Widgets (1)

GtkLabel, GtkButton and GtkBox

GtkLabel

We made a window and showed it on the screen in the previous section. Now we go on to the next topic: widgets. The simplest widget is GtkLabel. It is a widget with text in it.

#include <gtk/gtk.h>

static void
app_activate (GApplication *app) {
  GtkWidget *win;
  GtkWidget *lab;

  win = gtk_application_window_new (GTK_APPLICATION (app));
  gtk_window_set_title (GTK_WINDOW (win), "lb1");
  gtk_window_set_default_size (GTK_WINDOW (win), 400, 300);

  lab = gtk_label_new ("Hello.");
  gtk_window_set_child (GTK_WINDOW (win), lab);

  gtk_window_present (GTK_WINDOW (win));
}

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

  app = gtk_application_new ("com.github.ToshioCP.lb1", G_APPLICATION_DEFAULT_FLAGS);
  g_signal_connect (app, "activate", G_CALLBACK (app_activate), NULL);
  stat =g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);
  return stat;
}

Save this program to a file lb1.c. (You can use src/misc/lb1.c if you’ve downloaded this repository.) Then compile and run it.

$ comp lb1
$ ./a.out

A window with a message “Hello.” appears.

There are only a few changes between pr4.c and lb1.c. A program diff is useful to know the difference.

$ cd misc; diff pr4.c lb1.c
4c4
< app_activate (GApplication *app, gpointer user_data) {
---
> app_activate (GApplication *app) {
5a6
>   GtkWidget *lab;
8c9
<   gtk_window_set_title (GTK_WINDOW (win), "pr4");
---
>   gtk_window_set_title (GTK_WINDOW (win), "lb1");
9a11,14
> 
>   lab = gtk_label_new ("Hello.");
>   gtk_window_set_child (GTK_WINDOW (win), lab);
> 
18c23
<   app = gtk_application_new ("com.github.ToshioCP.pr4", G_APPLICATION_DEFAULT_FLAGS);
---
>   app = gtk_application_new ("com.github.ToshioCP.lb1", G_APPLICATION_DEFAULT_FLAGS);

This tells us:

• A signal handler app_activate doesn’t have user_data parameter. If the fourth argument of g_signal_connect is NULL, you can leave out user_data.
• The definition of a new variable lab is added.
• The title of the window is changed.
• A label is created and connected to the window as a child.

The function gtk_window_set_child (GTK_WINDOW (win), lab) makes the label lab a child widget of the window win. Be careful. A child widget is different from a child object. Objects have parent-child relationships and widgets also have parent-child relationships. But these two relationships are totally different. Don’t be confused. In the program lb1.c, lab is a child widget of win. Child widgets are always located in their parent widget on the screen. See how the window has appeared on the screen. The application window includes the label.

The window win doesn’t have any parents. We call such a window top-level window. An application can have more than one top-level windows.

GtkButton

The next widget is GtkButton. It displays a button on the screen with a label or icon on it. In this subsection, we will make a button with a label. When the button is clicked, it emits a “clicked” signal. The following program shows how to catch the signal and do something.

#include <gtk/gtk.h>

static void
click_cb (GtkButton *btn) {
  g_print ("Clicked.\n");
}

static void
app_activate (GApplication *app) {
  GtkWidget *win;
  GtkWidget *btn;

  win = gtk_application_window_new (GTK_APPLICATION (app));
  gtk_window_set_title (GTK_WINDOW (win), "lb2");
  gtk_window_set_default_size (GTK_WINDOW (win), 400, 300);

  btn = gtk_button_new_with_label ("Click me");
  gtk_window_set_child (GTK_WINDOW (win), btn);
  g_signal_connect (btn, "clicked", G_CALLBACK (click_cb), NULL);

  gtk_window_present (GTK_WINDOW (win));
}

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

  app = gtk_application_new ("com.github.ToshioCP.lb2", G_APPLICATION_DEFAULT_FLAGS);
  g_signal_connect (app, "activate", G_CALLBACK (app_activate), NULL);
  stat =g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);
  return stat;
}

Look at the line 17 to 19. First, it creates a GtkButton instance btn with a label “Click me”. Then, adds the button to the window win as a child. Finally, connects the “clicked” signal of the button to the handler click_cb. So, if btn is clicked, the function click_cb is invoked. The suffix “cb” means “call back”.

Name the program lb2.c and save it. Now compile and run it.

A window with the button appears. Click the button (it is a large button, you can click everywhere in the window), then a string “Clicked.” appears on the terminal. It shows the handler was invoked by clicking the button.

It’s good that we’ve made sure that the clicked signal was caught and the handler was invoked by using g_print. However, using g_print is out of harmony with GTK, which is a GUI library. So, we will change the handler. The following code is extracted from lb3.c.

static void
click_cb (GtkButton *btn, GtkWindow *win) {
  gtk_window_destroy (win);
}

static void
app_activate (GApplication *app) {
  GtkWidget *win;
  GtkWidget *btn;

  win = gtk_application_window_new (GTK_APPLICATION (app));
  gtk_window_set_title (GTK_WINDOW (win), "lb3");
  gtk_window_set_default_size (GTK_WINDOW (win), 400, 300);

  btn = gtk_button_new_with_label ("Close");
  gtk_window_set_child (GTK_WINDOW (win), btn);
  g_signal_connect (btn, "clicked", G_CALLBACK (click_cb), win);

  gtk_window_present (GTK_WINDOW (win));
}

And the difference between lb2.c and lb3.c is as follows.

