♻️ Introduces the XZbarDecoder

✅ Also adds tests
This is a follow up for #42

Author: AndreMiras

src/kivy_garden/zbarcam/zbarcam.py

@@ -14,6 +14,10 @@
 
 
 class ZBarDecoder:
+    @classmethod
+    def is_usable(cls):
+        return False
+
     def validate_code_types(self, code_types):
         available_code_types = self.get_available_code_types()
 
@@ -91,22 +95,36 @@ def decode(self, image, code_types):
         ]
 
 
-available_implementations = {
-    'pyzbar': PyZBarDecoder,
-    'zbarlight': ZBarLightDecoder,
-}
+class XZbarDecoder(ZBarDecoder):
+    """Proxy-like that deals with all the implementations."""
+    available_implementations = {
+        'pyzbar': PyZBarDecoder,
+        'zbarlight': ZBarLightDecoder,
+    }
+    zbar_decoder = None
+
+    def __init__(self):
+        # making it a singleton so it gets initialized once
+        XZbarDecoder.zbar_decoder = (
+            self.zbar_decoder or self._get_implementation())
+
+    def _get_implementation(self):
+        for name, implementation in self.available_implementations.items():
+            if implementation.is_usable():
+                zbar_decoder = implementation()
+                Logger.info('ZBarCam: Using implementation %s', name)
+                return zbar_decoder
+        else:
+            raise ImportError(
+                'No zbar implementation available '
+                f'(tried {", ".join(self.available_implementations.keys())})'
+            )
 
+    def get_available_code_types(self):
+        return self.zbar_decoder.get_available_code_types()
 
-for name, implementation in available_implementations.items():
-    if implementation.is_usable():
-        zbar_decoder = implementation()
-        Logger.info('ZBarCam: Using implementation %s', name)
-        break
-else:
-    raise ImportError(
-        'No zbar implementation available '
-        f'(tried {", ".join(available_implementations.keys())})'
-    )
+    def decode(self, image, code_types):
+        return self.zbar_decoder.decode(image, code_types)
 
 
 class ZBarCam(AnchorLayout):
@@ -119,7 +137,7 @@ class ZBarCam(AnchorLayout):
     symbols = ListProperty([])
     Symbol = namedtuple('Symbol', ['type', 'data'])
     # checking all possible types by default
-    code_types = ListProperty(zbar_decoder.get_available_code_types())
+    code_types = ListProperty(XZbarDecoder().get_available_code_types())
 
     def __init__(self, **kwargs):
         # lazy loading the kv file rather than loading at module level,
@@ -170,7 +188,7 @@ def _detect_qrcode_frame(cls, texture, code_types):
         pil_image = PIL.Image.frombytes(mode='RGBA', size=size,
                                         data=image_data)
         pil_image = fix_android_image(pil_image)
-        return zbar_decoder.decode(pil_image, code_types)
+        return XZbarDecoder().decode(pil_image, code_types)
 
     @property
     def xcamera(self):

tests/kivy_garden/zbarcam/test_zbarcam.py

@@ -1,11 +1,12 @@
 import os
-import unittest
 from unittest import mock
 
+import pytest
 from kivy.base import EventLoop
 from kivy.core.image import Image
 
 from kivy_garden.zbarcam import ZBarCam
+from kivy_garden.zbarcam.zbarcam import XZbarDecoder
 
 FIXTURE_DIR = os.path.join(
     os.path.abspath(
@@ -16,9 +17,14 @@
 EventLoop.ensure_window()
 
 
-class TestZBarCam(unittest.TestCase):
+def patch_is_usable(implementation, m_is_usable):
+    return mock.patch(
+        f'kivy_garden.zbarcam.zbarcam.{implementation}.is_usable', m_is_usable)
 
-    def setUp(self):
+
+class TestZBarCam:
+
+    def setup_method(self):
         with mock.patch('kivy.uix.anchorlayout.AnchorLayout.__init__'):
             self.zbarcam = ZBarCam()
 
@@ -70,3 +76,28 @@ def test_detect_qrcode_frame_two_qrcodes(self):
             Symbol(type='QRCODE', data=b'second zbarlight test qr code'),
             Symbol(type='QRCODE', data=b'zbarlight test qr code'),
         ]
+
+
+class TestXZbarDecoder:
+
+    def test_singleton(self):
+        """
+        New instances of XZbarDecoder should share the same instance of
+        zbar_decoder.
+        """
+        xzbar_decoder = XZbarDecoder()
+        zbar_decoder = xzbar_decoder.zbar_decoder
+        assert zbar_decoder == XZbarDecoder().zbar_decoder
+
+    def test_no_zbar_implementation_available(self):
+        """
+        Makes sure `ImportError` is raised on no available implementations.
+        """
+        # resets the singleton instance to force reprobing
+        XZbarDecoder.zbar_decoder = None
+        m_is_usable = mock.Mock(return_value=False)
+        with patch_is_usable("PyZBarDecoder", m_is_usable), \
+                patch_is_usable("ZBarLightDecoder", m_is_usable), \
+                pytest.raises(
+                    ImportError, match="No zbar implementation available"):
+            XZbarDecoder()