ property: any Property) -> ObservableProperty {
+        let identifier = ObjectIdentifier(property)
+        if let observableProperty = properties[identifier] { return observableProperty }
+        let observableProperty = ObservableProperty(property)
+        properties[identifier] = observableProperty
+        return observableProperty
+    }
+
+    func access(_ property: any Property) {
+        observableProperty(property).access()
+    }
+
+    func willSet(_ property: any Property) {
+        observableProperty(property).willSet()
+    }
+
+    func didSet(_ property: any Property) {
+        observableProperty(property).didSet()
+    }
+}
+
+/// Helper object that turns an external `Property` into a Swift observable property.
+@available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *)
+private final class ObservableProperty: Observable {
+
+    private let registrar = ObservationRegistrar()
+    private let property: any Property
+
+    init(_ property: any Property) {
+        self.property = property
+    }
+
+    var value: Any? { property.value }
+
+    func access() {
+        registrar.access(self, keyPath: \.value)
+    }
+
+    func willSet() {
+        registrar.willSet(self, keyPath: \.value)
+    }
+
+    func didSet() {
+        registrar.didSet(self, keyPath: \.value)
+    }
+}

KMPObservableViewModelCore/ObservableViewModelPublisher.swift

@@ -18,18 +18,53 @@ public final class ObservableViewModelPublisher: Combine.Publisher, KMPObservabl
     private let publisher: ObservableObjectPublisher
     private let subscriptionCount: any SubscriptionCount
 
+    private var _observableProperties: Any? = nil
+    @available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *)
+    private var observableProperties: ObservableProperties? {
+        _observableProperties as! ObservableProperties?
+    }
+
     internal init(_ viewModel: any ViewModel) {
         self.publisher = viewModel.objectWillChange
         self.subscriptionCount = viewModel.viewModelScope.subscriptionCount
+        if #available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *), viewModel is Observable {
+            _observableProperties = ObservableProperties()
+        }
     }
 
     public func receive<S>(subscriber: S) where S : Subscriber, Never == S.Failure, Void == S.Input {
         subscriptionCount.increase()
         publisher.receive(subscriber: ObservableViewModelSubscriber(subscriptionCount, subscriber))
     }
 
-    public func send() {
-        publisher.send()
+    public func access(_ property: any Property) {
+        if #available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *), let observableProperties {
+            observableProperties.access(property)
+        }
+    }
+
+    public func willSet(_ property: any Property) {
+        if #available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *), let observableProperties {
+            observableProperties.willSet(property)
+        } else {
+            publisher.send()
+        }
+    }
+
+    public func didSet(_ property: any Property) {
+        if #available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *), let observableProperties {
+            observableProperties.didSet(property)
+        }
+    }
+}
+
+internal extension KMPObservableViewModelCoreObjC.Publisher {
+    /// Casts this `Publisher` to an `ObservableViewModelPublisher`.
+    func cast() -> ObservableViewModelPublisher {
+        guard let publisher = self as? ObservableViewModelPublisher else {
+            fatalError("Publisher must be an ObservableViewModelPublisher")
+        }
+        return publisher
     }
 }
 

KMPObservableViewModelCore/ViewModel.swift

@@ -22,7 +22,7 @@ public protocol ViewModel: ObservableObject where ObjectWillChangePublisher == O
 public extension ViewModel {
     var viewModelWillChange: ObservableViewModelPublisher {
         if let publisher = viewModelScope.publisher {
-            return publisher as! ObservableViewModelPublisher
+            return publisher.cast()
         }
         let publisher = ObservableViewModelPublisher(self)
         viewModelScope.publisher = publisher

KMPObservableViewModelCoreObjC/KMPOVMProperty.h

@@ -0,0 +1,22 @@
+//
+//  KMPOVMProperty.h
+//  KMPObservableViewModelCoreObjC
+//
+//  Created by Rick Clephas on 25/10/2025.
+//
+
+#ifndef KMPOVMProperty_h
+#define KMPOVMProperty_h
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+__attribute__((swift_name("Property")))
+@protocol KMPOVMProperty
+@property (readonly) id _Nullable value;
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif /* KMPOVMProperty_h */

