GTK 4 tutorial

The tfe program in the previous section is not so good because many things are crammed into tfepplication.c. Many static variables in tfepplication.c shows that.

static GtkDialog *pref;
static GtkFontButton *fontbtn;
static GSettings *settings;
static GtkDialog *alert;
static GtkLabel *lb_alert;
static GtkButton *btn_accept;

static gulong pref_close_request_handler_id = 0;
static gulong alert_close_request_handler_id = 0;
static gboolean is_quit;

Generally, if there are many global or static variables in the program, it is not a good program. Such programs are difficult to maintain.

The file tfeapplication.c should be divided into several files.

tfeapplication.c only has codes related to GtkApplication.
A file for GtkApplicationWindow
A file for a preference dialog
A file for an alert dialog

The preference dialog is defined by a ui file. And it has GtkBox, GtkLabel and GtkFontButton in it. Such widget is called composite widget. Composite widget is a child object (not child widget) of a widget. For example, the preference composite widget is a child object of GtkDialog. Composite widget can be built from template XML. Next subsection shows how to build a preference dialog.

Preference dialog

First, write a template XML file.

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="TfePref" parent="GtkDialog">
    <property name="title">Preferences</property>
    <property name="resizable">FALSE</property>
    <property name="modal">TRUE</property>
    <child internal-child="content_area">
      <object class="GtkBox" id="content_area">
        <child>
          <object class="GtkBox" id="pref_boxh">
            <property name="orientation">GTK_ORIENTATION_HORIZONTAL</property>
            <property name="spacing">12</property>
            <property name="margin-start">12</property>
            <property name="margin-end">12</property>
            <property name="margin-top">12</property>
            <property name="margin-bottom">12</property>
            <child>
              <object class="GtkLabel" id="fontlabel">
                <property name="label">Font:</property>
                <property name="xalign">1</property>
              </object>
            </child>
            <child>
              <object class="GtkFontButton" id="fontbtn">
              </object>
            </child>
          </object>
        </child>
      </object>
    </child>
  </template>
</interface>

3: Template tag specifies a composite widget. The value of a class attribute is the object name of the composite widget. This XML file names the object “TfePref”. It is defined in a C source file and it will be shown later. A parent attribute specifies the direct parent object of the composite widget. TfePref is a child object of GtkDialog. Therefore the value of the attribute is “GtkDialog”. A parent attribute is optional but it is recommended to specify.

Other lines are the same as before. The object TfePref is defined in tfepref.h and tfepref.c.

#ifndef __TFE_PREF_H__
#define __TFE_PREF_H__

#include <gtk/gtk.h>

#define TFE_TYPE_PREF tfe_pref_get_type ()
G_DECLARE_FINAL_TYPE (TfePref, tfe_pref, TFE, PREF, GtkDialog)

GtkWidget *
tfe_pref_new (GtkWindow *win);

#endif /* __TFE_PREF_H__ */

6-7: When you define a new object, you need to write these two lines. Refer to Section 8.
9-10: tfe_pref_new creates a new TfePref object. It has a parameter win which is used as a transient parent window to show the dialog.

#include "tfepref.h"

struct _TfePref
{
  GtkDialog parent;
  GSettings *settings;
  GtkFontButton *fontbtn;
};

G_DEFINE_TYPE (TfePref, tfe_pref, GTK_TYPE_DIALOG);

static void
tfe_pref_dispose (GObject *gobject) {
  TfePref *pref = TFE_PREF (gobject);

  g_clear_object (&pref->settings);
  G_OBJECT_CLASS (tfe_pref_parent_class)->dispose (gobject);
}

static void
tfe_pref_init (TfePref *pref) {
  gtk_widget_init_template (GTK_WIDGET (pref));
  pref->settings = g_settings_new ("com.github.ToshioCP.tfe");
  g_settings_bind (pref->settings, "font", pref->fontbtn, "font", G_SETTINGS_BIND_DEFAULT);
}

static void
tfe_pref_class_init (TfePrefClass *class) {
  GObjectClass *object_class = G_OBJECT_CLASS (class);

  object_class->dispose = tfe_pref_dispose;
  gtk_widget_class_set_template_from_resource (GTK_WIDGET_CLASS (class), "/com/github/ToshioCP/tfe/tfepref.ui");
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), TfePref, fontbtn);
}

