How to get notified of a DBus property change in Vala?

Development
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1

Hi,

this seems like a very basic question, but weirdly I haven’t been able to find an answer in the documentation. I’m trying to write a DBus client in Vala and I’d like to be notified when a property on a DBus object changes (assuming that it sends out the PropertiesChanged signal as expected). Following the many examples, I define an interface in Vala:

[DBus (name = "org.example.SimpleService", timeout = 120000)]
public interface SimpleService : GLib.Object {
	[DBus (name = "ExampleProperty")]
	public abstract bool example_property { get; set; }
}

This works, and I can retrieve and change example_property – also, if its value changes, it is properly updated. However, I have a hard time figuring out how to connect to a changed signal for the property. I’ve tried the following (where obj is an instance of the above class):

  • obj.notify[“example_property”].connect(…) compiles, but no notification
  • obj.notify.connect(…) compiles, but no notification
  • obj.g_properties_changed.connect(…) does not compile (signal not known)
  • obj.example_property.notify.connect(…) does not compile (bool does not have signals)
  • obj.example_property_changed.connect(…) does not compile (just tried at random)

What works however:

  • defining the g_properties_changed()signal myself as a member of the SimpleService class (feels very weird)
  • deriving the SimpleService class from GLib.DBusProxy instead of GLib.Object. This makes more sense, but all examples I have seen always derive the proxy object from GLib.Object so I’m wondering if this introduces additional problems (I’ve tried looking at the generated C code, and it seems fine, although I don’t fully understand it. Also, this requires “unpacking” the changed properties from a GLib.Variant, which seems contrary to the goal of using a proxy object.

So my question is: is there a Vala-native (or at least more Vala-like) way of getting notified of DBus property changes? If not, is deriving the proxy object explicitly from GLib.DBusProxy the recommended way to do this?

Thanks!

2

I’m wondering if this is actually the same as the following issue:

which has an MR as well: Draft: dbus: Bind properties with the GDBusProxy directly (!155) · Merge requests · GNOME / vala · GitLab

3

Yes, that’s exactly the issue you’re hitting, and the patch that would fix it.

You can work around the lack of support with something like this:

((DBusProxy) object).g_properties_changed.connect ((obj, changed_properties, invalidated_properties) => {
	if ("ExampleProperty" in invalidated_properties) {
		print ("ExampleProperty invalidated\n");
	} else if (changed_properties.lookup_value ("ExampleProperty", null) != null) {
		print ("ExampleProperty changed\n");
	}
});
4

Thank you for confirming!