KMPObservableViewModelCoreObjC/KMPOVMPublisher.h

@@ -9,12 +9,15 @@
 #define KMPOVMPublisher_h
 
 #import <Foundation/Foundation.h>
+#import "KMPOVMProperty.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
 __attribute__((swift_name("Publisher")))
 @protocol KMPOVMPublisher
-- (void)send;
+- (void)access:(id<KMPOVMProperty>)property;
+- (void)willSet:(id<KMPOVMProperty>)property;
+- (void)didSet:(id<KMPOVMProperty>)property;
 @end
 
 NS_ASSUME_NONNULL_END

KMPObservableViewModelCoreObjC/KMPObservableViewModelCoreObjC.h

@@ -5,6 +5,7 @@
 //  Created by Rick Clephas on 27/11/2022.
 //
 
+#import "KMPOVMProperty.h"
 #import "KMPOVMPublisher.h"
 #import "KMPOVMSubscriptionCount.h"
 #import "KMPOVMViewModelScope.h"

README.md

@@ -177,7 +177,7 @@ Create a `KMPObservableViewModel.swift` file with the following contents:
 import KMPObservableViewModelCore
 import shared // This should be your shared KMP module
 
-extension Kmp_observableviewmodel_coreViewModel: ViewModel { }
+extension Kmp_observableviewmodel_coreViewModel: @retroactive ViewModel { }
 ```
 
 After that you can use your view model almost as if it were an `ObservableObject`.   
@@ -211,6 +211,31 @@ class TimeTravelViewModel: shared.TimeTravelViewModel {
 }
 ```
 
+### Observation support
+
+When using `ObservableObject`s your SwiftUI views will likely rerender a lot.
+Luckily the [Observation](https://developer.apple.com/documentation/Observation) framework brings significant 
+improvements in terms of change tracking, allowing for more efficient rerendering.
+
+To add support for the Observation framework to all your Kotlin view models,
+simply add the following to your `KMPObservableViewModel.swift` file:
+```swift
+import Observation
+import shared // This should be your shared KMP module
+
+extension Kmp_observableviewmodel_coreViewModel: @retroactive Observable { }
+```
+
+Alternatively you can add the `Observable` conformance to specific Kotlin view models instead.
+
+> [!NOTE]
+> The `Observable` conformance is automatically added if you subclass your view model and use the `@Observable` macro.
+
+> [!NOTE]
+> As an added bonus your Kotlin view models will benefit from the Observable framework on supported OS versions
+> regardless of your app's deployment target. E.g. you can benefit from Observation support on iOS 17 and above
+> while still supporting iOS 16 and below using `ObservableObject`s.
+
 ### Child view models
 
 You'll need some additional logic if your `ViewModel`s expose child view models.

kmp-observableviewmodel-core/src/androidxMain/kotlin/com/rickclephas/kmp/observableviewmodel/ViewModel.kt

@@ -8,7 +8,7 @@ import kotlinx.coroutines.CoroutineScope
 import kotlin.reflect.KClass
 
 /**
- * A Kotlin Multiplatform Mobile ViewModel.
+ * A Kotlin Multiplatform ViewModel.
  */
 public actual abstract class ViewModel: AndroidXViewModel {
 

kmp-observableviewmodel-core/src/appleMain/kotlin/com/rickclephas/kmp/observableviewmodel/NativeViewModelScope.kt

@@ -8,7 +8,7 @@ import platform.darwin.NSObject
  * Implementation of [ViewModelScope] for Apple platforms.
  * @property coroutineScope The [CoroutineScope] associated with the [ViewModel].
  */
-internal class NativeViewModelScope internal constructor(
+internal class NativeViewModelScope(
     val coroutineScope: CoroutineScope
 ): NSObject(), ViewModelScope {
 

kmp-observableviewmodel-core/src/appleMain/kotlin/com/rickclephas/kmp/observableviewmodel/ObservableStateFlow.kt