GtkWidget *
tfe_pref_new (GtkWindow *win) {
  return GTK_WIDGET (g_object_new (TFE_TYPE_PREF, "transient-for", win, NULL));
}

3-8: The structure of an instance of this object. It has two variables, settings and fontbtn.
10: G_DEFINE_TYPE macro. This macro registers the TfePref type.
12-18: Dispose handler. This handler is called when the instance is destroyed. The destruction process has two stages, disposing and finalizing. When disposing, the instance releases all the references (to the other instances). TfePref object holds a reference to the GSettings instance. It is released in line 16. After that parents dispose handler is called in line 17. For further information about destruction process, refer to Section 11.
27-34: Class initialization function.
31: Set the dispose handler.
32: gtk_widget_class_set_template_from_resource function associates the description in the XML file (tfepref.ui) with the widget. At this moment no instance is created. It just make the class to know the structure of the object. That’s why the top level tag is not <object> but <template> in the XML file.
33: gtk_widget_class_bind_template_child function binds a private variable of the object with a child object in the template. This function is a macro. The name of the private variable (fontbtn in line 7) and the id fontbtn in the XML file (line 24) must be the same. The pointer to the instance will be assigned to the variable fontbtn when the instance is created.
20-25: Instance initialization function.
22: Creates the instance based on the template in the class. The template has been made during the class initialization process.
23: Create GSettings instance with the id “com.github.ToshioCP.tfe”.
24: Bind the font key in the GSettings object to the font property in the GtkFontButton.
36-39: The function tfe_pref_new creates an instance of TfePref. The parameter win is a transient parent.

Now, It is very simple to use this dialog. A caller just creates this object and shows it.

TfePref *pref;
pref = tfe_pref_new (win) /* win is the top-level window */
gtk_widget_show (GTK_WINDOW (win));

This instance is automatically destroyed when a user clicks on the close button. That’s all. If you want to show the dialog again, just create and show it.

Alert dialog

It is almost same as preference dialog.

Its XML file is:

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="TfeAlert" parent="GtkDialog">
    <property name="title">Are you sure?</property>
    <property name="resizable">FALSE</property>
    <property name="modal">TRUE</property>
    <child internal-child="content_area">
      <object class="GtkBox">
        <child>
          <object class="GtkBox">
            <property name="orientation">GTK_ORIENTATION_HORIZONTAL</property>
            <property name="spacing">12</property>
            <property name="margin-start">12</property>
            <property name="margin-end">12</property>
            <property name="margin-top">12</property>
            <property name="margin-bottom">12</property>
            <child>
              <object class="GtkImage">
                <property name="icon-name">dialog-warning</property>
                <property name="icon-size">GTK_ICON_SIZE_LARGE</property>
              </object>
            </child>
            <child>
              <object class="GtkLabel" id="lb_alert">
              </object>
            </child>
          </object>
        </child>
      </object>
    </child>
    <child type="action">
      <object class="GtkButton" id="btn_cancel">
        <property name="label">Cancel</property>
      </object>
    </child>
    <child type="action">
      <object class="GtkButton" id="btn_accept">
        <property name="label">Close</property>
      </object>
    </child>
    <action-widgets>
      <action-widget response="cancel" default="true">btn_cancel</action-widget>
      <action-widget response="accept">btn_accept</action-widget>
    </action-widgets>
  </template>
</interface>

The header file is:

#ifndef __TFE_ALERT_H__
#define __TFE_ALERT_H__

#include <gtk/gtk.h>

#define TFE_TYPE_ALERT tfe_alert_get_type ()
G_DECLARE_FINAL_TYPE (TfeAlert, tfe_alert, TFE, ALERT, GtkDialog)

void
tfe_alert_set_message (TfeAlert *alert, const char *message);

void
tfe_alert_set_button_label (TfeAlert *alert, const char *label);

