add some comments

backend/playbackmanager.go

@@ -18,6 +18,9 @@ const (
 	ReplayGainTrack = string(player.ReplayGainTrack)
 )
 
+// A high-level Subsonic-aware playback backend.
+// Manages loading tracks into the Player queue,
+// sending callbacks on play time updates and track changes.
 type PlaybackManager struct {
 	ctx           context.Context
 	cancelPollPos context.CancelFunc

backend/servermanager.go

@@ -85,10 +85,12 @@ func (s *ServerManager) Logout() {
 	}
 }
 
+// Sets a callback that is invoked when a server is connected to.
 func (s *ServerManager) OnServerConnected(cb func()) {
 	s.onServerConnected = append(s.onServerConnected, cb)
 }
 
+// Sets a callback that is invoked when the user logs out of a server.
 func (s *ServerManager) OnLogout(cb func()) {
 	s.onLogout = append(s.onLogout, cb)
 }

backend/updatechecker.go

@@ -9,6 +9,8 @@ import (
 	"time"
 )
 
+// A component to check for updates from the
+// Github releases/latest URL.
 type UpdateChecker struct {
 	OnUpdatedVersionFound func()
 
@@ -26,6 +28,8 @@ func NewUpdateChecker(appVersionTag, latestReleaseURL string, lastCheckedTag *st
 	}
 }
 
+// Start automatically polling for updates in the background.
+// Quits when the given ctx's Done channel returns a value.
 func (u *UpdateChecker) Start(ctx context.Context, interval time.Duration) {
 	go func() {
 		u.checkForUpdate() // check once at startup