Expose observe overloads for separate tracking and application of changes

Sources/SwiftNavigation/NSObject+Observe.swift

@@ -110,6 +110,52 @@
       observe { _ in apply() }
     }
 
+    /// Observe access to properties of an observable (or perceptible) object.
+    ///
+    /// This tool allows you to set up an observation loop so that you can access fields from an
+    /// observable model in order to populate your view, and also automatically track changes to
+    /// any fields accessed in the tracking parameter so that the view is always up-to-date.
+    ///
+    /// - Parameter tracking: A closure that contains properties to track
+    /// - Parameter onChange: Invoked when the value of a property changes
+    /// - Returns: A cancellation token.
+    @discardableResult
+    public func observe(
+      _ tracking: @escaping @MainActor @Sendable () -> Void,
+      onChange apply: @escaping @MainActor @Sendable () -> Void
+    ) -> ObserveToken {
+      observe { _ in apply() }
+    }
+
+    /// Observe access to properties of an observable (or perceptible) object.
+    ///
+    /// A version of ``observe(_:)`` that is passed the current transaction.
+    ///
+    /// - Parameter tracking: A closure that contains properties to track
+    /// - Parameter onChange: Invoked when the value of a property changes
+    /// - Returns: A cancellation token.
+    @discardableResult
+    public func observe(
+      _ tracking: @escaping @MainActor @Sendable (_ transaction: UITransaction) -> Void,
+      onChange apply: @escaping @MainActor @Sendable (_ transaction: UITransaction) -> Void
+    ) -> ObserveToken {
+      let token = SwiftNavigation.observe { transaction in
+        MainActor._assumeIsolated {
+          tracking(transaction)
+        }
+      } onChange: { transaction in
+        MainActor._assumeIsolated {
+          apply(transaction)
+        }
+      } task: { transaction, work in
+        DispatchQueue.main.async {
+          withUITransaction(transaction, work)
+        }
+      }
+      tokens.append(token)
+      return token
+    }
+
     /// Observe access to properties of an observable (or perceptible) object.
     ///
     /// A version of ``observe(_:)`` that is passed the current transaction.

Sources/SwiftNavigation/Observe.swift

@@ -61,6 +61,72 @@ import ConcurrencyExtras
     observe { _ in Result(catching: apply).get() }
   }
 
