Composite templates lands in Vala

This is another journal entry relating to the composite template saga. It’s been a little over a month now since Openismus was kind enough to sponsor my work on embedding of GtkBuilder xml directly into GtkWidget class data, allowing the automation of composite widget classes.

As of recently, composite template support has been merged into Vala master. This Vala improvement was brought to you by Luca Bruno, so if you find yourself saving a lot of time in your daily Vala development practice, please be encouraged to transfer at least part of these savings into Luca Bruno’s personal beer fund.

This is really exciting for me because this integration in high level languages is really what I’ve had in mind while working towards this goal. Of course, the developer experience would be greatly improved by IDE tools which use Glade’s core and additionally have knowledge of Vala syntax and provide some glue features to help automate these associations (having both Glade’s core and Vala’s parser and object structures in the same memory space would allow some awesome integration features)… at any rate, this is still a major milestone on the road to a bleeding edge developer experience.

From now on your UI developer experience should look like this

The new Vala constructs allowing one to integrate with GtkBuilder files and create composite widgets in Vala are documented here.

Using the Vala compiler in GNOME git master today, it is possible to compile this full demo.

And here is just a sample from that demo:

using Gtk;

[GtkTemplate (ui = "/org/foo/my/mywidget.ui")]
public class MyWidget : Box {
        public string text {
                get { return entry.text; }
                set { entry.text = value; }

        private Entry entry;

        public MyWidget (string text) {
                this.text = text;

        private void button_clicked (Button button) {
                print ("The button was clicked with entry text: %s\n", entry.text);

        private void entry_changed (Button button) {
                print ("The entry text changed: %s\n", entry.text);

                notify_property ("text");

The above is basically what an example composite widget looks like in Vala code. Notice that there are three attributes which can be used to bind your class to a GtkBuilder xml file.

  • GtkTemplate: This attribute tells the compiler to bind your class with the specified .ui resource, it is up to you to ensure that the specified resource is compiled into your program (using GResource).
  • GtkChild: This indicates that the instance variable refers to an object defined in the template GtkBuilder xml
  • GtkCallback: This defines one of your object class methods as a callback suitable to be referred to in the GtkBuilder xml in <signal> statements. If a class method is defined as a [GtkCallback], then you can simply specify the method name as a handler in Glade’s signal editor and your method will be called in response to the signal emission defined in the GtkBuilder file.

A small challenge

Over recent years, we’ve been hearing various stories about what GNOME should ‘choose for an official binding’, my money has always been on Vala. This is not because I know how to write code in vala, it’s not because I feel comfortable with vala or anything of the sort.

My reasoning is simple:

  • Vala is a formalism that we control, since it is based on GObject it’s the most valid candidate to deliver the features which are specific to our platform in a comfortable way. Examples of this are the implied syntaxes which Vala provides to connect callbacks to GSignals, built-in language features for using asynchronous callbacks from GIO, and now, my favourite addition is of course the [GtkTemplate] syntax.
  • A programming language is a window into a feature set available in a given platform. It took me all of 2 days to learn Objective-C and after doing so it makes perfect sense that one would choose Objective-C with it’s platform specific feature set to integrate with the NextStep environment, because hey, that’s what it was created for (so why try to use basic C code, or Java to integrate with the NextStep environment if a customized Objective-C variant was explicitly created to make your developer experience richer for that platform ?)

So the challenge is this: Prove me wrong.

Do you have another favourite language binding for GNOME ?

If so, can you please extend your language’s formalism to define composite widget classes ?

Whether this is really a challenge or not is yet to be seen, I’m really curious ;-)