Introduce SubscriptionCount class

Author: rickclephas

KMPObservableViewModelCore/ObservableViewModelPublisher.swift

@@ -14,12 +14,14 @@ public final class ObservableViewModelPublisher: Publisher {
     public typealias Failure = Never
 
     internal weak var viewModel: (any ViewModel)?
+    private let subscriptionCount: any SubscriptionCount
 
     private let publisher = ObservableObjectPublisher()
     private var objectWillChangeCancellable: AnyCancellable? = nil
 
     internal init(_ viewModel: any ViewModel, _ objectWillChange: ObservableObjectPublisher) {
         self.viewModel = viewModel
+        self.subscriptionCount = viewModel.viewModelScope.subscriptionCount
         viewModel.viewModelScope.setSendObjectWillChange { [weak self] in
             self?.publisher.send()
         }
@@ -29,8 +31,8 @@ public final class ObservableViewModelPublisher: Publisher {
     }
 
     public func receive<S>(subscriber: S) where S : Subscriber, Never == S.Failure, Void == S.Input {
-        viewModel?.viewModelScope.increaseSubscriptionCount()
-        publisher.receive(subscriber: ObservableViewModelSubscriber(self, subscriber))
+        subscriptionCount.increase()
+        publisher.receive(subscriber: ObservableViewModelSubscriber(subscriptionCount, subscriber))
     }
 
     deinit {
@@ -47,16 +49,16 @@ private class ObservableViewModelSubscriber<S>: Subscriber where S : Subscriber,
     typealias Input = Void
     typealias Failure = Never
 
-    private let publisher: ObservableViewModelPublisher
+    private let subscriptionCount: any SubscriptionCount
     private let subscriber: S
 
-    init(_ publisher: ObservableViewModelPublisher, _ subscriber: S) {
-        self.publisher = publisher
+    init(_ subscriptionCount: any SubscriptionCount, _ subscriber: S) {
+        self.subscriptionCount = subscriptionCount
         self.subscriber = subscriber
     }
 
     func receive(subscription: Subscription) {
-        subscriber.receive(subscription: ObservableViewModelSubscription(publisher, subscription))
+        subscriber.receive(subscription: ObservableViewModelSubscription(subscriptionCount, subscription))
     }
 
     func receive(_ input: Void) -> Subscribers.Demand {
@@ -71,24 +73,21 @@ private class ObservableViewModelSubscriber<S>: Subscriber where S : Subscriber,
 /// Subscription for `ObservableViewModelPublisher` that decreases the subscription count upon cancellation.
 private class ObservableViewModelSubscription: Subscription {
 
-    private let publisher: ObservableViewModelPublisher
+    private var subscriptionCount: (any SubscriptionCount)?
     private let subscription: Subscription
 
-    init(_ publisher: ObservableViewModelPublisher, _ subscription: Subscription) {
-        self.publisher = publisher
+    init(_ subscriptionCount: any SubscriptionCount, _ subscription: Subscription) {
+        self.subscriptionCount = subscriptionCount
         self.subscription = subscription
     }
 
     func request(_ demand: Subscribers.Demand) {
         subscription.request(demand)
     }
 
-    private var cancelled = false
-    
     func cancel() {
         subscription.cancel()
-        guard !cancelled else { return }
-        cancelled = true
-        publisher.viewModel?.viewModelScope.decreaseSubscriptionCount()
+        subscriptionCount?.decrease()
+        subscriptionCount = nil
     }
 }

KMPObservableViewModelCoreObjC/KMPOVMSubscriptionCount.h

@@ -0,0 +1,23 @@
+//
+//  KMPOVMSubscriptionCount.h
+//  KMPObservableViewModel
+//
+//  Created by Rick Clephas on 09/06/2025.
+//
+
+#ifndef KMPOVMSubscriptionCount_h
+#define KMPOVMSubscriptionCount_h
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+__attribute__((swift_name("SubscriptionCount")))
+@protocol KMPOVMSubscriptionCount
+- (void)increase;
+- (void)decrease;
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif /* KMPOVMSubscriptionCount_h */

