refactor and cleanup [skip ci]

submodules/Postbox/Sources/UnsentMessageHistoryView.swift

@@ -0,0 +1,39 @@
+import Foundation
+
+final class UnsentMessageHistoryView {
+    var ids: Set<MessageId>
+    
+    init(ids: [MessageId]) {
+        self.ids = Set(ids)
+    }
+    
+    func refreshDueToExternalTransaction(fetchUnsentMessageIds: () -> [MessageId]) -> Bool {
+        let ids = Set(fetchUnsentMessageIds())
+        if ids != self.ids {
+            self.ids = ids
+            return true
+        } else {
+            return false
+        }
+    }
+    
+    func replay(_ operations: [IntermediateMessageHistoryUnsentOperation]) -> Bool {
+        var updated = false
+        for operation in operations {
+            switch operation {
+                case let .Insert(id):
+                    if !self.ids.contains(id) {
+                        self.ids.insert(id)
+                        updated = true
+                    }
+                case let .Remove(id):
+                    if self.ids.contains(id) {
+                        self.ids.remove(id)
+                        updated = true
+                    }
+            }
+        }
+        
+        return updated
+    }
+}