event callback does not get executed in custom gtk3 widget

Question

Currently trying to create a custom widget which is based directly on GtkWidget, marks it as drawable and draws content with cairo. So much for the context.

As soon as I try to handle events (so I can implement zoom) - especially the scroll event - just does not work and I am not sure as of why.

The callback does get executed on focus-in/focus out (good old print statement proves that), but I do never ever get any scroll wheel activity detected by that callback (button clicks are key press/release do not work either).

I tried to hook up to the event signal by

• using the function pointer and assigning callback function (which I think is the right thing to do)
• using g_signal_connect (mywidget, "event",..) from foo_new or foo_init

Neither did work.

---

The Foo init:

foo_init (Foo *self)
{
    GtkWidget *widget = GTK_WIDGET (self);
    gtk_widget_set_has_window (widget, FALSE);

    self->priv = FOO_GET_PRIVATE (self);

    gtk_widget_add_events (widget, GDK_ALL_EVENTS_MASK);
    g_assert ((gtk_widget_get_events (widget) & GDK_SCROLL_MASK) != 0); //just fine

    /* added some stuff I also tried but did not work */
    gtk_widget_set_sensitive (widget, TRUE);
    gtk_widget_set_can_focus (widget, TRUE);
    gtk_widget_grab_focus (widget);
    ...

---

How can I get all the events of my widget?

---

Assigning widget_class->key_press_event = my_handler_callback; actually works as expected and I get the keys I press, but the very same widget_class->button_press_event = my_handler_callback; or widget_class->scroll_event = my_handler_callback; assignments do not work!

widget_class->key_press_event = my_handler_callback; // works
widget_class->key_release_event = my_handler_callback; // works
widget_class->button_press_event = my_handler_callback; // NOT
widget_class->button_release_event = my_handler_callback; // NOT
widget_class->scroll_event = my_handler_callback; // NOT

---

This made me suspicious.

To receive this signal, the GdkWindow associated to the widget needs to enable the GDK_BUTTON_PRESS_MASK mask.

Is it necessary to realize the widget before gtk_widget_add_events "works"...?

---

Update: Tried to call gtk_widget_add_events after gtk_widget_show_all. No change.

---

Update: fully compileable example

#ifndef __FOO_H__
#define __FOO_H__

#include <gtk/gtk.h>

G_BEGIN_DECLS

#define FOO_TYPE            (foo_get_type ())
#define FOO(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), FOO_TYPE, Foo))
#define FOO_CONST(obj)      (G_TYPE_CHECK_INSTANCE_CAST ((obj), FOO_TYPE, Foo const))
#define FOO_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), FOO_TYPE, FooClass))
#define FOO_IS(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), FOO_TYPE))
#define FOO_IS_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), FOO_TYPE))
#define FOO_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), FOO_TYPE, FooClass))

typedef struct _Foo         Foo;
typedef struct _FooClass    FooClass;
typedef struct _FooPrivate  FooPrivate;

struct _Foo
{
    GtkWidget parent;

    FooPrivate *priv;
};

struct _FooClass
{
    GtkWidgetClass parent_class;
};

GType foo_get_type (void) G_GNUC_CONST;
Foo *foo_new (void);


G_END_DECLS

#endif /* __FOO_H__ */

---

#include "foo.h"


gboolean
scroll_hook (GtkWidget *widget, GdkEventScroll *event)
{
    g_print ("%p registered a scroll event\n");
    return TRUE;
}


#define FOO_GET_PRIVATE(object)(G_TYPE_INSTANCE_GET_PRIVATE((object), FOO_TYPE, FooPrivate))

struct _FooPrivate
{
    char to_silence_warning;
};

G_DEFINE_TYPE (Foo, foo, GTK_TYPE_WIDGET)

static void
foo_finalize (GObject *object)
{
    G_OBJECT_CLASS (foo_parent_class)->finalize (object);
}

static void
foo_class_init (FooClass *klass)
{
    GObjectClass *object_class = G_OBJECT_CLASS (klass);
    object_class->finalize = foo_finalize;

    GtkWidgetClass *widget_class = GTK_WIDGET_CLASS(klass);
    widget_class->scroll_event = scroll_hook;
    g_type_class_add_private (object_class, sizeof (FooPrivate));
}

static void
foo_init (Foo *self)
{
    self->priv = FOO_GET_PRIVATE (self);
    gtk_widget_set_has_window(GTK_WIDGET(self), FALSE);
}

Foo *
foo_new ()
{
    return g_object_new (FOO_TYPE, NULL);
}

---

#include <gtk/gtk.h>
#include "foo.h"
#include <stdlib.h>

gboolean
chicken_out (GtkWidget *widget, GdkEvent *event, gpointer user_data)
{
    g_print ("bye");
    gtk_main_quit();
    return TRUE;
}

int
main (int argc, char *argv[])
{
    GtkWidget *window;
    Foo *my;
    int i;

    gtk_init (&argc, &argv);

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    my = foo_new ();

    gtk_container_add (GTK_CONTAINER (window), GTK_WIDGET (my));
    gtk_widget_show_all (window);

    gtk_widget_add_events (GTK_WIDGET (my), GDK_ALL_EVENTS_MASK);
    g_assert ((gtk_widget_get_events (GTK_WIDGET (my)) & GDK_SCROLL_MASK) != 0);

    g_signal_connect (window, "delete-event", G_CALLBACK(chicken_out), NULL);
    gtk_main ();

    return EXIT_SUCCESS;
}

Use

gcc `pkg-config --cflags --libs gtk+-3.0` -I. ./foo.c ./foo_test.c  -o foo.bin

to compile (granted, all files are in your cwd)

Answer 1

Moving from GtkWidget to GtkDrawingArea was required, as without it would crash. The root cause was that the ->window private variable of widget is not populated without a proper realize default callback - which I did not implement.