sidebar: Create a TreeListModel helper function

I can't inherit from Gtk.TreeListModel since the new() constructor is
the only way to set the create_child_func. Instead, use this wrapper
function to do the same thing.

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

sidebar/model.py

@@ -1,6 +1,7 @@
 # Copyright 2021 (c) Anna Schumaker.
 from gi.repository import GObject
 from gi.repository import Gio
+from gi.repository import Gtk
 
 class ChildModel(GObject.GObject, Gio.ListModel):
     def __init__(self, parent):
@@ -14,3 +15,11 @@ class ChildModel(GObject.GObject, Gio.ListModel):
 
     def children_changed(self, item, pos, rm, add):
         self.emit("items-changed", pos, rm, add)
+
+
+def create_child_func(item):
+    return ChildModel(item) if item.has_children() else None
+
+def TreeListModel(table):
+    return Gtk.TreeListModel.new(table, passthrough=False, autoexpand=False,
+                                        create_func=create_child_func)

sidebar/test_model.py

@@ -4,6 +4,7 @@ import db
 import unittest
 from gi.repository import GObject
 from gi.repository import Gio
+from gi.repository import Gtk
 from . import model
 
 class TestChildModel(unittest.TestCase):
@@ -33,3 +34,20 @@ class TestChildModel(unittest.TestCase):
         self.assertEqual(child.get_n_items(), 1)
         self.assertEqual(child.get_item(0), album)
         self.assertEqual(self.changed, (0, 0, 1))
+
+
+class TestTreeListModel(unittest.TestCase):
+    def test_init(self):
+        tree = model.TreeListModel(db.artist.Table)
+        self.assertIsInstance(tree, Gtk.TreeListModel)
+        self.assertEqual(tree.get_model(), db.artist.Table)
+        self.assertFalse(tree.get_autoexpand())
+        self.assertFalse(tree.get_passthrough())
+
+    def test_create_child_func(self):
+        artist = db.artist.Table.find("Test Artist", "Test Sort")
+        child = model.create_child_func(artist)
+        self.assertEqual(child.parent, artist)
+
+        genre = db.genre.Table.find("Test Genre")
+        self.assertIsNone(model.create_child_func(genre))