texture: Add a Texture Cache

The Texture Cache will be used to map filenames to Gdk.Textures loaded
into memory. The application can then re-use textures instead of making
expensive filesystem calls and loading the same images multiple times.

Implements: #51 ("Texture Cache")
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

emmental/texture.py

@@ -0,0 +1,26 @@
+# Copyright 2023 (c) Anna Schumaker.
+"""A cache to hold Gdk.Textures used by cover art."""
+import pathlib
+from gi.repository import Gdk
+
+
+class _TextureCache(dict):
+    """A custom dictionary for storing texture files."""
+
+    def __missing__(self, path: pathlib.Path | None) -> Gdk.Texture:
+        """Load a cache item from disk or add a new item entirely."""
+        texture = Gdk.Texture.new_from_filename(str(path))
+        self.__setitem__(path, texture)
+        return texture
+
+    def __getitem__(self, path: pathlib.Path | None) -> Gdk.Texture | None:
+        """Get a Gdk.Texture cache item from the cache."""
+        if path is not None and path.is_file():
+            return super().__getitem__(path)
+
+    def drop(self, path: pathlib.Path | None) -> None:
+        """Drop a single cache item from the cache."""
+        self.pop(path, None)
+
+
+CACHE = _TextureCache()

tests/test_texture.py

@@ -0,0 +1,41 @@
+# Copyright 2023 (c) Anna Schumaker.
+"""Tests our Gdk.Texture cache."""
+import emmental.texture
+import pathlib
+import tests.util
+import unittest
+from gi.repository import Gdk
+
+
+class TestTextureCache(unittest.TestCase):
+    """Test our custom cache dictionary."""
+
+    def setUp(self):
+        """Set up common variables."""
+        self.cache = emmental.texture._TextureCache()
+
+    def test_init(self):
+        """Test that the cache dict is initialized properly."""
+        self.assertIsInstance(emmental.texture.CACHE,
+                              emmental.texture._TextureCache)
+        self.assertDictEqual(emmental.texture.CACHE, {})
+
+        self.assertIsInstance(self.cache, dict)
+        self.assertDictEqual(self.cache, {})
+
+    def test_drop(self):
+        """Test dropping items from the cache."""
+        self.cache[tests.util.COVER_JPG]
+        self.cache.drop(tests.util.COVER_JPG)
+        self.assertDictEqual(self.cache, {})
+
+    def test_getitem(self):
+        """Test getting and creating items in the cache dict."""
+        self.assertIsNone(self.cache[None])
+        self.assertIsNone(self.cache[pathlib.Path("/no/such/path")])
+        self.assertDictEqual(self.cache, {})
+
+        texture = self.cache[tests.util.COVER_JPG]
+        self.assertIsInstance(texture, Gdk.Texture)
+        self.assertDictEqual(self.cache, {tests.util.COVER_JPG: texture})
+        self.assertEqual(self.cache[tests.util.COVER_JPG], texture)