add Spirc.seek_offset command

- preparation for MPRIS support

connect/src/spirc.rs

@@ -126,6 +126,7 @@ enum SpircCommand {
     RepeatTrack(bool),
     Disconnect { pause: bool },
     SetPosition(u32),
+    SeekOffset(i32),
     SetVolume(u16),
     Activate,
     Load(LoadRequest),
@@ -385,6 +386,13 @@ impl Spirc {
         Ok(self.commands.send(SpircCommand::Load(command))?)
     }
 
+    /// Seek to given offset.
+    ///
+    /// Does nothing if we are not the active device.
+    pub fn seek_offset(&self, offset_ms: i32) -> Result<(), Error> {
+        Ok(self.commands.send(SpircCommand::SeekOffset(offset_ms))?)
+    }
+
     /// Disconnects the current device and pauses the playback according the value.
     ///
     /// Does nothing if we are not the active device.
@@ -651,6 +659,7 @@ impl SpircTask {
             SpircCommand::Repeat(repeat) => self.handle_repeat_context(repeat)?,
             SpircCommand::RepeatTrack(repeat) => self.handle_repeat_track(repeat),
             SpircCommand::SetPosition(position) => self.handle_seek(position),
+            SpircCommand::SeekOffset(offset) => self.handle_seek_offset(offset),
             SpircCommand::SetVolume(volume) => self.set_volume(volume),
             SpircCommand::Load(command) => self.handle_load(command, None).await?,
         };
@@ -1461,6 +1470,25 @@ impl SpircTask {
         };
     }
 
+    fn handle_seek_offset(&mut self, offset_ms: i32) {
+        let position_ms = match self.play_status {
+            SpircPlayStatus::Stopped => return,
+            SpircPlayStatus::LoadingPause { position_ms }
+            | SpircPlayStatus::LoadingPlay { position_ms }
+            | SpircPlayStatus::Paused { position_ms, .. } => position_ms,
+            SpircPlayStatus::Playing {
+                nominal_start_time, ..
+            } => {
+                let now = self.now_ms();
+                (now - nominal_start_time) as u32
+            }
+        };
+
+        let position_ms = ((position_ms as i32) + offset_ms).max(0) as u32;
+
+        self.handle_seek(position_ms);
+    }
+
     fn handle_shuffle(&mut self, shuffle: bool) -> Result<(), Error> {
         self.player.emit_shuffle_changed_event(shuffle);
         self.connect_state.handle_shuffle(shuffle)