video: stop video in post when it disappears from view

Changelog-Fixes: Stop video when it disappears from view

damus/Views/Video/DamusVideoPlayer.swift

@@ -43,12 +43,14 @@ struct DamusVideoPlayer: View {
     }
     
     var body: some View {
+        GeometryReader { geo in
+            let localFrame = geo.frame(in: .local)
+            let localCenter = CGPoint(x: localFrame.midX, y: localFrame.midY)
+            let globalCenter = geo.frame(in: .global).origin.applying(.init(translationX: localCenter.x, y: localCenter.y))
+            let centerY = globalCenter.y
+
             ZStack(alignment: .bottomTrailing) {
                 VideoPlayer(url: url, model: model)
-                .onAppear {
-                    model.start()
-                }
-            
                 if model.has_audio == true {
                     MuteIcon
                         .zIndex(11.0)
@@ -63,6 +65,13 @@ struct DamusVideoPlayer: View {
                 }
                 video_size = size
             }
+            .onChange(of: centerY) { _ in
+                let screenHeight = UIScreen.main.bounds.height
+                let screenMidY = screenHeight / 2
+                let tol = 0.20 * screenHeight /// tolerance - can vary  to taste ie.,  %  of screen height of a centered box in which video plays
+                model.play = centerY > screenMidY - tol && centerY < screenMidY + tol /// video plays when inside tolerance box
+            }
+        }
     }
 }
 struct DamusVideoPlayer_Previews: PreviewProvider {