Implement home screen activity indicators

Riot/Modules/Common/ActivityIndicator/AppActivityIndicatorPresenter.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 UIKit
+import MatrixSDK
+import CommonKit
+
+/// Presenter which displays activity / loading indicators using app-wide `AppNavigator`, thus displaying them in a unified way,
+/// and `ActivityCenter`/`Activity`, which ensures that only one activity is shown at a given time.
+///
+/// Note: clients can skip using `AppActivityIndicatorPresenter` and instead coordiinate with `AppNavigatorProtocol` directly.
+/// The presenter exists mostly as a transition for view controllers already using `ActivityIndicatorPresenterType` and / or view controllers
+/// written in objective-c.
+@objc final class AppActivityIndicatorPresenter: NSObject, ActivityIndicatorPresenterType {
+    private let appNavigator: AppNavigatorProtocol
+    private var activity: Activity?
+    
+    init(appNavigator: AppNavigatorProtocol) {
+        self.appNavigator = appNavigator
+    }
+
+    @objc func presentActivityIndicator() {
+        activity = appNavigator.addLoadingActivity()
+    }
+    
+    @objc func removeCurrentActivityIndicator(animated: Bool, completion: (() -> Void)?) {
+        activity = nil
+    }
+    
+    func presentActivityIndicator(on view: UIView, animated: Bool, completion: (() -> Void)?) {
+        MXLog.error("[AppActivityIndicatorPresenter] Shared activity indicator does not support presenting from custom views")
+    }
+}