From c589c7405278452d8c74747223248a93112f3e00 Mon Sep 17 00:00:00 2001
From: Justin Mazzocchi <2831158+jzzocc@users.noreply.github.com>
Date: Tue, 6 Oct 2020 16:12:11 -0700
Subject: [PATCH] Scroll position maintenance

---
 View Controllers/TableViewController.swift       |  6 ++++++
 .../ViewModels/CollectionItemsViewModel.swift    | 16 ++++++++++++++++
 .../Sources/ViewModels/CollectionViewModel.swift |  1 +
 .../Sources/ViewModels/ProfileViewModel.swift    |  4 ++++
 4 files changed, 27 insertions(+)

diff --git a/View Controllers/TableViewController.swift b/View Controllers/TableViewController.swift
index 879a867..ab8ae3e 100644
--- a/View Controllers/TableViewController.swift	
+++ b/View Controllers/TableViewController.swift	
@@ -210,6 +210,12 @@ private extension TableViewController {
             self.sizeTableHeaderFooterViews()
         }
         .store(in: &cancellables)
+
+        tableView.publisher(for: \.contentOffset)
+            .compactMap { [weak self] _ in self?.tableView.indexPathsForVisibleRows?.first }
+            .removeDuplicates()
+            .sink { [weak self] in self?.viewModel.viewedAtTop(indexPath: $0) }
+            .store(in: &cancellables)
     }
 
     func update(items: [[CollectionItemIdentifier]]) {
diff --git a/ViewModels/Sources/ViewModels/CollectionItemsViewModel.swift b/ViewModels/Sources/ViewModels/CollectionItemsViewModel.swift
index 6362c99..6aeed0e 100644
--- a/ViewModels/Sources/ViewModels/CollectionItemsViewModel.swift
+++ b/ViewModels/Sources/ViewModels/CollectionItemsViewModel.swift
@@ -15,6 +15,7 @@ final public class CollectionItemsViewModel: ObservableObject {
     private var viewModelCache = [CollectionItem: (CollectionItemViewModel, AnyCancellable)]()
     private let navigationEventsSubject = PassthroughSubject<NavigationEvent, Never>()
     private let loadingSubject = PassthroughSubject<Bool, Never>()
+    private var topVisibleIndexPath = IndexPath(item: 0, section: 0)
     private var lastSelectedLoadMore: LoadMore?
     private var cancellables = Set<AnyCancellable>()
 
@@ -80,6 +81,10 @@ extension CollectionItemsViewModel: CollectionViewModel {
         }
     }
 
+    public func viewedAtTop(indexPath: IndexPath) {
+        topVisibleIndexPath = indexPath
+    }
+
     public func canSelect(indexPath: IndexPath) -> Bool {
         switch items.value[indexPath.section][indexPath.item] {
         case let .status(_, configuration):
@@ -180,6 +185,17 @@ private extension CollectionItemsViewModel {
                     }
                 }
             }
+
+            if items.value.count > topVisibleIndexPath.section,
+               items.value[topVisibleIndexPath.section].count > topVisibleIndexPath.item {
+                let topVisibleItem = items.value[topVisibleIndexPath.section][topVisibleIndexPath.item]
+
+                if newItems.count > topVisibleIndexPath.section,
+                   let newIndex = newItems[topVisibleIndexPath.section].firstIndex(of: topVisibleItem),
+                   newIndex > topVisibleIndexPath.item {
+                    return .init(item: topVisibleItem)
+                }
+            }
         }
 
         return nil
diff --git a/ViewModels/Sources/ViewModels/CollectionViewModel.swift b/ViewModels/Sources/ViewModels/CollectionViewModel.swift
index bd8b7c4..5af1b69 100644
--- a/ViewModels/Sources/ViewModels/CollectionViewModel.swift
+++ b/ViewModels/Sources/ViewModels/CollectionViewModel.swift
@@ -12,6 +12,7 @@ public protocol CollectionViewModel {
     var nextPageMaxId: String? { get }
     var maintainScrollPositionOfItem: CollectionItemIdentifier? { get }
     func request(maxId: String?, minId: String?)
+    func viewedAtTop(indexPath: IndexPath)
     func select(indexPath: IndexPath)
     func canSelect(indexPath: IndexPath) -> Bool
     func viewModel(indexPath: IndexPath) -> CollectionItemViewModel
diff --git a/ViewModels/Sources/ViewModels/ProfileViewModel.swift b/ViewModels/Sources/ViewModels/ProfileViewModel.swift
index cf9e464..ac7a5d4 100644
--- a/ViewModels/Sources/ViewModels/ProfileViewModel.swift
+++ b/ViewModels/Sources/ViewModels/ProfileViewModel.swift
@@ -85,6 +85,10 @@ extension ProfileViewModel: CollectionViewModel {
         collectionViewModel.value.request(maxId: maxId, minId: minId)
     }
 
+    public func viewedAtTop(indexPath: IndexPath) {
+        collectionViewModel.value.viewedAtTop(indexPath: indexPath)
+    }
+
     public func select(indexPath: IndexPath) {
         collectionViewModel.value.select(indexPath: indexPath)
     }