KMPObservableViewModelCoreObjC/KMPOVMViewModelScope.h

@@ -9,12 +9,17 @@
 #define KMPOVMViewModelScope_h
 
 #import <Foundation/Foundation.h>
+#import "KMPOVMSubscriptionCount.h"
+
+NS_ASSUME_NONNULL_BEGIN
 
 __attribute__((swift_name("ViewModelScope")))
 @protocol KMPOVMViewModelScope
-- (void)increaseSubscriptionCount;
-- (void)decreaseSubscriptionCount;
+@property (readonly) id<KMPOVMSubscriptionCount> subscriptionCount;
+
 - (void)setSendObjectWillChange:(void (^ _Nonnull)(void))sendObjectWillChange;
 @end
 
+NS_ASSUME_NONNULL_END
+
 #endif /* KMPOVMViewModelScope_h */

KMPObservableViewModelCoreObjC/KMPObservableViewModelCoreObjC.h

@@ -5,4 +5,5 @@
 //  Created by Rick Clephas on 27/11/2022.
 //
 
+#import "KMPOVMSubscriptionCount.h"
 #import "KMPOVMViewModelScope.h"

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

@@ -16,7 +16,7 @@ internal class CombinedSubscriptionCount private constructor(
 ): StateFlow<Int> {
 
     constructor(viewModelScope: NativeViewModelScope, stateFlow: MutableStateFlow<*>) : this(
-        viewModelScopeCount = viewModelScope.subscriptionCount,
+        viewModelScopeCount = viewModelScope.subscriptionCount().stateFlow,
         stateFlowCount = stateFlow.subscriptionCount
     )
 

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

@@ -1,10 +1,6 @@
 package com.rickclephas.kmp.observableviewmodel
 
 import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.flow.MutableStateFlow
-import kotlinx.coroutines.flow.StateFlow
-import kotlinx.coroutines.flow.asStateFlow
-import kotlinx.coroutines.flow.update
 import platform.darwin.NSObject
 
 /**
@@ -15,19 +11,8 @@ internal class NativeViewModelScope internal constructor(
     val coroutineScope: CoroutineScope
 ): NSObject(), ViewModelScope {
 
-    private val _subscriptionCount = MutableStateFlow(0)
-    /**
-     * A [StateFlow] that emits the number of subscribers to the [ViewModel].
-     */
-    val subscriptionCount: StateFlow<Int> = _subscriptionCount.asStateFlow()
-
-    override fun increaseSubscriptionCount() {
-        _subscriptionCount.update { it + 1 }
-    }
-
-    override fun decreaseSubscriptionCount() {
-        _subscriptionCount.update { it - 1 }
-    }
+    private val _subscriptionCount = SubscriptionCount()
+    override fun subscriptionCount(): SubscriptionCount = _subscriptionCount
 
     private var sendObjectWillChange: (() -> Unit)? = null
 

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

@@ -0,0 +1,20 @@
+package com.rickclephas.kmp.observableviewmodel
+
+import com.rickclephas.kmp.observableviewmodel.objc.KMPOVMSubscriptionCountProtocol
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.update
+import platform.darwin.NSObject
+
+/**
+ * [KMPOVMSubscriptionCountProtocol] implementation that uses a [StateFlow] to store the subscription count.
+ */
+internal class SubscriptionCount: NSObject(), KMPOVMSubscriptionCountProtocol {
+
+    private val _stateFlow = MutableStateFlow(0)
+    val stateFlow: StateFlow<Int> = _stateFlow.asStateFlow()
+
+    override fun increase(): Unit = _stateFlow.update { it + 1 }
+    override fun decrease(): Unit = _stateFlow.update { it - 1 }
+}

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

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