Use ITypeInfo#to_callback_ffitype to define callback FFI argument types

lib/gir_ffi/info_ext/i_arg_info.rb

@@ -6,6 +6,11 @@ def to_ffitype
         return :pointer if direction != :in
         return argument_type.to_ffitype
       end
+
+      def to_callback_ffitype
+        return :pointer if direction != :in
+        return argument_type.to_callback_ffitype
+      end
     end
   end
 end

lib/gir_ffi/info_ext/i_callback_info.rb

@@ -6,8 +6,18 @@ def to_ffitype
         Builder.build_class(self)
       end
 
+      def argument_ffi_types
+        args.map { |arg| arg.to_callback_ffitype }
+      end
+
       def return_ffi_type
-        return_type.to_callback_ffitype
+        result = return_type.to_callback_ffitype
+        # FIXME: Should this be in ITypeInfo#to_callback_ffitype?
+        if result == GLib::Boolean
+          :bool
+        else
+          result
+        end
       end
     end
   end

lib/gir_ffi/info_ext/i_signal_info.rb

@@ -45,7 +45,7 @@ def gvalue_for_return_value
       # types as well?
       def ffi_callback_argument_types
         types = args.map do |arg|
-          arg.argument_type.to_callback_ffitype
+          arg.to_callback_ffitype
         end
         types.unshift(:pointer).push(:pointer)
       end

test/integration/generated_gio_test.rb

@@ -92,5 +92,11 @@
       pass
     end
   end
+
+  describe "the SocketSourceFunc callback" do
+    it "can be cast to a native function" do
+      Gio::SocketSourceFunc.new {|*args| p args}.to_native
+    end
+  end
 end