GtkWidget *
tfe_alert_new (GtkWindow *win);

#endif /* __TFE_ALERT_H__ */

There are three public functions. The functions tfe_alert_set_message and tfe_alert_set_button_label sets the label and button name of the alert dialog. For example, if you want to show an alert that the user tries to close without saving the content, set them like:

tfe_alert_set_message (alert, "Are you really close without saving?"); /* alert points to a TfeAlert instance */
tfe_alert_set_button_label (alert, "Close");

The function tfe_alert_new creates a TfeAlert dialog.

The C source file is:

#include "tfealert.h"

struct _TfeAlert
{
  GtkDialog parent;
  GtkLabel *lb_alert;
  GtkButton *btn_accept;
};

G_DEFINE_TYPE (TfeAlert, tfe_alert, GTK_TYPE_DIALOG);

void
tfe_alert_set_message (TfeAlert *alert, const char *message) {
  gtk_label_set_text (alert->lb_alert, message);
}

void
tfe_alert_set_button_label (TfeAlert *alert, const char *label) {
  gtk_button_set_label (alert->btn_accept, label);
}

static void
tfe_alert_init (TfeAlert *alert) {
  gtk_widget_init_template (GTK_WIDGET (alert));
}

static void
tfe_alert_class_init (TfeAlertClass *class) {
  gtk_widget_class_set_template_from_resource (GTK_WIDGET_CLASS (class), "/com/github/ToshioCP/tfe/tfealert.ui");
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), TfeAlert, lb_alert);
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), TfeAlert, btn_accept);
}

GtkWidget *
tfe_alert_new (GtkWindow *win) {
  return GTK_WIDGET (g_object_new (TFE_TYPE_ALERT, "transient-for", win, NULL));
}

The program is almost same as tfepref.c.

The instruction how to use this object is as follows.

Write a “response” signal handler.
Create a TfeAlert object.
Connect “response” signal to a handler
Show the dialog
In the signal handler, do something with regard to the response-id. Then destroy the dialog.

Top-level window

In the same way, create a child object of GtkApplicationWindow. The object name is “TfeWindow”.

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="TfeWindow" parent="GtkApplicationWindow">
    <property name="title">file editor</property>
    <property name="default-width">600</property>
    <property name="default-height">400</property>
    <child>
      <object class="GtkBox" id="boxv">
        <property name="orientation">GTK_ORIENTATION_VERTICAL</property>
        <child>
          <object class="GtkBox" id="boxh">
            <property name="orientation">GTK_ORIENTATION_HORIZONTAL</property>
            <child>
              <object class="GtkLabel" id="dmy1">
                <property name="width-chars">10</property>
              </object>
            </child>
            <child>
              <object class="GtkButton" id="btno">
                <property name="label">Open</property>
                <property name="action-name">win.open</property>
              </object>
            </child>
            <child>
              <object class="GtkButton" id="btns">
                <property name="label">Save</property>
                <property name="action-name">win.save</property>
              </object>
            </child>
            <child>
              <object class="GtkLabel" id="dmy2">
                <property name="hexpand">TRUE</property>
              </object>
            </child>
            <child>
              <object class="GtkButton" id="btnc">
                <property name="label">Close</property>
                <property name="action-name">win.close</property>
              </object>
            </child>
            <child>
              <object class="GtkMenuButton" id="btnm">
                <property name="direction">down</property>
                <property name="halign">start</property>
                <property name="icon-name">open-menu-symbolic</property>
              </object>
            </child>
            <child>
              <object class="GtkLabel" id="dmy3">
                <property name="width-chars">10</property>
              </object>
            </child>
          </object>
        </child>
        <child>
          <object class="GtkNotebook" id="nb">
            <property name="scrollable">TRUE</property>
            <property name="hexpand">TRUE</property>
            <property name="vexpand">TRUE</property>
          </object>
        </child>
      </object>
    </child>
  </template>
</interface>

This XML file is almost same as before except template tag and “action-name” property in buttons.