+  /// Tracks access to properties of an observable model.
+  ///
+  /// This function allows one to minimally observe changes in a model in order to
+  /// react to those changes. For example, if you had an observable model like so:
+  ///
+  /// ```swift
+  /// @Observable
+  /// class FeatureModel {
+  ///   var count = 0
+  /// }
+  /// ```
+  ///
+  /// Then you can use `observe` to observe changes in the model. For example, in UIKit you can
+  /// update a `UILabel`:
+  ///
+  /// ```swift
+  /// observe { _ = model.value } onChange: { [weak self] in
+  ///   guard let self else { return }
+  ///   countLabel.text = "Count: \(model.count)"
+  /// }
+  /// ```
+  ///
+  /// Anytime the `count` property of the model changes the trailing closure will be invoked again,
+  /// allowing you to update the view. Further, only changes to properties accessed in the trailing
+  /// closure will be observed.
+  ///
+  /// > Note: If you are targeting Apple's older platforms (anything before iOS 17, macOS 14,
+  /// > tvOS 17, watchOS 10), then you can use our
+  /// > [Perception](http://github.com/pointfreeco/swift-perception) library to replace Swift's
+  /// > Observation framework.
+  ///
+  /// This function also works on non-Apple platforms, such as Windows, Linux, Wasm, and more. For
+  /// example, in a Wasm app you could observe changes to the `count` property to update the inner
+  /// HTML of a tag:
+  ///
+  /// ```swift
+  /// import JavaScriptKit
+  ///
+  /// var countLabel = document.createElement("span")
+  /// _ = document.body.appendChild(countLabel)
+  ///
+  /// let token = observe { _ = model.count } onChange: {
+  ///   countLabel.innerText = .string("Count: \(model.count)")
+  /// }
+  /// ```
+  ///
+  /// And you can also build your own tools on top of `observe`.
+  ///
+  /// - Parameters:
+  ///   - isolation: The isolation of the observation.
+  ///   - tracking: A closure that contains properties to track.
+  ///   - onChange: A closure that is triggered after some tracked property has changed
+  /// - Returns: A token that keeps the subscription alive. Observation is cancelled when the token
+  ///   is deallocated.
+  public func observe(
+    isolation: (any Actor)? = #isolation,
+    @_inheritActorContext _ tracking: @escaping @Sendable () -> Void,
+    @_inheritActorContext onChange apply: @escaping @Sendable () -> Void
+  ) -> ObserveToken {
+    observe(
+      isolation: isolation,
+      { _ in tracking() },
+      onChange: { _ in apply() }
+    )
+  }
+
   /// Tracks access to properties of an observable model.
   ///
   /// A version of ``observe(isolation:_:)`` that is handed the current ``UITransaction``.
@@ -83,12 +149,43 @@ import ConcurrencyExtras
       }
     )
   }
+
+
+/// Tracks access to properties of an observable model.
+///
+/// A version of ``observe(isolation:_:)`` that is handed the current ``UITransaction``.
+///
+/// - Parameters:
+///   - isolation: The isolation of the observation.
+///   - tracking: A closure that contains properties to track.
+///   - onChange: A closure that is triggered after some tracked property has changed
+/// - Returns: A token that keeps the subscription alive. Observation is cancelled when the token
+///   is deallocated.
+  public func observe(
+    isolation: (any Actor)? = #isolation,
+    @_inheritActorContext _ tracking: @escaping @Sendable (UITransaction) -> Void,
+    @_inheritActorContext onChange apply: @escaping @Sendable (_ transaction: UITransaction) -> Void
+  ) -> ObserveToken {
+    let actor = ActorProxy(base: isolation)
+    return observe(
+      tracking,
+      onChange: apply,
+      task: { transaction, operation in
+        Task {
+          await actor.perform {
+            operation()
+          }
+        }
+      }
+    )
+  }
 #endif
 
 func _observe(
   _ apply: @escaping @Sendable (_ transaction: UITransaction) -> Void,
   task: @escaping @Sendable (
-    _ transaction: UITransaction, _ operation: @escaping @Sendable () -> Void
+    _ transaction: UITransaction,
+    _ operation: @escaping @Sendable () -> Void
   ) -> Void = {
     Task(operation: $1)
   }
@@ -121,6 +218,48 @@ func _observe(
   return token
 }
 
+func observe(
+  _ tracking: @escaping @Sendable (_ transaction: UITransaction) -> Void,
+  onChange apply: @escaping @Sendable (_ transaction: UITransaction) -> Void,
+  task: @escaping @Sendable (
+    _ transaction: UITransaction,
+    _ operation: @escaping @Sendable () -> Void
+  ) -> Void = {
+    Task(operation: $1)
+  }
+) -> ObserveToken {
+  let token = ObserveToken()
+  SwiftNavigation.onChange(
+    of: { [weak token] transaction in
+      guard let token, !token.isCancelled else { return }
+      tracking(transaction)
+    },
+    perform: { [weak token] transaction in
+      guard
+        let token,
+        !token.isCancelled
+      else { return }
+
+      var perform: @Sendable () -> Void = { apply(transaction) }
+      for key in transaction.storage.keys {
+        guard let keyType = key.keyType as? any _UICustomTransactionKey.Type
+        else { continue }
+        func open<K: _UICustomTransactionKey>(_: K.Type) {
+          perform = { [perform] in
+            K.perform(value: transaction[K.self]) {
+              perform()
+            }
+          }
+        }
+        open(keyType)
+      }
+      perform()
+    },
+    task: task
+  )
+  return token
+}
+
 private func onChange(
   _ apply: @escaping @Sendable (_ transaction: UITransaction) -> Void,
   task: @escaping @Sendable (
@@ -136,6 +275,29 @@ private func onChange(
   }
 }
 
