wacom: add PacketHandlers for handling special commands

For data packets other than basic motion/pressure strokes, add a PacketHandler
that can deal with that particular packet. Those handlers are part of the
class declaration, when we instantiate a protocol we pull them all into a
packet_handlers list and go through them. The first one to return True is
considered the right handler and we expect something has been done with the
packet.

This patch includes the handlers for the end-of-stroke and end-of-sequence
packets, the rest to be added later.

tuhi/wacom.py

@@ -15,6 +15,7 @@
 import binascii
 import calendar
 import enum
+import inspect
 import logging
 import threading
 import time
@@ -220,6 +221,30 @@ class WacomPacket(GObject.Object):
         return " ".join(debug_data)
 
 
+class WacomPacketHandler(GObject.Object):
+    def process(self, packet, drawing):
+        raise NotImplementedError('This method must be implemented in the subclass')
+
+
+class WacomPacketHandlerEndOfSequence(WacomPacketHandler):
+    def process(self, packet, drawing):
+        if bytes(packet.args) != b'\xff\xff\xff\xff\xff\xff\xff\xff':
+            return False
+
+        logger.info(f'end of sequence')
+        return True
+
+
+class WacomPacketHandlerEndOfStroke(WacomPacketHandler):
+    def process(self, packet, drawing):
+        if bytes(packet.args) != b'\x00\x00\xff\xff\xff\xff\xff\xff':
+            return False
+
+        logger.info(f'end of stroke')
+        drawing.current_stroke.seal()
+        return True
+
+
 class WacomProtocolLowLevelComm(GObject.Object):
     '''
     Internal class to handle the communication with the Wacom device.
@@ -405,6 +430,26 @@ class WacomProtocolBase(WacomProtocolLowLevelComm):
         device.connect_gatt_value(WACOM_OFFLINE_CHRC_PEN_DATA_UUID,
                                   self._on_pen_data_received)
 
+        # Instantiate all packet_handlers from our current object and its
+        # parent classes
+        self.packet_handlers = []
+        parents = inspect.getmro(self.__class__)
+        child = None
+        for cls in parents:
+            if cls.__name__ == 'WacomProtocolBase':
+                break
+
+            if (child is not None and
+                    child.packet_handlers and
+                    child.packet_handlers == cls.packet_handlers):
+                raise NotImplementedError(f'Subclass {child} must override packet_handlers')
+
+            for handler in cls.packet_handlers:
+                h = handler()
+                self.packet_handlers.append(h)
+
+            child = cls
+
     def _on_pen_data_changed(self, name, value):
         logger.debug(binascii.hexlify(bytes(value)))
 
@@ -671,16 +716,17 @@ class WacomProtocolBase(WacomProtocolLowLevelComm):
             logger.debug(f'packet: {packet}')
             offset += packet.length
 
+            has_handler = False
+            for handler in self.packet_handlers:
+                if handler.process(packet, drawing):
+                    has_handler = True
+                    break
+            if has_handler:
+                continue
+
             if self.parse_next_stroke_prefix(packet.opcode, packet.bytes):
                 stroke = drawing.new_stroke()
                 continue
-            if bytes(packet.args) == b'\xff\xff\xff\xff\xff\xff\xff\xff':
-                logger.info(f'end of sequence')
-                continue
-            if bytes(packet.args) == b'\x00\x00\xff\xff\xff\xff\xff\xff':
-                logger.info(f'end of stroke')
-                stroke.seal()
-                continue
 
             stroke = drawing.current_stroke
             if stroke is None:
@@ -784,6 +830,8 @@ class WacomProtocolSpark(WacomProtocolBase):
     width = 21600
     height = 14800
     protocol = Protocol.SPARK
+    packet_handlers = [WacomPacketHandlerEndOfStroke,
+                       WacomPacketHandlerEndOfSequence]
 
 
 class WacomProtocolSlate(WacomProtocolSpark):
@@ -797,6 +845,7 @@ class WacomProtocolSlate(WacomProtocolSpark):
     width = 21600
     height = 14800
     protocol = Protocol.SLATE
+    packet_handlers = []
 
     def __init__(self, device, uuid):
         super().__init__(device, uuid)
@@ -896,6 +945,7 @@ class WacomProtocolIntuosPro(WacomProtocolSlate):
     width = 44800
     height = 29600
     protocol = Protocol.INTUOS_PRO
+    packet_handlers = []
 
     def __init__(self, device, uuid):
         super().__init__(device, uuid)