[ios] Remove bookmark model from observer list.

iphone/Maps/Bookmarks/Categories/BMCView/BMCViewController.swift

@@ -42,8 +42,20 @@ final class BMCViewController: MWMViewController {
 
   override func viewDidAppear(_ animated: Bool) {
     super.viewDidAppear(animated)
+    // Disable all notifications in BM on appearance of this view.
+    // It allows to significantly improve performance in case of bookmarks
+    // modification. All notifications will be sent on controller's disappearance.
+    viewModel.setNotificationsEnabled(false)
+    viewModel.addToObserverList()
     viewModel.convertAllKMLIfNeeded()
   }
+  
+  override func viewDidDisappear(_ animated: Bool) {
+    super.viewDidDisappear(animated)
+    // Allow to send all notifications in BM.
+    viewModel.setNotificationsEnabled(true)
+    viewModel.removeFromObserverList()
+  }
 
   private func updateCategoryName(category: BMCCategory?) {
     let isNewCategory = (category == nil)

iphone/Maps/Bookmarks/Categories/BMCViewModel/BMCDefaultViewModel.swift

@@ -25,7 +25,6 @@ final class BMCDefaultViewModel: NSObject {
 
   override init() {
     super.init()
-    BM.add(self)
     loadData()
   }
 
@@ -207,6 +206,14 @@ extension BMCDefaultViewModel: BMCViewModel {
       }
     }
   }
+
+  func addToObserverList() {
+    BM.add(self)
+  }
+
+  func removeFromObserverList() {
+    BM.remove(self)
+  }
 }
 
 extension BMCDefaultViewModel: MWMBookmarksObserver {

iphone/Maps/Bookmarks/Categories/BMCViewModel/BMCViewModel.swift

@@ -42,4 +42,10 @@ protocol BMCViewModel: AnyObject {
   func grant(permission: BMCPermission?)
 
   func convertAllKMLIfNeeded();
+
+  func addToObserverList()
+  func removeFromObserverList()
+
+  func setNotificationsEnabled(_ enabled: Bool)
+  func areNotificationsEnabled() -> Bool
 }