$ cd misc; diff lb2.c lb3.c
4,5c4,5
< click_cb (GtkButton *btn) {
<   g_print ("Clicked.\n");
---
> click_cb (GtkButton *btn, GtkWindow *win) {
>   gtk_window_destroy (win);
14c14
<   gtk_window_set_title (GTK_WINDOW (win), "lb2");
---
>   gtk_window_set_title (GTK_WINDOW (win), "lb3");
17c17
<   btn = gtk_button_new_with_label ("Click me");
---
>   btn = gtk_button_new_with_label ("Close");
19c19
<   g_signal_connect (btn, "clicked", G_CALLBACK (click_cb), NULL);
---
>   g_signal_connect (btn, "clicked", G_CALLBACK (click_cb), win);
29c29
<   app = gtk_application_new ("com.github.ToshioCP.lb2", G_APPLICATION_DEFAULT_FLAGS);
---
>   app = gtk_application_new ("com.github.ToshioCP.lb3", G_APPLICATION_DEFAULT_FLAGS);
35d34
< 

The changes are:

• The function g_print in lb2.c was deleted and two lines are inserted.
  • click_cb has the second parameter, which comes from the fourth argument of the g_signal_connect at line 17. One thing to be careful is the types are different between the second parameter of click_cb and the fourth argument of g_signal_connect. The former is GtkWindow * and the latter is GtkWidget *. The compiler doesn’t complain because g_signal_connect uses gpointer (general type of pointer). In this program the instance pointed by win is a GtkApplicationWindow object. It is a descendant of GtkWindow and GtkWidget class, so both GtkWindow * and GtkWidget * are correct types of the instance.
  • gtk_destroy (win) destroys the top-level window. Then the application quits.
• The label of btn is changed from “Click me” to “Close”.
• The fourth argument of g_signal_connect is changed from NULL to win.

The most important change is the fourth argument of the g_signal_connect. This argument is described as “data to pass to the handler” in the definition of g_signal_connect.

GtkBox

GtkWindow and GtkApplicationWindow can have only one child. If you want to add two or more widgets in a window, you need a container widget. GtkBox is one of the containers. It arranges two or more child widgets into a single row or column. The following procedure shows the way to add two buttons in a window.

• Create a GtkApplicationWindow instance.
• Create a GtkBox instance and add it to the GtkApplicationWindow as a child.
• Create a GtkButton instance and append it to the GtkBox.
• Create another GtkButton instance and append it to the GtkBox.

After this, the Widgets are connected as the following diagram.

The program lb4.c is as follows.

#include <gtk/gtk.h>

static void
click1_cb (GtkButton *btn) {
  const char *s;

  s = gtk_button_get_label (btn);
  if (g_strcmp0 (s, "Hello.") == 0)
    gtk_button_set_label (btn, "Good-bye.");
  else
    gtk_button_set_label (btn, "Hello.");
}

static void
click2_cb (GtkButton *btn, GtkWindow *win) {
  gtk_window_destroy (win);
}

static void
app_activate (GApplication *app) {
  GtkWidget *win;
  GtkWidget *box;
  GtkWidget *btn1;
  GtkWidget *btn2;

  win = gtk_application_window_new (GTK_APPLICATION (app));
  gtk_window_set_title (GTK_WINDOW (win), "lb4");
  gtk_window_set_default_size (GTK_WINDOW (win), 400, 300);

  box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 5);
  gtk_box_set_homogeneous (GTK_BOX (box), TRUE);
  gtk_window_set_child (GTK_WINDOW (win), box);

  btn1 = gtk_button_new_with_label ("Hello.");
  g_signal_connect (btn1, "clicked", G_CALLBACK (click1_cb), NULL);

  btn2 = gtk_button_new_with_label ("Close");
  g_signal_connect (btn2, "clicked", G_CALLBACK (click2_cb), win);

  gtk_box_append (GTK_BOX (box), btn1);
  gtk_box_append (GTK_BOX (box), btn2);

  gtk_window_present (GTK_WINDOW (win));
}

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

  app = gtk_application_new ("com.github.ToshioCP.lb4", G_APPLICATION_DEFAULT_FLAGS);
  g_signal_connect (app, "activate", G_CALLBACK (app_activate), NULL);
  stat =g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);
  return stat;
}

Look at the function app_activate.

After the creation of a GtkApplicationWindow instance, a GtkBox instance is created.

box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 5);
gtk_box_set_homogeneous (GTK_BOX (box), TRUE);

The first argument arranges the children of the box vertically. The orientation constants are defined like this:

• GTK_ORIENTATION_VERTICAL: the children widgets are arranged vertically
• GTK_ORIENTATION_HORIZONTAL: the children widgets are arranged horizontally

The second argument is the size of the space between the children. The unit of the length is pixel.

The next function fills the box with the children, giving them the same space.

After that, two buttons btn1 and btn2 are created and the signal handlers are set. Then, these two buttons are appended to the box.

static void
click1_cb (GtkButton *btn) {
  const char *s;

  s = gtk_button_get_label (btn);
  if (g_strcmp0 (s, "Hello.") == 0)
    gtk_button_set_label (btn, "Good-bye.");
  else
    gtk_button_set_label (btn, "Hello.");
}

The function gtk_button_get_label returns a text from the label. The string is owned by the button and you can’t modify or free it. The const qualifier is necessary for the string s. If you change the string, your compiler will give you a waring.

You always need to be careful with the const qualifier when you see the GTK 4 API reference.

The handler corresponding to btn1 toggles its label. The handler corresponding to btn2 destroys the top-level window and the application quits.