playlist: Create a way to switch tags in the TagModel

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

playlist/model.py

@@ -20,3 +20,8 @@ class TagModel(GObject.GObject, Gio.ListModel):
 
     def do_get_object(self, n):
         return self.tag[n]
+
+    def switch_tag(self, tag):
+        prev = len(self.tag) if self.tag else 0
+        self.tag = tag
+        self.items_changed(0, prev, len(self.tag))

playlist/test_model.py

@@ -7,6 +7,12 @@ import unittest
 
 
 class TestTagModel(unittest.TestCase):
+    def setUp(self):
+        self.changed = (None, None, None)
+
+    def items_changed_cb(self, model, position, removed, added):
+        self.changed = (position, removed, added)
+
     def test_tag_model_init(self):
         t = tag.Tag("Test")
         m = model.TagModel(t)
@@ -27,3 +33,20 @@ class TestTagModel(unittest.TestCase):
 
         self.assertEqual(m.get_n_items(), 0)
         self.assertIsNone(m.get_item(0))
+
+    def test_tag_model_switch_tag(self):
+        t = tag.Tag("A")
+        t.add_track(fake.Track(1))
+        t.add_track(fake.Track(2))
+
+        u = tag.Tag("B")
+        u.add_track(fake.Track(3))
+
+        m = model.TagModel()
+        m.connect("items-changed", self.items_changed_cb)
+
+        m.switch_tag(t)
+        self.assertEqual(self.changed, (0, 0, 2))
+
+        m.switch_tag(u)
+        self.assertEqual(self.changed, (0, 2, 1))