+private func onChange(
+  of tracking: @escaping @Sendable (_ transaction: UITransaction) -> Void,
+  perform operation: @escaping @Sendable (_ transaction: UITransaction) -> Void,
+  task: @escaping @Sendable (
+    _ transaction: UITransaction,
+    _ operation: @escaping @Sendable () -> Void
+  ) -> Void
+) {
+  operation(.current)
+
+  withPerceptionTracking {
+    tracking(.current)
+  } onChange: {
+    task(.current) {
+      onChange(
+        of: tracking,
+        perform: operation,
+        task: task
+      )
+    }
+  }
+}
+
 /// A token for cancelling observation.
 ///
 /// When this token is deallocated it cancels the observation it was associated with. Store this

Tests/SwiftNavigationTests/ObserveTests+Nesting.swift

@@ -0,0 +1,132 @@
+import SwiftNavigation
+import Perception
+import XCTest
+
+class NestingObserveTests: XCTestCase {
+  #if swift(>=6)
+    func testIsolation() async {
+      await MainActor.run {
+        var count = 0
+        let token = SwiftNavigation.observe {
+          count = 1
+        }
+        XCTAssertEqual(count, 1)
+        _ = token
+      }
+    }
+  #endif
+
+  #if !os(WASI)
+    @MainActor
+    func testNestedObservation() async {
+      let object = ParentObject()
+      let model = ParentObject.Model()
+
+      MockTracker.shared.entries.removeAll()
+      object.bind(model)
+
+      XCTAssertEqual(
+        MockTracker.shared.entries.map(\.label),
+        [
+          "ParentObject.bind",
+          "ParentObject.value.didSet",
+          "ChildObject.bind",
+          "ChildObject.value.didSet",
+        ]
+      )
+
+      MockTracker.shared.entries.removeAll()
+      model.child.value = 1
+
+      await Task.yield()
+
+      XCTAssertEqual(
+        MockTracker.shared.entries.map(\.label),
+        [
+          "ChildObject.Model.value.didSet",
+          "ChildObject.value.didSet",
+        ]
+      )
+    }
+  #endif
+}
+
+#if !os(WASI)
+  fileprivate class ParentObject: @unchecked Sendable {
+    var tokens: Set<ObserveToken> = []
+    let child: ChildObject = .init()
+
+    var value: Int = 0 {
+      didSet { MockTracker.shared.track(value, with: "ParentObject.value.didSet") }
+    }
+
+    func bind(_ model: Model) {
+      MockTracker.shared.track((), with: "ParentObject.bind")
+
+      tokens = [
+        observe { _ = model.value } onChange: { [weak self] in
+          self?.value = model.value
+        },
+        observe { _ = model.child } onChange: { [weak self] in
+          self?.child.bind(model.child)
+        }
+      ]
+    }
+
+    @Perceptible
+    class Model: @unchecked Sendable {
+      var value: Int = 0 {
+        didSet { MockTracker.shared.track(value, with: "ParentObject.Model.value.didSet") }
+      }
+
+      var child: ChildObject.Model = .init() {
+        didSet { MockTracker.shared.track(value, with: "ParentObject.Model.value.didSet") }
+      }
+    }
+  }
+
+  fileprivate class ChildObject: @unchecked Sendable {
+    var tokens: Set<ObserveToken> = []
+
+    var value: Int = 0 {
+      didSet { MockTracker.shared.track(value, with: "ChildObject.value.didSet") }
+    }
+
+    func bind(_ model: Model) {
+      MockTracker.shared.track((), with: "ChildObject.bind")
+
+      tokens = [
+        observe { _ = model.value } onChange: { [weak self] in
+          self?.value = model.value
+        }
+      ]
+    }
+
+    @Perceptible
+    class Model: @unchecked Sendable {
+      var value: Int = 0 {
+        didSet { MockTracker.shared.track(value, with: "ChildObject.Model.value.didSet") }
+      }
+    }
+  }
+
+  fileprivate final class MockTracker: @unchecked Sendable {
+    static let shared = MockTracker()
+
+    struct Entry {
+      var label: String
+      var value: Any
+    }
+
+    var entries: [Entry] = []
+
+    init() {}
+
+    func track(
+      _ value: Any,
+      with label: String
+    ) {
+      entries.append(.init(label: label, value: value))
+    }
+  }
+#endif