Support strongly typed signals

In addition to properties and methods, the arguments of a signal have also
types which can be defined with `TypedSignal`, a subclass of `signal`.

pydbus/strong_typing.py

@@ -1,6 +1,7 @@
 """Decorators for methods and properties to strongly typed the values."""
 import inspect
 
+from pydbus.generic import signal
 from pydbus.xml_generator import verify_arguments
 
 
@@ -34,3 +35,10 @@ def decorate(func):
 		verify_arguments(func)
 		return func
 	return decorate
+
+
+class TypedSignal(signal):
+
+	def __init__(self, arguments):
+		super(TypedSignal, self).__init__()
+		self.arg_types = arguments

pydbus/xml_generator.py

@@ -5,6 +5,8 @@
 from itertools import islice
 from xml.etree import ElementTree
 
+from pydbus.generic import signal
+
 
 PROPERTY_EMITS_SIGNAL = "org.freedesktop.DBus.Property.EmitsChangedSignal"
 
@@ -149,7 +151,7 @@ def valid_member(member):
 		"""Only select members with the correct type and name."""
 		if isinstance(member, property):
 			member = member.fget
-		elif not ismethod(member):
+		elif not ismethod(member) and not isinstance(member, signal):
 			return False
 		return member.__name__[0].isupper()
 
@@ -190,6 +192,17 @@ def valid_member(member):
 						entry, "annotation",
 						{"name": PROPERTY_EMITS_SIGNAL, "value": "true"})
 				attributes["access"] = "readwrite"
+		elif isinstance(value, signal):
+			if hasattr(value, "arguments"):
+				arguments = value.arguments
+			elif require_strong_typing:
+				raise ValueError(
+					"No argument definitions for signal "
+					"'{}'".format(value.__name__))
+			entry = ElementTree.SubElement(value, "signal")
+			for arg, arg_type in arguments:
+				ElementTree.SubElement(
+					entry, "arg", {"name": arg, "type": arg_type})
 		elif ismethod(value):
 			arg_types, ret_type = verify_arguments(value)
 			entry = ElementTree.SubElement(get_interface(value), "method")