Implement caching for thread data sources

Riot/Modules/Room/DataSources/ThreadDataSource.swift

@@ -15,15 +15,31 @@
 //
 
 import Foundation
+import UIKit
 
 @objcMembers
 public class ThreadDataSource: RoomDataSource {
+
+    private typealias ThreadID = String
+
+    /// Map of cached data sources. Keys are thread identifiers.
+    private static var dataSourceCache: [ThreadID: ThreadDataSource] = [:]
     
     public override func finalizeInitialization() {
         super.finalizeInitialization()
         showReadMarker = false
         showBubbleReceipts = false
         showTypingRow = false
+
+        NotificationCenter.default.addObserver(self,
+                                               selector: #selector(didReceiveMemoryWarning),
+                                               name: UIApplication.didReceiveMemoryWarningNotification,
+                                               object: nil)
+
+        NotificationCenter.default.addObserver(self,
+                                               selector: #selector(didLeaveRoom(_:)),
+                                               name: .mxSessionDidLeaveRoom,
+                                               object: nil)
     }
     
     public override var showReadMarker: Bool {
@@ -41,5 +57,47 @@ public class ThreadDataSource: RoomDataSource {
             _ = newValue
         }
     }
+
+    public override class func load(withRoomId roomId: String!,
+                                    initialEventId: String!,
+                                    threadId: String!,
+                                    andMatrixSession mxSession: MXSession!,
+                                    onComplete: ((Any?) -> Void)!) {
+        if let dataSource = dataSourceCache[threadId] {
+            onComplete(dataSource)
+            return
+        }
+        super.load(withRoomId: roomId, initialEventId: initialEventId, threadId: threadId, andMatrixSession: mxSession) { dataSource in
+            if let dataSource = dataSource as? ThreadDataSource {
+                Self.dataSourceCache[threadId] = dataSource
+            }
+            onComplete(dataSource)
+        }
+    }
+
+    @objc
+    private func didReceiveMemoryWarning() {
+        MXLog.debug("[ThreadDataSource] didReceiveMemoryWarning. Will reload not active data sources in cache.")
+
+        Self.dataSourceCache.forEach {
+            if $1.delegate == nil {
+                $1.reload()
+            }
+        }
+    }
+
+    @objc
+    private func didLeaveRoom(_ notification: Notification) {
+        MXLog.debug("[ThreadDataSource] didReceiveMemoryWarning. Will reload not active data sources in cache.")
+
+        guard let session = notification.object as? MXSession,
+              session == mxSession,
+              let roomId = notification.userInfo?[kMXSessionNotificationRoomIdKey] as? String else {
+            return
+        }
+
+        let threadIds = Array(Self.dataSourceCache.filter { $1.roomId == roomId }.keys)
+        threadIds.forEach { Self.dataSourceCache.removeValue(forKey: $0) }
+    }
     
 }