Add comments for SpatialExternalSurface in xr-alpha04

Author: jkleinert

xr/src/main/java/com/example/xr/compose/SpatialExternalSurface.kt

@@ -38,23 +38,32 @@ fun SpatialExternalSurfaceContent() {
     Subspace {
         SpatialExternalSurface(
             modifier = SubspaceModifier
-                .width(1200.dp)
-                .height(676.dp),
+                .width(1200.dp) // Default width is 400.dp if no width modifier is specified
+                .height(676.dp), // Default height is 400.dp if no height modifier is specified
+            // Use StereoMode.Mono, StereoMode.SideBySide, or StereoMode.TopBottom, depending
+            // upon which type of content you are rendering: monoscopic content, side-by-side stereo
+            // content, or top-bottom stereo content
             stereoMode = StereoMode.SideBySide,
         ) {
             val exoPlayer = remember { ExoPlayer.Builder(context).build() }
             val videoUri = Uri.Builder()
                 .scheme(ContentResolver.SCHEME_ANDROID_RESOURCE)
+                // Represents a side-by-side stereo video, where each frame contains a pair of
+                // video frames arranged side-by-side. The frame on the left represents the left
+                // eye view, and the frame on the right represents the right eye view.
                 .path("sbs_video.mp4")
                 .build()
             val mediaItem = MediaItem.fromUri(videoUri)
 
+            // onSurfaceCreated is invoked only one time, when the Surface is created
             onSurfaceCreated { surface ->
                 exoPlayer.setVideoSurface(surface)
                 exoPlayer.setMediaItem(mediaItem)
                 exoPlayer.prepare()
                 exoPlayer.play()
             }
+            // onSurfaceDestroyed is invoked when the SpatialExternalSurface composable and its
+            // associated Surface are destroyed
             onSurfaceDestroyed { exoPlayer.release() }
         }
     }