Implement initial structure and core functionality for Duck Preview application

tests/test_dispatcher.py

@@ -0,0 +1,47 @@
+from duck_preview.dispatcher.frame_dispatcher import FrameDispatcher
+
+
+def test_subscribe_notified():
+    dispatcher = FrameDispatcher()
+    received = []
+
+    def cb(frame):
+        received.append(frame)
+
+    dispatcher.subscribe(cb)
+    dispatcher.on_frame("test")
+
+    assert received == ["test"]
+
+
+def test_unsubscribe_not_notified():
+    dispatcher = FrameDispatcher()
+    received = []
+
+    def cb(frame):
+        received.append(frame)
+
+    dispatcher.subscribe(cb)
+    dispatcher.unsubscribe(cb)
+    dispatcher.on_frame("test")
+
+    assert received == []
+
+
+def test_multiple_subscribers():
+    dispatcher = FrameDispatcher()
+    received_1 = []
+    received_2 = []
+
+    def cb1(frame):
+        received_1.append(frame)
+
+    def cb2(frame):
+        received_2.append(frame)
+
+    dispatcher.subscribe(cb1)
+    dispatcher.subscribe(cb2)
+    dispatcher.on_frame("test")
+
+    assert received_1 == ["test"]
+    assert received_2 == ["test"]