Add FramePreferenceKey for use in ViewFrameReader.

RiotSwiftUI/Modules/Common/ViewFrameReader/FramePreferenceKey.swift

@@ -0,0 +1,27 @@
+// 
+// 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 SwiftUI
+
+/// A SwiftUI `PreferenceKey` for `CGRect` values such as a view's frame.
+@available(iOS 14.0, *)
+struct FramePreferenceKey: PreferenceKey {
+    static var defaultValue: CGRect = .zero
+    
+    static func reduce(value: inout CGRect, nextValue: () -> CGRect) {
+        value = nextValue()
+    }
+}

RiotSwiftUI/Modules/Common/ViewFrameReader/ViewFrameReader.swift

@@ -31,11 +31,13 @@ struct ViewFrameReader: View {
     @Binding var frame: CGRect
     
     var body: some View {
-        GeometryReader { geo -> Color in
-            DispatchQueue.main.async {
-                frame = geo.frame(in: .local)
-            }
-            return .clear
+        GeometryReader { geometry in
+            Color.clear
+                .preference(key: FramePreferenceKey.self,
+                            value: geometry.frame(in: .local))
+        }
+        .onPreferenceChange(FramePreferenceKey.self) {
+            frame = $0
         }
     }
 }