From 7ac47b63e5e0e9b7838b1b5ec27bd075c01c114c Mon Sep 17 00:00:00 2001
From: Stephen Celis <stephen@stephencelis.com>
Date: Wed, 16 Oct 2024 10:09:09 -0700
Subject: [PATCH] Support eager navigation destination resolution

Currently we steer folks to using `viewDidLoad` for all observation, but
this laziness can lead to issues with deep-linking in type-erased
navigation stacks, where a destination may declare a sub-destination in
its `viewDidLoad`, causing the first drill-down to occur lazily over
multiple animated steps instead of a single one. This behavior can be
seen in the type-erased navigation case study in the repository.

This PR is a proof of concept to show that the library can populate a
"current navigation stack" in the UI transaction so that a destination's
initializer can declare its dependent destinations, fixing the case
study behavior.
---
 .../ErasedNavigationStackController.swift     | 16 +++++++--------
 .../NavigationStackController.swift           | 20 +++++++++++++++++--
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/Examples/CaseStudies/UIKit/ErasedNavigationStackController.swift b/Examples/CaseStudies/UIKit/ErasedNavigationStackController.swift
index c77aae3e3..e2eb3fb37 100644
--- a/Examples/CaseStudies/UIKit/ErasedNavigationStackController.swift
+++ b/Examples/CaseStudies/UIKit/ErasedNavigationStackController.swift
@@ -81,6 +81,10 @@ private class NumberFeatureViewController: UIViewController {
     self.number = number
     super.init(nibName: nil, bundle: nil)
     title = "Feature \(number)"
+
+    navigationDestination(for: String.self) { string in
+      StringFeatureViewController(string: string)
+    }
   }
   required init?(coder: NSCoder) {
     fatalError("init(coder:) has not been implemented")
@@ -89,10 +93,6 @@ private class NumberFeatureViewController: UIViewController {
     super.viewDidLoad()
     view.backgroundColor = .systemBackground
 
-    navigationDestination(for: String.self) { string in
-      StringFeatureViewController(string: string)
-    }
-
     let numberButton = UIButton(
       type: .system,
       primaryAction: UIAction { [weak self] _ in
@@ -137,6 +137,10 @@ private class StringFeatureViewController: UIViewController {
     self.string = string
     super.init(nibName: nil, bundle: nil)
     title = "Feature '\(string)'"
+
+    navigationDestination(for: Bool.self) { bool in
+      BoolFeatureViewController(bool: bool)
+    }
   }
   required init?(coder: NSCoder) {
     fatalError("init(coder:) has not been implemented")
@@ -145,10 +149,6 @@ private class StringFeatureViewController: UIViewController {
     super.viewDidLoad()
     view.backgroundColor = .systemBackground
 
-    navigationDestination(for: Bool.self) { bool in
-      BoolFeatureViewController(bool: bool)
-    }
-
     let numberButton = UIButton(
       type: .system,
       primaryAction: UIAction { [weak self] _ in
diff --git a/Sources/UIKitNavigation/Navigation/NavigationStackController.swift b/Sources/UIKitNavigation/Navigation/NavigationStackController.swift
index 54cc23f48..feda12692 100644
--- a/Sources/UIKitNavigation/Navigation/NavigationStackController.swift
+++ b/Sources/UIKitNavigation/Navigation/NavigationStackController.swift
@@ -338,14 +338,19 @@
         )
         return
       }
-      stackController.path.append(.lazy(.element(value)))
+      withUITransaction(\.stackController, stackController) {
+        stackController.path.append(.lazy(.element(value)))
+      }
     }
 
     public func navigationDestination<D: Hashable>(
       for data: D.Type,
       destination: @escaping (D) -> UIViewController
     ) {
-      guard let navigationController = navigationController ?? self as? UINavigationController
+      guard
+        let navigationController = UITransaction.current.stackController
+          ?? navigationController
+          ?? self as? UINavigationController
       else {
         reportIssue(
           """
@@ -442,4 +447,15 @@
       }
     }
   }
+
+  private extension UITransaction {
+    var stackController: NavigationStackController? {
+      get { self[NavigationStackControllerKey.self] }
+      set { self[NavigationStackControllerKey.self] = newValue }
+    }
+  }
+
+  private enum NavigationStackControllerKey: UITransactionKey {
+    static let defaultValue: NavigationStackController? = nil
+  }
 #endif