GtkButton implements GtkActionable interface, which has “action-name” property. If this property is set, GtkButton activates the action when it is clicked. For example, if an open button is clicked, “win.open” action will be activated and open_activated handler will be invoked.

This action is also used by “<Control>o” accelerator (See the source code of tfewindow.c below). If you use “clicked” signal for the button, you need its signal handler. Then, there are two handlers:

a handler for the “clicked” signal on the button
a handler for the “activate” signal on the “win.open” action, to which “<Control>o” accelerator is connected

These two handlers do almost same thing. It is inefficient. Connecting buttons to actions is a good way to reduce unnecessary codes.

#ifndef __TFE_WINDOW_H__
#define __TFE_WINDOW_H__

#include <gtk/gtk.h>

#define TFE_TYPE_WINDOW tfe_window_get_type ()
G_DECLARE_FINAL_TYPE (TfeWindow, tfe_window, TFE, WINDOW, GtkApplicationWindow)

void
tfe_window_notebook_page_new (TfeWindow *win);

void
tfe_window_notebook_page_new_with_files (TfeWindow *win, GFile **files, int n_files);

GtkWidget *
tfe_window_new (GtkApplication *app);

#endif /* __TFE_WINDOW_H__ */

There are three public functions. The function tfe_window_notebook_page_new creates a new notebook page. This is a wrapper function for notebook_page_new. It is called by GtkApplication object. The function tfe_window_notebook_page_new_with_files creates notebook pages with a contents read from the given files. The function tfe_window_new creates a TfeWindow instance.

#include "tfewindow.h"
#include "tfenotebook.h"
#include "tfepref.h"
#include "tfealert.h"
#include "css.h"

struct _TfeWindow {
  GtkApplicationWindow parent;
  GtkMenuButton *btnm;
  GtkNotebook *nb;
  GSettings *settings;
  gboolean is_quit;
};

G_DEFINE_TYPE (TfeWindow, tfe_window, GTK_TYPE_APPLICATION_WINDOW);

/* alert response signal handler */
static void
alert_response_cb (GtkDialog *alert, int response_id, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);

  if (response_id == GTK_RESPONSE_ACCEPT) {
    if (win->is_quit)
      gtk_window_destroy(GTK_WINDOW (win));
    else
      notebook_page_close (win->nb);
  }
  gtk_window_destroy (GTK_WINDOW (alert));
}

/* ----- action activated handlers ----- */
static void
open_activated (GSimpleAction *action, GVariant *parameter, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);

  notebook_page_open (GTK_NOTEBOOK (win->nb));
}

static void
save_activated (GSimpleAction *action, GVariant *parameter, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);

  notebook_page_save (GTK_NOTEBOOK (win->nb));
}

static void
close_activated (GSimpleAction *action, GVariant *parameter, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);
  TfeAlert *alert;

  if (has_saved (win->nb))
    notebook_page_close (win->nb);
  else {
    win->is_quit = false;
    alert = TFE_ALERT (tfe_alert_new (GTK_WINDOW (win)));
    tfe_alert_set_message (alert, "Contents aren't saved yet.\nAre you sure to close?");
    tfe_alert_set_button_label (alert, "Close");
    g_signal_connect (GTK_DIALOG (alert), "response", G_CALLBACK (alert_response_cb), win);
    gtk_widget_show (GTK_WIDGET (alert));
  }
}

static void
new_activated (GSimpleAction *action, GVariant *parameter, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);

  notebook_page_new (GTK_NOTEBOOK (win->nb));
}

static void
saveas_activated (GSimpleAction *action, GVariant *parameter, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);

  notebook_page_saveas (GTK_NOTEBOOK (win->nb));
}

static void
pref_activated (GSimpleAction *action, GVariant *parameter, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);
  GtkWidget *pref;

  pref = tfe_pref_new (GTK_WINDOW (win));
  gtk_widget_show (pref);
}

static void
quit_activated (GSimpleAction *action, GVariant *parameter, gpointer user_data) {
  TfeWindow *win = TFE_WINDOW (user_data);

