From 288a0eac9fb76d466a79bda02651672eaf9ba5ef Mon Sep 17 00:00:00 2001
From: Thomas Ricouard <ricouard77@gmail.com>
Date: Sat, 4 Feb 2023 14:42:10 +0100
Subject: [PATCH] Save / Restore latest seen statuses

---
 .../Sources/Timeline/TimelineCache.swift      |  8 ++++++++
 .../Sources/Timeline/TimelineViewModel.swift  | 19 ++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/Packages/Timeline/Sources/Timeline/TimelineCache.swift b/Packages/Timeline/Sources/Timeline/TimelineCache.swift
index 7eeb7a4b..39a1a272 100644
--- a/Packages/Timeline/Sources/Timeline/TimelineCache.swift
+++ b/Packages/Timeline/Sources/Timeline/TimelineCache.swift
@@ -41,4 +41,12 @@ actor TimelineCache {
       return nil
     }
   }
+  
+  func setLatestSeenStatuses(ids: [String], for client: Client) {
+    UserDefaults.standard.set(ids, forKey: client.id)
+  }
+  
+  func getLatestSeenStatus(for client: Client) -> [String]? {
+    UserDefaults.standard.array(forKey: client.id) as? [String]
+  }
 }
diff --git a/Packages/Timeline/Sources/Timeline/TimelineViewModel.swift b/Packages/Timeline/Sources/Timeline/TimelineViewModel.swift
index 0119ac61..7cb53c74 100644
--- a/Packages/Timeline/Sources/Timeline/TimelineViewModel.swift
+++ b/Packages/Timeline/Sources/Timeline/TimelineViewModel.swift
@@ -17,6 +17,7 @@ class TimelineViewModel: ObservableObject {
   // Internal source of truth for a timeline.
   private var statuses: [Status] = []
   private var visibileStatusesIds = Set<String>()
+  
   var scrollToTopVisible: Bool = false {
     didSet {
       if scrollToTopVisible {
@@ -168,8 +169,18 @@ extension TimelineViewModel: StatusesFetcher {
         !cachedStatuses.isEmpty,
        timeline == .home {
       statuses = cachedStatuses
-      withAnimation {
+      if let latestSeenId = await cache.getLatestSeenStatus(for: client)?.last,
+         let index = statuses.firstIndex(where: { $0.id == latestSeenId }),
+         index > 0 {
+        // Restore cache and scroll to latest seen status.
         statusesState = .display(statuses: statuses, nextPageState: statuses.count < 20 ? .none : .hasNextPage)
+        scrollToIndexAnimated = false
+        scrollToIndex = index + 1
+      } else {
+        // Restore cache and scroll to top.
+        withAnimation {
+          statusesState = .display(statuses: statuses, nextPageState: statuses.count < 20 ? .none : .hasNextPage)
+        }
       }
       // And then we fetch statuses again toget newest statuses from there.
       await fetchStatuses()
@@ -311,6 +322,12 @@ extension TimelineViewModel: StatusesFetcher {
   func statusDidAppear(status: Status) {
     pendingStatusesObserver.removeStatus(status: status)
     visibileStatusesIds.insert(status.id)
+    
+    if let client, timeline == .home {
+      Task {
+        await cache.setLatestSeenStatuses(ids: visibileStatusesIds.map{ $0 }, for: client)
+      }
+    }
   }
 
   func statusDidDisappear(status: Status) {