Add yieldLoading to SharedSubscriber (#125)

Author: mhayes853

Sources/Sharing/Internal/Reference.swift

@@ -224,7 +224,10 @@ final class _PersistentReference<Key: SharedReaderKey>:
       : .initialValue(initialValue)
     self.subscription = key.subscribe(
       context: context,
-      subscriber: SharedSubscriber(callback: callback)
+      subscriber: SharedSubscriber(
+        callback: callback,
+        onLoading: { [weak self] in self?.isLoading = $0 }
+      )
     )
   }
 

Sources/Sharing/SharedContinuations.swift

@@ -67,9 +67,14 @@ public struct LoadContinuation<Value>: Sendable {
 /// an external system can be shared.
 public struct SharedSubscriber<Value>: Sendable {
   let callback: @Sendable (Result<Value?, any Error>) -> Void
+  let onLoading: (@Sendable (Bool) -> Void)?
 
-  public init(callback: @escaping @Sendable (Result<Value?, any Error>) -> Void) {
+  public init(
+    callback: @escaping @Sendable (Result<Value?, any Error>) -> Void,
+    onLoading: (@Sendable (Bool) -> Void)? = nil
+  ) {
     self.callback = callback
+    self.onLoading = onLoading
   }
 
   /// Yield an updated value from an external source.
@@ -79,6 +84,13 @@ public struct SharedSubscriber<Value>: Sendable {
     yield(with: .success(value))
   }
 
+  /// Yield a loading state from an external source.
+  ///
+  /// - Parameter isLoading: Whether the external source is loading.
+  public func yieldLoading(_ isLoading: Bool = true) {
+    onLoading?(isLoading)
+  }
+
   /// Yield the initial value provided to the property wrapper when none exists in the external
   /// source.
   ///

Tests/SharingTests/IsLoadingTests.swift

@@ -64,6 +64,49 @@ import Testing
     #expect($value.isLoading == false)
     #expect(value == 42)
   }
+
+  @Test func isLoadingSubcriberYieldValue() {
+    struct Key: SharedReaderKey {
+      let id = UUID()
+      let testScheduler: TestSchedulerOf<DispatchQueue>
+      func load(context: LoadContext<Int>, continuation: LoadContinuation<Int>) {
+        continuation.resumeReturningInitialValue()
+      }
+      func subscribe(
+        context: LoadContext<Int>, subscriber: SharedSubscriber<Int>
+      ) -> SharedSubscription {
+        subscriber.yieldLoading()
+        testScheduler.schedule { subscriber.yield(42) }
+        return SharedSubscription {}
+      }
+    }
+    @SharedReader(Key(testScheduler: testScheduler)) var value = 0
+    #expect($value.isLoading == true)
+    testScheduler.advance()
+    #expect($value.isLoading == false)
+    #expect(value == 42)
+  }
+
+  @Test func isLoadingSubcriberYieldIsLoading() {
+    struct Key: SharedReaderKey {
+      let id = UUID()
+      let testScheduler: TestSchedulerOf<DispatchQueue>
+      func load(context: LoadContext<Int>, continuation: LoadContinuation<Int>) {
+        continuation.resumeReturningInitialValue()
+      }
+      func subscribe(
+        context: LoadContext<Int>, subscriber: SharedSubscriber<Int>
+      ) -> SharedSubscription {
+        subscriber.yieldLoading()
+        testScheduler.schedule { subscriber.yieldLoading(false) }
+        return SharedSubscription {}
+      }
+    }
+    @SharedReader(Key(testScheduler: testScheduler)) var value = 0
+    #expect($value.isLoading == true)
+    testScheduler.advance()
+    #expect($value.isLoading == false)
+  }
 }
 
 private struct Key: SharedReaderKey {