  TfeAlert *alert;

  if (has_saved_all (GTK_NOTEBOOK (win->nb)))
    gtk_window_destroy (GTK_WINDOW (win));
  else {
    win->is_quit = true;
    alert = TFE_ALERT (tfe_alert_new (GTK_WINDOW (win)));
    tfe_alert_set_message (alert, "Contents aren't saved yet.\nAre you sure to quit?");
    tfe_alert_set_button_label (alert, "Quit");
    g_signal_connect (GTK_DIALOG (alert), "response", G_CALLBACK (alert_response_cb), win);
    gtk_widget_show (GTK_WIDGET (alert));
  }
}

/* gsettings changed::font signal handler */
static void
changed_font_cb (GSettings *settings, char *key, gpointer user_data) {
  GtkWindow *win = GTK_WINDOW (user_data); 
  char *font;
  PangoFontDescription *pango_font_desc;

  font = g_settings_get_string (settings, "font");
  pango_font_desc = pango_font_description_from_string (font);
  g_free (font);
  set_font_for_display_with_pango_font_desc (win, pango_font_desc);
}

/* --- public functions --- */

void
tfe_window_notebook_page_new (TfeWindow *win) {
  notebook_page_new (win->nb);
}

void
tfe_window_notebook_page_new_with_files (TfeWindow *win, GFile **files, int n_files) {
  int i;

  for (i = 0; i < n_files; i++)
    notebook_page_new_with_file (win->nb, files[i]);
  if (gtk_notebook_get_n_pages (win->nb) == 0)
    notebook_page_new (win->nb);
}

/* --- TfeWindow object construction/destruction --- */ 
static void
tfe_window_dispose (GObject *gobject) {
  TfeWindow *window = TFE_WINDOW (gobject);

  g_clear_object (&window->settings);
  G_OBJECT_CLASS (tfe_window_parent_class)->dispose (gobject);
}

static void
tfe_window_init (TfeWindow *win) {
  GtkBuilder *build;
  GMenuModel *menu;

  gtk_widget_init_template (GTK_WIDGET (win));

  build = gtk_builder_new_from_resource ("/com/github/ToshioCP/tfe/menu.ui");
  menu = G_MENU_MODEL (gtk_builder_get_object (build, "menu"));
  gtk_menu_button_set_menu_model (win->btnm, menu);
  g_object_unref(build);

  win->settings = g_settings_new ("com.github.ToshioCP.tfe");
  g_signal_connect (win->settings, "changed::font", G_CALLBACK (changed_font_cb), win);

/* ----- action ----- */
  const GActionEntry win_entries[] = {
    { "open", open_activated, NULL, NULL, NULL },
    { "save", save_activated, NULL, NULL, NULL },
    { "close", close_activated, NULL, NULL, NULL },
    { "new", new_activated, NULL, NULL, NULL },
    { "saveas", saveas_activated, NULL, NULL, NULL },
    { "pref", pref_activated, NULL, NULL, NULL },
    { "close-all", quit_activated, NULL, NULL, NULL }
  };
  g_action_map_add_action_entries (G_ACTION_MAP (win), win_entries, G_N_ELEMENTS (win_entries), win);

  changed_font_cb(win->settings, "font", win);
}

static void
tfe_window_class_init (TfeWindowClass *class) {
  GObjectClass *object_class = G_OBJECT_CLASS (class);

  object_class->dispose = tfe_window_dispose;
  gtk_widget_class_set_template_from_resource (GTK_WIDGET_CLASS (class), "/com/github/ToshioCP/tfe/tfewindow.ui");
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), TfeWindow, btnm);
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), TfeWindow, nb);
}

GtkWidget *
tfe_window_new (GtkApplication *app) {
  return GTK_WIDGET (g_object_new (TFE_TYPE_WINDOW, "application", app, NULL));
}

