Connecting to scroll_event in Vala

Question

I've been struggling to connect to the scroll_event of a TextView widget. I can connect a lambda to it, and then run my method, but I'd like to understand why directly connecting doesn't work. I've been using the code below

using Gtk;

public class TextFileViewer : Gtk.Window {

    private TextView text_view;

    public TextFileViewer () {
        this.title = "Text File Viewer";
        this.position = WindowPosition.CENTER;
        set_default_size (400, 300);

        this.text_view = new TextView ();
        this.text_view.editable = true;
        this.text_view.cursor_visible = true;

        var scroll = new ScrolledWindow (null, null);
        scroll.set_policy (PolicyType.AUTOMATIC, PolicyType.AUTOMATIC);
    scroll.add (this.text_view);

        this.text_view.scroll_event.connect (on_scroll_event);

        var vbox = new VBox (true, 0);
        vbox.pack_start (this.text_view, true, true, 0);
        add (vbox);
    }

    private void on_scroll_event () {
            stderr.printf("We scrollin breds");
    }

    public static int main (string[] args) {
        Gtk.init (ref args);

        var window = new TextFileViewer ();
        window.destroy.connect (Gtk.main_quit);
        window.show_all ();

        Gtk.main ();
        return 0;
    }
}

That code gives me the error:

gtkviewer.vala:20.46-20.60: error: Argument 1: Cannot convert from `TextFileViewer.on_scroll_event' to `Gtk.Widget.scroll_event'
    scroll.scroll_event.connect (on_scroll_event);

vala is at version 0.12.0

Answer 1

Check the scroll-event signal arguments:

public virtual signal bool scroll_event (Gdk.EventScroll event);

private bool on_scroll_event (Gdk.EventScroll e) {
        stderr.printf("We scrollin breds");
        return true;
}