Cross-signing: Create key verification setup banner.

Riot/Modules/KeyVerification/Banners/KeyVerificationBannerPreferences.swift

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+/*
+ Copyright 2020 New Vector Ltd
+ 
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+ 
+ http://www.apache.org/licenses/LICENSE-2.0
+ 
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
+
+import Foundation
+
+/// Key verification banner user preferences.
+@objcMembers
+final class KeyVerificationBannerPreferences: NSObject {
+    
+    // MARK: - Constants
+    
+    private enum UserDefaultsKeys {
+        static let hideSetupBanner = "KeyVerificationBannerPreferencesHideSetupBanner"
+    }
+    
+    static let shared = KeyVerificationBannerPreferences()
+    
+    // MARK: - Properties
+    
+    // MARK: - Public
+    
+    /// Remember to hide key backup setup banner.
+    var hideSetupBanner: Bool {
+        get {
+            return UserDefaults.standard.bool(forKey: UserDefaultsKeys.hideSetupBanner)
+        }
+        set {
+            UserDefaults.standard.set(newValue, forKey: UserDefaultsKeys.hideSetupBanner)
+        }
+    }
+    
+    /// Reset key backup banner preferences to default values
+    func reset() {
+        self.hideSetupBanner = false
+    }
+}