playlist: Give Playlists a can-go-next property

This property is updated whenever the items in the Playlist change or if
the current-track property is changed. It can be used to know in advance
if calling next_track() can be expected to return a valid Track
instance.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

emmental/playlist/playlist.py

@@ -10,6 +10,8 @@ FLAGS = GObject.ParamFlags.READWRITE | GObject.ParamFlags.EXPLICIT_NOTIFY
 class Playlist(model.TrackidModel):
     """A TrackidModel with extra Playlist features."""
 
+    can_go_next = GObject.Property(type=bool, default=False)
+
     def __init__(self, sql: db.Connection,
                  playlist: db.playlist.Playlist = None):
         """Initialize the Playlist instance."""
@@ -23,6 +25,9 @@ class Playlist(model.TrackidModel):
         if playlist is not None:
             self.playlist = playlist
 
+        self.connect("items-changed", self.__update_can_go_next)
+        self.connect("notify::current-track", self.__update_can_go_next)
+
     def __get_nth_track(self, n: int | None) -> db.tracks.Track | None:
         return self[n] if n is not None and n < len(self.trackids) else None
 
@@ -42,6 +47,13 @@ class Playlist(model.TrackidModel):
                 self.on_trackids_reset(plist.tracks)
                 self.notify("sort-order")
 
+    def __update_can_go_next(self, *args) -> None:
+        if len(self.trackids) == 0:
+            self.can_go_next = False
+        else:
+            current = self.__playlist.current_trackid
+            self.can_go_next = current != self.trackids[-1]
+
     def __track_moved(self, track: db.tracks.Track, *, offset: int) -> None:
         index = self.index(track)
         new_pos = index + offset

tests/playlist/test_playlist.py

@@ -56,6 +56,25 @@ class TestPlaylist(tests.util.TestCase):
         plist2 = emmental.playlist.playlist.Playlist(self.sql, self.db_plist)
         self.assertEqual(plist2.playlist, self.db_plist)
 
+    def test_can_go_next(self):
+        """Test the playlist can_go_next() property."""
+        self.assertFalse(self.playlist.can_go_next)
+
+        self.playlist.playlist = self.db_plist
+        self.playlist.add_track(self.track1)
+        self.assertTrue(self.playlist.can_go_next)
+
+        self.playlist.add_track(self.track2)
+        self.playlist.next_track()
+        self.assertTrue(self.playlist.can_go_next)
+        self.playlist.next_track()
+        self.assertFalse(self.playlist.can_go_next)
+
+        self.playlist.move_track_up(self.track2)
+        self.assertTrue(self.playlist.can_go_next)
+        self.playlist.move_track_down(self.track2)
+        self.assertFalse(self.playlist.can_go_next)
+
     def test_add_track(self):
         """Test the playlist add_track() function."""
         self.playlist.add_track(self.track1)