Track VCs being popped from stack from UIKit

Tapping the back button or interactively popping via gesture can
conflict with observation when the view controller being popped to
executes a mutation in its `will`- or `didAppear`. While this isn't
common in vanilla UIKit, in TCA sending even a no-op action in these
lifecycle hooks can lead to immediately re-pushing the view controller
that was just popped.

This commit does what it can to detect when the back button was pushed
or the interactive pop gesture was invoked so that if/when the pop is
committed, we can update the path more eagerly, avoiding the
re-presentation.

Sources/SwiftNavigation/UINavigationPath.swift

@@ -8,7 +8,7 @@ public struct UINavigationPath: Equatable {
   public var elements: [Element] = []
 
   @_spi(Internals)
-  public enum Element: Equatable {
+  public enum Element: Hashable {
     case eager(AnyHashable)
     case lazy(Lazy)
 
@@ -52,6 +52,15 @@ public struct UINavigationPath: Equatable {
         return CodableRepresentation.Element(eager) == lazy
       }
     }
+
+    public func hash(into hasher: inout Hasher) {
+      switch self {
+      case let .eager(value), let .lazy(.element(value)):
+        hasher.combine(value)
+      case let .lazy(.codable(value)):
+        hasher.combine(value.decode())
+      }
+    }
   }
 
   /// The number of elements in this path.

Sources/UIKitNavigation/Navigation/NavigationStackController.swift

@@ -19,6 +19,7 @@
         }
       }
     }
+    private var didPop: Set<UINavigationPath.Element> = []
     private let pathDelegate = PathDelegate()
     private var root: UIViewController?
 
@@ -62,6 +63,10 @@
 
       super.delegate = pathDelegate
 
+      interactivePopGestureRecognizer?.addTarget(
+        self, action: #selector(interactivePopGestureRecognizerAction)
+      )
+
       if #available(iOS 17, macOS 14, tvOS 17, watchOS 10, *) {
         traitOverrides.push = UIPushAction { [weak self] value in
           self?._push(value: value)
@@ -154,6 +159,17 @@
       }
     }
 
+    @objc private func interactivePopGestureRecognizerAction(
+      _ gesture: UIScreenEdgePanGestureRecognizer
+    ) {
+      guard
+        gesture.state == .began,
+        let last = path.last,
+        !viewControllers.compactMap(\.navigationID).contains(last)
+      else { return }
+      didPop.insert(last)
+    }
+
     fileprivate func viewController(
       for navigationID: UINavigationPath.Element
     ) -> UIViewController? {
@@ -310,6 +326,29 @@
     }
   }
 
+  extension NavigationStackController: UINavigationBarDelegate {
+    public func navigationBar(
+      _ navigationBar: UINavigationBar, shouldPop item: UINavigationItem
+    ) -> Bool {
+      if let navigationID = viewControllers
+        .first(where: { $0.navigationItem == item })?
+        .navigationID
+      {
+        didPop.insert(navigationID)
+      }
+      return true
+    }
+
+    public func navigationBar(_ navigationBar: UINavigationBar, didPop item: UINavigationItem) {
+      path.removeAll(where: { didPop.contains($0) })
+      didPop.removeAll()
+    }
+
+    public func navigationBar(_ navigationBar: UINavigationBar, didPush item: UINavigationItem) {
+      didPop.removeAll()
+    }
+  }
+
   extension UIViewController {
     @available(iOS, deprecated: 17, renamed: "traitCollection.push")
     @available(macOS, deprecated: 14, renamed: "traitCollection.push")