Various performance tweaks (#474)

* Store and reuse room list placeholder avatars and last messages

* Cache and reuse HomeScreenRooms

* Reduce RoomSummaryProvider diff collection time

* Promote more logs to info

* Slighty tweak RustTracing to make it easier to configure

* Move TimelineProvider and RoomTimelineController item processing to background queues

* Prevent the timeline from stopping an ogoing decelerating scroll when starting backpaginating

ElementX/Sources/Screens/HomeScreen/HomeScreenModels.swift

@@ -95,6 +95,9 @@ struct HomeScreenViewStateBindings {
 }
 
 struct HomeScreenRoom: Identifiable, Equatable {
+    private static let placeholderLastMessage = AttributedString("Last message")
+    private static let placeholderAvatar = UIImage(systemName: "photo")
+    
     /// The list item identifier can be a real room identifier, a custom one for invalidated entries
     /// or a completely unique one for empty items and skeletons
     let id: String
@@ -102,11 +105,11 @@ struct HomeScreenRoom: Identifiable, Equatable {
     /// The real room identifier this item points to
     let roomId: String?
     
-    let name: String
+    var name = ""
     
-    let hasUnreads: Bool
+    var hasUnreads = false
     
-    let timestamp: String?
+    var timestamp: String?
     
     var lastMessage: AttributedString?
     
@@ -122,8 +125,8 @@ struct HomeScreenRoom: Identifiable, Equatable {
                        name: "Placeholder room name",
                        hasUnreads: false,
                        timestamp: "Now",
-                       lastMessage: AttributedString("Last message"),
-                       avatar: UIImage(systemName: "photo"),
+                       lastMessage: Self.placeholderLastMessage,
+                       avatar: Self.placeholderAvatar,
                        isPlaceholder: true)
     }
 }

ElementX/Sources/Screens/HomeScreen/HomeScreenViewModel.swift

@@ -31,6 +31,8 @@ class HomeScreenViewModel: HomeScreenViewModelType, HomeScreenViewModelProtocol
     
     private let visibleItemRangePublisher = CurrentValueSubject<Range<Int>, Never>(0..<0)
     
+    private var roomsForIdentifiers = [String: HomeScreenRoom]()
+    
     var callback: ((HomeScreenViewModelAction) -> Void)?
     
     // MARK: - Setup
@@ -178,7 +180,7 @@ class HomeScreenViewModel: HomeScreenViewModelType, HomeScreenViewModelProtocol
     // MARK: - Private
     
     private func loadDataForRoomIdentifier(_ identifier: String) {
-        guard let room = state.rooms.first(where: { $0.roomId == identifier }),
+        guard let room = roomsForIdentifiers[identifier],
               room.avatar == nil,
               let avatarURL = room.avatarURL else {
             return
@@ -206,6 +208,8 @@ class HomeScreenViewModel: HomeScreenViewModelType, HomeScreenViewModelProtocol
             return
         }
         
+        MXLog.info("Updating rooms")
+        
         var rooms = [HomeScreenRoom]()
         
         // Try merging together results from both the visibleRoomsSummaryProvider and the allRoomsSummaryProvider
@@ -235,24 +239,34 @@ class HomeScreenViewModel: HomeScreenViewModelType, HomeScreenViewModelProtocol
         }
         
         state.rooms = rooms
+        
+        MXLog.info("Finished updating rooms")
     }
     
     private func buildRoom(with details: RoomSummaryDetails) -> HomeScreenRoom {
-        let avatarImage = details.avatarURL.flatMap { userSession.mediaProvider.imageFromURL($0, avatarSize: .room(on: .home)) }
+        var room: HomeScreenRoom! = roomsForIdentifiers[details.id]
         
-        var timestamp: String?
-        if let lastMessageTimestamp = details.lastMessageTimestamp {
-            timestamp = lastMessageTimestamp.formatted(date: .omitted, time: .shortened)
+        if room == nil {
+            room = HomeScreenRoom(id: details.id,
+                                  roomId: details.id,
+                                  avatarURL: details.avatarURL)
         }
         
-        return HomeScreenRoom(id: details.id,
-                              roomId: details.id,
-                              name: details.name,
-                              hasUnreads: details.unreadNotificationCount > 0,
-                              timestamp: timestamp,
-                              lastMessage: details.lastMessage,
-                              avatarURL: details.avatarURL,
-                              avatar: avatarImage)
+        room.name = details.name
+        room.hasUnreads = details.unreadNotificationCount > 0
+        room.lastMessage = details.lastMessage
+        
+        if let avatarURL = details.avatarURL {
+            room.avatar = userSession.mediaProvider.imageFromURL(avatarURL, avatarSize: .room(on: .home))
+        }
+        
+        if let lastMessageTimestamp = details.lastMessageTimestamp {
+            room.timestamp = lastMessageTimestamp.formatted(date: .omitted, time: .shortened)
+        }
+        
+        roomsForIdentifiers[details.id] = room
+        
+        return room
     }
     
     private func updateVisibleRange(_ range: Range<Int>) {

ElementX/Sources/Screens/RoomScreen/View/TimelineTableViewController.swift

@@ -69,7 +69,6 @@ class TimelineTableViewController: UIViewController {
         didSet {
             // Paginate again if the threshold hasn't been satisfied.
             paginateBackwardsPublisher.send(())
-            applySnapshot()
         }
     }