Remove fromViewController, it's irrelevant for what the code proclaim…

… to do:

handlePopBack (by Customer through backButtonItem) to some UIVC instance and giving a chance to the subclass to do something about it.

Added lots of comments and split the file into groups using MARK-ing

Coordinator/NavigationCoordinator.swift

@@ -12,16 +12,25 @@ open class NavigationCoordinator: Coordinator<UINavigationController>, UINavigat
 	//	References to actual UIViewControllers managed by this Coordinator instance.
 	open var viewControllers: [UIViewController] = []
 
-	///	This method is implemented to detect when "pop" happens.
-	///	`popViewController` must be detected in order to remove popped VC from Coordinator's `viewControllers` array.
+	///	This method is implemented to detect when customer "pop" back using UINC's backButtonItem.
+	///	(Need to detect that in order to remove popped VC from Coordinator's `viewControllers` array.)
 	///
 	///	It is strongly advised to *not* override this method, but it's allowed to do so in case you really need to.
 	///	What you likely want to override is `handlePopBack(to:)` method.
 	open func navigationController(_ navigationController: UINavigationController, didShow viewController: UIViewController, animated: Bool) {
-		let fromViewController = navigationController.transitionCoordinator?.viewController(forKey: .from)
-		self.didShowController(viewController, fromViewController: fromViewController)
+		self.didShowController(viewController)
 	}
 
+	///	If you subclass NavigationCoordinator, then override this method if you need to
+	///	do something special when customer taps the UIKit's backButton in the navigationBar.
+	///
+	///	By default, this does nothing.
+	open func handlePopBack(to vc: UIViewController?) {
+	}
+
+
+	//	MARK:- Presenting
+
 	public func present(_ vc: UIViewController) {
 		rootViewController.present(vc, animated: true, completion: nil)
 	}
@@ -30,6 +39,9 @@ open class NavigationCoordinator: Coordinator<UINavigationController>, UINavigat
 		rootViewController.dismiss(animated: true, completion: nil)
 	}
 
+
+	//	MARK:- Navigating
+
 	///	Main method to push supplied UIVC to the navigation stack.
 	///	First it adds the `vc` to the Coordinator's `viewControllers` then calls `show(vc)` on the root.
 	public func show(_ vc: UIViewController) {
@@ -68,12 +80,8 @@ open class NavigationCoordinator: Coordinator<UINavigationController>, UINavigat
 		rootViewController.popToViewController(vc, animated: animated)
 	}
 
-	///	If you subclass NavigationCoordinator, then override this method if you need to
-	///	do something special when customer taps the UIKit's backButton in the navigationBar.
-	///
-	///	By default, this does nothing.
-	open func handlePopBack(to vc: UIViewController?) {
-	}
+
+	//	MARK:- Coordinator lifecycle
 
 	open override func start(with completion: @escaping () -> Void) {
 		//	assign itself as UINavigationControllerDelegate
@@ -108,33 +116,47 @@ open class NavigationCoordinator: Coordinator<UINavigationController>, UINavigat
 }
 
 private extension NavigationCoordinator {
-	func didShowController(_ viewController: UIViewController, fromViewController: UIViewController?) {
-        if let fromViewController = fromViewController {
-            guard viewControllers.contains(fromViewController) else { return }
-            guard let last = viewControllers.last, last === fromViewController else { return }
-
-            if let index = viewControllers.firstIndex(of: viewController) {
-                let lastPosition = viewControllers.count - 1
-                viewControllers = Array(viewControllers.dropLast(lastPosition - index))
-                handlePopBack(to: viewController)
-            } else {
-                viewControllers.removeLast()
-                handlePopBack(to: last)
-            }
-        } else {
-            guard viewController !== viewControllers.last else { return }
-            guard let index = viewControllers.firstIndex(of: viewController) else { return }
-
-            let lastPosition = viewControllers.count - 1
-            viewControllers = Array(viewControllers.dropLast(lastPosition - index))
-            handlePopBack(to: viewController)
-        }
+	func didShowController(_ viewController: UIViewController) {
+		//	Note: various sanity checks are done below, explained in comments.
+		//	You would want to add some log calls for each one
+		//	(or at least add a breakpoint so you know when it happens)
+		//	since those checks point to logical errors in the app's architecture flows.
+
+
+		//	If VC, which was just shown, is the last in this Coordinator's stack,
+		//	then just bail out, because popped VC was not in this Coordinator's domain.
+		//		| If this actually happens, it likely points to a mistake somewhere else.
+		//		| (It means we had some `show(vc)` happen that _did not_ update this Coordinator's viewControllers,
+		//		|  nor it switched to some other Coordinator which should have become UINC.delegate)
+		if viewController === viewControllers.last {
+			return
+		}
+
+		//	Check: just shown VC should be present in Coordinator's viewControllers sequence.
+		//	If it's not there, then bail out.
+		//		| Again, this should not happen,
+		//		| since this Coordinator should then not be UINCDelegate.
+		guard let index = viewControllers.firstIndex(of: viewController) else {
+			return
+		}
+
+		//	Note: using firstIndex(of:) and not .last nicely
+		//	handles if you programatically pop more than one UIVC.
+
+
+		let lastIndex = viewControllers.count - 1
+		if lastIndex <= index {
+			return
+		}
+		viewControllers = Array(viewControllers.dropLast(lastIndex - index))
+		handlePopBack(to: viewController)
+
+
 
 		//	is there any controller left shown in this Coordinator?
 		if viewControllers.count == 0 {
 			//	inform the parent Coordinator that this child Coordinator has no more VCs
 			parent?.coordinatorDidFinish(self, completion: {})
-			return
 		}
 	}
 }