Getting Started with GTK+

Basics
Packing
Building user interfaces
Building applications
A trivial application
Populating the window
Opening files
An application menu
A preference dialog
Adding a search bar
Adding a side bar
Properties
Header bar
Custom Drawing

GTK+ is a widget toolkit. Each user interface created by GTK+ consists of widgets. This is implemented in C using GObject, an object-oriented framework for C. Widgets are organized in a hierachy. The window widget is the main container. The user interface is then built by adding buttons, drop-down menus, input fields, and other widgets to the window. If you are creating complex user interfaces it is recommended to use GtkBuilder and its GTK-specific markup description language, instead of assembling the interface manually. You can also use a visual user interface editor, like Glade.

GTK+ is event-driven. The toolkit listens for events such as a click on a button, and passes the event to your application.

This chapter contains some tutorial information to get you started with GTK+ programming. It assumes that you have GTK+, its dependencies and a C compiler installed and ready to use. If you need to build GTK+ itself first, refer to the Compiling the GTK+ libraries section in this reference.

Basics

To begin our introduction to GTK, we'll start with a simple signal-based Gtk application. This program will create an empty 200 × 200 pixel window.

Create a new file with the following content named example-0.c.

#include <gtk/gtk.h>

static void
activate (GtkApplication* app,
          gpointer        user_data)
{
  GtkWidget *window;

  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");
  gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);
  gtk_widget_show_all (window);
}

int
main (int    argc,
      char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

You can compile the program above with GCC using:


        gcc `pkg-config --cflags gtk+-3.0` -o example-0 example-0.c `pkg-config --libs gtk+-3.0`
      

For more information on how to compile a GTK+ application, please refer to the Compiling GTK+ Applications section in this reference.

All GTK+ applications will, of course, include gtk/gtk.h, which declares functions, types and macros required by GTK+ applications.

Even if GTK+ installs multiple header files, only the top-level gtk/gtk.h header can be directly included by third party code. The compiler will abort with an error if any other header is directly included.

In a GTK+ application, the purpose of the main() function is to create a GtkApplication object and run it. In this example a GtkApplication pointer named app is called and then initialized using gtk_application_new().

When creating a GtkApplication you need to pick an application identifier (a name) and input to gtk_application_new() as parameter. For this example org.gtk.example is used but for choosing an identifier for your application see this guide. Lastly gtk_application_new() takes a GApplicationFlags as input for your application, if your application would have special needs.

Next the activate signal is connected to the activate() function above the main() functions. The activate signal will be sent when your application is launched with g_application_run() on the line below. The gtk_application_run() also takes as arguments the pointers to the command line arguments counter and string array; this allows GTK+ to parse specific command line arguments that control the behavior of GTK+ itself. The parsed arguments will be removed from the array, leaving the unrecognized ones for your application to parse.

Within g_application_run the activate() signal is sent and we then proceed into the activate() function of the application. Inside the activate() function we want to construct our GTK window, so that a window is shown when the application is launched. The call to gtk_application_window_new() will createa a new GtkWindow and store it inside the window pointer. The window will have a frame, a title bar, and window controls depending on the platform.

A window title is set using gtk_window_set_title(). This function takes a GtkWindow* pointer and a string as input. As our window pointer is a GtkWidget pointer, we need to cast it to GtkWindow*. But instead of casting window via (GtkWindow*), window can be cast using the macro GTK_WINDOW(). GTK_WINDOW() will check if the pointer is an instance of the GtkWindow class, before casting, and emit a warning if the check fails. More information about this convention can be found here.

Finally the window size is set using gtk_window_set_default_size and the window is then shown by GTK via gtk_widget_show_all().

When you exit the window, by for example pressing the X, the g_application_run() in the main loop returns with a number which is saved inside an integer named "status". Afterwards, the GtkApplication object is freed from memory with g_object_unref(). Finally the status integer is returned and the GTK application exits.

While the program is running, GTK+ is receiving events. These are typically input events caused by the user interacting with your program, but also things like messages from the window manager or other applications. GTK+ processes these and as a result, signals may be emitted on your widgets. Connecting handlers for these signals is how you normally make your program do something in response to user input.

The following example is slightly more complex, and tries to showcase some of the capabilities of GTK+.

In the long tradition of programming languages and libraries, it is called Hello, World.

Example 1. Hello World in GTK+

Create a new file with the following content named example-1.c.

#include <gtk/gtk.h>

/* This is a callback function. The data arguments are ignored
 * in this example. More on callbacks below.
 */
static void
print_hello (GtkWidget *widget,
             gpointer   data)
{
  g_print ("Hello World\n");
}

static gboolean
on_delete_event (GtkWidget *widget,
                 GdkEvent  *event,
                 gpointer   data)
{
  /* If you return FALSE in the "delete_event" signal handler,
   * GTK will emit the "destroy" signal. Returning TRUE means
   * you don't want the window to be destroyed.
   *
   * This is useful for popping up 'are you sure you want to quit?'
   * type dialogs.
   */

  g_print ("delete event occurred\n");

  return TRUE;
}

int
main (int   argc,
      char *argv[])
{
  /* GtkWidget is the storage type for widgets */
  GtkWidget *window;
  GtkWidget *button;

  /* This is called in all GTK applications. Arguments are parsed
   * from the command line and are returned to the application.
   */
  gtk_init (&argc, &argv);

  /* create a new window, and set its title */
  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_window_set_title (GTK_WINDOW (window), "Hello");

  /* When the window emits the "delete-event" signal (which is emitted
   * by GTK+ in response to an event coming from the window manager,
   * usually as a result of clicking the "close" window control), we
   * ask it to call the on_delete_event() function as defined above.
   *
   * The data passed to the callback function is NULL and is ignored
   * in the callback function.
   */
  g_signal_connect (window, "delete-event", G_CALLBACK (on_delete_event), NULL);

  /* Here we connect the "destroy" event to the gtk_main_quit() function.
   *
   * This signal is emitted when we call gtk_widget_destroy() on the window,
   * or if we return FALSE in the "delete_event" callback.
   */
  g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);

  /* Sets the border width of the window. */
  gtk_container_set_border_width (GTK_CONTAINER (window), 10);

  /* Creates a new button with the label "Hello World". */
  button = gtk_button_new_with_label ("Hello World");

  /* When the button receives the "clicked" signal, it will call the
   * function print_hello() passing it NULL as its argument.
   *
   * The print_hello() function is defined above.
   */
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  /* The g_signal_connect_swapped() function will connect the "clicked" signal
   * of the button to the gtk_widget_destroy() function; instead of calling it
   * using the button as its argument, it will swap it with the user data
   * argument. This will cause the window to be destroyed by calling
   * gtk_widget_destroy() on the window.
   */
  g_signal_connect_swapped (button, "clicked", G_CALLBACK (gtk_widget_destroy), window);

  /* This packs the button into the window. A GtkWindow inherits from GtkBin,
   * which is a special container that can only have one child
   */
  gtk_container_add (GTK_CONTAINER (window), button);

  /* The final step is to display this newly created widget... */
  gtk_widget_show (button);

  /* ... and the window */
  gtk_widget_show (window);

  /* All GTK applications must have a gtk_main(). Control ends here
   * and waits for an event to occur (like a key press or a mouse event),
   * until gtk_main_quit() is called.
   */
  gtk_main ();

  return 0;
}

You can compile the program above with GCC using:


        gcc `pkg-config --cflags gtk+-3.0` -o example-1 example-1.c `pkg-config --libs gtk+-3.0`