curds: Add a next() function to the PlaylistManager

I'll eventually make this more complex when I add in more playlists, but
for now it just calls next on the Collection playlist

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

curds/playlist/manager.py

@@ -28,3 +28,9 @@ class PlaylistManager(dict):
     def __init__(self):
         self["Collection"] = collection.CollectionPlaylist()
         self["Library"]    = LibraryManager()
+        self.current       = self["Collection"]
+        self.track         = None
+
+    def next(self):
+        self.track = self.current.next()
+        return self.track

curds/playlist/test_manager.py

@@ -31,6 +31,9 @@ class TestPlaylistManager(unittest.TestCase):
         self.assertIsInstance(playman["Library"],    manager.LibraryManager)
         self.assertIsInstance(playman["Library"],    list)
 
+        self.assertEqual(playman.current, playman["Collection"])
+        self.assertEqual(playman.track,   None)
+
     def on_new_library(self, plist):
         self.cb_plist = plist
 
@@ -73,3 +76,18 @@ class TestPlaylistManager(unittest.TestCase):
         self.assertEqual(len(plist), 1250)
         self.assertEqual(len(alist), 12)
         self.assertEqual(len(playman["Collection"]), len(alist) + len(plist))
+
+    def test_manager_next(self):
+        playman = manager.PlaylistManager()
+        self.assertIsNone(playman.next())
+        self.assertIsNone(playman.track)
+
+        clist = playman["Collection"]
+        plist = playman["Library"].add(test_library)
+        library.library_thread.join()
+
+        track = playman.next()
+        self.assertEqual(playman.track, track)
+        self.assertEqual(clist.current, 0)
+        self.assertEqual(track,   clist[0])
+