Extract and start to split theme.

Riot/ModulesSwiftUI/Common/Theme/ThemeObserver.swift

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+// 
+// Copyright 2021 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
+import Combine
+
+@available(iOS 14.0, *)
+class ThemeObserver: ObservableObject {
+    
+    static let shared = ThemeObserver()
+    
+    init() {
+        NotificationCenter.default.publisher(for: NSNotification.Name.themeServiceDidChangeTheme).map { _ in
+            ThemeService.shared().theme
+        }.assign(to: &$theme)
+    }
+    @Published var theme: Theme = ThemeService.shared().theme
+}
+
+
+class ThemePublisher: ObservableObject {
+    static let shared = ThemePublisher()
+}