feat: abstract base classes for endpoints

Introduces EndpointBase and ReceiverBase abstract base classes. Implements a ReceiverBase with the MainReceiver.

ref: N25B-168

src/robot_interface/state.py

@@ -0,0 +1,58 @@
+import logging
+import signal
+import threading
+
+
+class State(object):
+    """
+    Do not create an instance of this class directly: use the instance `state` below. This state must be initiated once,
+    probably when your program starts.
+
+    This class is used to share state between threads. For example, when the program is quit, that all threads can
+    detect this via the `exit_event` property being set.
+    """
+    def __init__(self):
+        self.is_initialized = False
+        self.exit_event = None
+        self.endpoints = []  # type: List[EndpointBase]
+
+    def initialize(self):
+        if self.is_initialized:
+            logging.warn("Already initialized")
+            return
+
+        self.exit_event = threading.Event()
+        def handle_exit(_, __):
+            logging.info("Exiting.")
+            self.exit_event.set()
+        signal.signal(signal.SIGINT, handle_exit)
+        signal.signal(signal.SIGTERM, handle_exit)
+
+        self.is_initialized = True
+
+    def deinitialize(self):
+        if not self.is_initialized: return
+
+        for endpoint in self.endpoints:
+            endpoint.close()
+
+        self.is_initialized = False
+
+    def __getattribute__(self, name):
+        # Enforce that the state is initialized before accessing any property (aside from the basic ones)
+        if name in ("initialize", "deinitialize", "is_initialized", "__dict__", "__class__"):
+            return object.__getattribute__(self, name)
+
+        if not object.__getattribute__(self, "is_initialized"):
+            # Special case for the exit_event: if the event is set, return it without an error
+            if name == "exit_event":
+                exit_event = object.__getattribute__(self, "exit_event")
+                if exit_event and exit_event.is_set(): return exit_event
+
+            raise RuntimeError("State must be initialized before accessing '%s'" % name)
+
+        return object.__getattribute__(self, name)
+
+
+# Must call `.initialize` before use
+state = State()