videos: improve precision & sensitivity of auto-pause mechanism

Closes: https://github.com/damus-io/damus/pull/1308
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/Video/DamusVideoPlayer.swift

@@ -7,10 +7,20 @@
 
 import SwiftUI
 
+/// get coordinates in Global reference frame given a Local point & geometry
+func globalCoordinate(localX x: CGFloat, localY y: CGFloat,
+                      localGeometry geo: GeometryProxy) -> CGPoint {
+    let localPoint = CGPoint(x: x, y: y)
+    return geo.frame(in: .global).origin.applying(
+        .init(translationX: localPoint.x, y: localPoint.y)
+    )
+}
+
 struct DamusVideoPlayer: View {
     var url: URL
     @ObservedObject var model: VideoPlayerModel
     @Binding var video_size: CGSize?
+    @EnvironmentObject private var orientationTracker: OrientationTracker
     
     var mute_icon: String {
         if model.has_audio == false || model.muted {
@@ -45,10 +55,8 @@ 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 centerY = globalCoordinate(localX: 0, localY: localFrame.midY, localGeometry: geo).y
-            let globalCenter = geo.frame(in: .global).origin.applying(.init(translationX: localCenter.x, y: localCenter.y))
+            let delta = localFrame.height / 2
-            let centerY = globalCenter.y
-
             ZStack(alignment: .bottomTrailing) {
                 VideoPlayer(url: url, model: model)
                 if model.has_audio == true {
@@ -66,10 +74,14 @@ struct DamusVideoPlayer: View {
                 video_size = size
             }
             .onChange(of: centerY) { _ in
-                let screenHeight = UIScreen.main.bounds.height
+                /// pause video when it is scrolled beyond visible range
-                let screenMidY = screenHeight / 2
+                let isBelowTop = centerY + delta > 100, /// 100 =~ approx. bottom (y) of ContentView's TabView
-                let tol = 0.20 * screenHeight /// tolerance - can vary  to taste ie.,  %  of screen height of a centered box in which video plays
+                    isAboveBottom = centerY - delta < orientationTracker.deviceMajorAxis
-                model.play = centerY > screenMidY - tol && centerY < screenMidY + tol /// video plays when inside tolerance box
+                if isBelowTop && isAboveBottom {
+                    model.start()
+                } else {
+                    model.stop()
+                }
             }
         }
     }

damus/damusApp.swift

@@ -20,11 +20,13 @@ struct damusApp: App {
 struct MainView: View {
     @State var needs_setup = false;
     @State var keypair: Keypair? = nil;
+    @StateObject private var orientationTracker = OrientationTracker()
     
     var body: some View {
         Group {
             if let kp = keypair, !needs_setup {
                 ContentView(keypair: kp)
+                    .environmentObject(orientationTracker)
             } else {
                 SetupView()
                     .onReceive(handle_notify(.login)) { notif in
@@ -38,7 +40,11 @@ struct MainView: View {
             try? clear_keypair()
             keypair = nil
         }
+        .onReceive(NotificationCenter.default.publisher(for: UIDevice.orientationDidChangeNotification)) { _ in
+            orientationTracker.setDeviceMajorAxis()
+        }
         .onAppear {
+            orientationTracker.setDeviceMajorAxis()
             keypair = get_saved_keypair()
         }
     }
@@ -65,3 +71,14 @@ class AppDelegate: NSObject, UIApplicationDelegate, UNUserNotificationCenterDele
         completionHandler()
     }
 }
+
+class OrientationTracker: ObservableObject {
+    var deviceMajorAxis: CGFloat = 0
+    func setDeviceMajorAxis() {
+        let bounds = UIScreen.main.bounds
+        let height = max(bounds.height, bounds.width) /// device's longest dimension
+        let width = min(bounds.height, bounds.width)  /// device's shortest dimension
+        let orientation = UIDevice.current.orientation
+        deviceMajorAxis = (orientation == .portrait || orientation == .unknown) ? height : width
+    }
+}