Add CommonKit and Activity

Signed-off-by: Andy Uhnak <andyuhnak@gmail.com>

CommonKit/Source/Activity/ActivityCenter.swift

@@ -0,0 +1,60 @@
+// 
+// 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
+
+/// A shared activity center with a single FIFO queue which will ensure only one activity is shown at a given time.
+///
+/// `ActivityCenter` offers a `shared` center that can be used by any clients, but clients are also allowed
+/// to create local `ActivityCenter` if the context requres multiple simultaneous activities.
+public class ActivityCenter {
+    private class Weak<T: AnyObject> {
+        weak var element: T?
+        init(_ element: T) {
+            self.element = element
+        }
+    }
+    
+    public static let shared = ActivityCenter()
+    private var queue = [Weak<Activity>]()
+    
+    /// Add a new activity to the queue by providing a request.
+    ///
+    /// The queue will start the activity right away, if there are no currently running activities,
+    /// otherwise the activity will be put on hold.
+    public func add(_ request: ActivityRequest) -> Activity {
+        let activity = Activity(request: request) { [weak self] in
+            self?.startNextIfIdle()
+        }
+        
+        queue.append(Weak(activity))
+        startNextIfIdle()
+        return activity
+    }
+    
+    private func startNextIfIdle() {
+        cleanup()
+        if let activity = queue.first?.element, activity.state == .pending {
+            activity.start()
+        }
+    }
+    
+    private func cleanup() {
+        queue.removeAll {
+            $0.element == nil || $0.element?.state == .completed
+        }
+    }
+}