@@ -9,25 +9,32 @@ import kotlinx.coroutines.flow.StateFlow
 
 /**
  * A [MutableStateFlow] wrapper that emits state change events through the [NativeViewModelScope]
- * and it accounts for the [NativeViewModelScope.subscriptionCount].
+ * and accounts for the [NativeViewModelScope.subscriptionCount].
  */
 @OptIn(ExperimentalForInheritanceCoroutinesApi::class)
 internal class ObservableMutableStateFlow<T>(
     private val viewModelScope: NativeViewModelScope,
     private val stateFlow: MutableStateFlow<T>
 ): MutableStateFlow<T> {
 
+    private val property = StateFlowProperty(stateFlow)
+
     override var value: T
-        get() = stateFlow.value
+        get() {
+            viewModelScope.publisher?.access(property)
+            return stateFlow.value
+        }
         set(value) {
-            if (stateFlow.value != value) {
-                viewModelScope.publisher?.send()
-            }
+            val publisher = viewModelScope.publisher?.takeIf { stateFlow.value != value }
+            publisher?.willSet(property)
             stateFlow.value = value
+            publisher?.didSet(property)
         }
 
+    // Same implementation as in StateFlowImpl, but we need to go through our own value property.
+    // https://github.com/Kotlin/kotlinx.coroutines/blob/6dfabf763fe9fc91fbb73eb0f2d5b488f53043f1/kotlinx-coroutines-core/common/src/flow/StateFlow.kt#L367
     override val replayCache: List<T>
-        get() = stateFlow.replayCache
+        get() = listOf(value)
 
     /**
      * The combined subscription count from the [NativeViewModelScope] and the actual [StateFlow].
@@ -38,10 +45,11 @@ internal class ObservableMutableStateFlow<T>(
         stateFlow.collect(collector)
 
     override fun compareAndSet(expect: T, update: T): Boolean {
-        if (stateFlow.value == expect && expect != update) {
-            viewModelScope.publisher?.send()
-        }
-        return stateFlow.compareAndSet(expect, update)
+        val publisher = viewModelScope.publisher?.takeIf { stateFlow.value == expect && expect != update }
+        publisher?.willSet(property)
+        val result = stateFlow.compareAndSet(expect, update)
+        publisher?.didSet(property)
+        return result
     }
 
     @ExperimentalCoroutinesApi

kmp-observableviewmodel-core/src/appleMain/kotlin/com/rickclephas/kmp/observableviewmodel/StateFlowProperty.kt

@@ -0,0 +1,14 @@
+package com.rickclephas.kmp.observableviewmodel
+
+import com.rickclephas.kmp.observableviewmodel.objc.KMPOVMPropertyProtocol
+import kotlinx.coroutines.flow.StateFlow
+import platform.darwin.NSObject
+
+/**
+ * A [Property][KMPOVMPropertyProtocol] that retrieves its value from a [StateFlow].
+ */
+internal class StateFlowProperty<out T>(
+    private val stateFlow: StateFlow<T>
+): NSObject(), KMPOVMPropertyProtocol {
+    override fun value(): T = stateFlow.value
+}

kmp-observableviewmodel-core/src/commonMain/kotlin/com/rickclephas/kmp/observableviewmodel/ViewModel.kt

@@ -3,7 +3,7 @@ package com.rickclephas.kmp.observableviewmodel
 import kotlinx.coroutines.CoroutineScope
 
 /**
- * A Kotlin Multiplatform Mobile ViewModel.
+ * A Kotlin Multiplatform ViewModel.
  */
 public expect abstract class ViewModel {
 

kmp-observableviewmodel-core/src/nativeInterop/cinterop/KMPObservableViewModelCoreObjC.def

@@ -1,3 +1,3 @@
 language = Objective-C
 package = com.rickclephas.kmp.observableviewmodel.objc
-headers = KMPOVMPublisher.h KMPOVMSubscriptionCount.h KMPOVMViewModelScope.h
+headers = KMPOVMProperty.h KMPOVMPublisher.h KMPOVMSubscriptionCount.h KMPOVMViewModelScope.h

kmp-observableviewmodel-core/src/nonAndroidxMain/kotlin/com/rickclephas/kmp/observableviewmodel/ViewModel.kt

@@ -4,7 +4,7 @@ import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.cancel
 
 /**
- * A Kotlin Multiplatform Mobile ViewModel.
+ * A Kotlin Multiplatform ViewModel.
  */
 public actual abstract class ViewModel {