17-29: alert_response_cb is a call back function of the “response” signal of TfeAlert dialog. This is the same as before except gtk_window_destroy(GTK_WINDOW (win)) is used instead of tfe_application_quit.
31-102: Handlers of action activated signal. The user_data is a pointer to TfeWindow instance.
104-115: A handler of “changed::font” signal of GSettings object.
111: Gets the font from GSettings data.
112: Gets a PangoFontDescription from the font. In the previous version, the program gets the font description from the GtkFontButton. The button data and GSettings data are the same. Therefore, the data got here is the same as the data in the GtkFontButton. In addition, we don’t need to worry about the preference dialog is alive or not thanks to the GSettings.
114: Sets CSS on the display with the font description.
117-132: Public functions.
134-141: Dispose handler. The GSettings object needs to be released.
143-171: Instance initialization function.
148: Creates a composite widget instance with the template.
151-153: Insert menu to the menu button.
155-156: Creates a GSettings instance with the id “com.github.ToshioCP.tfe”. Connects “changed::font” signal to the handler changed_font_cb. This signal emits when the GSettings data is changed. The second part “font” of the signal name “changed::font” is called details. Signals can have details. If a GSettings instance has more than one key, “changed” signal emits only if the key, which has the same name as the detail, changes its value. For example, Suppose a GSettings object has three keys “a”, “b” and “c”.
- “changed::a” is emitted when the value of the key “a” is changed. It isn’t emitted when the value of “b” or “c” is changed.
- “changed::b” is emitted when the value of the key “b” is changed. It isn’t emitted when the value of “a” or “c” is changed.
- “changed::c” is emitted when the value of the key “c” is changed. It isn’t emitted when the value of “a” or “b” is changed. In this version of tfe, there is only one key (“font”). So, even if the signal doesn’t have a detail, the result is the same. But in the future version, it will probably need details.
158-168: Creates actions.
170: Sets CSS font.
173-181: Class initialization function.
177: Sets the dispose handler.
178: Sets the composite widget template
179-180: Binds private variable with child objects in the template.
183-186: tfe_window_new. This function creates TfeWindow instance.

TfeApplication

The file tfeapplication.c is now very simple.

#include "tfewindow.h"

/* ----- activate, open, startup handlers ----- */
static void
app_activate (GApplication *application) {
  GtkApplication *app = GTK_APPLICATION (application);
  GtkWidget *win = GTK_WIDGET (gtk_application_get_active_window (app));

  tfe_window_notebook_page_new (TFE_WINDOW (win));
  gtk_widget_show (GTK_WIDGET (win));
}

static void
app_open (GApplication *application, GFile ** files, gint n_files, const gchar *hint) {
  GtkApplication *app = GTK_APPLICATION (application);
  GtkWidget *win = GTK_WIDGET (gtk_application_get_active_window (app));

  tfe_window_notebook_page_new_with_files (TFE_WINDOW (win), files, n_files);
  gtk_widget_show (win);
}

static void
app_startup (GApplication *application) {
  GtkApplication *app = GTK_APPLICATION (application);
  int i;

  tfe_window_new (app);

/* ----- accelerator ----- */ 
  struct {
    const char *action;
    const char *accels[2];
  } action_accels[] = {
    { "win.open", { "<Control>o", NULL } },
    { "win.save", { "<Control>s", NULL } },
    { "win.close", { "<Control>w", NULL } },
    { "win.new", { "<Control>n", NULL } },
    { "win.saveas", { "<Shift><Control>s", NULL } },
    { "win.close-all", { "<Control>q", NULL } },
  };

  for (i = 0; i < G_N_ELEMENTS(action_accels); i++)
    gtk_application_set_accels_for_action(GTK_APPLICATION(app), action_accels[i].action, action_accels[i].accels);
}

/* ----- main ----- */

#define APPLICATION_ID "com.github.ToshioCP.tfe"

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

  app = gtk_application_new (APPLICATION_ID, G_APPLICATION_HANDLES_OPEN);

  g_signal_connect (app, "startup", G_CALLBACK (app_startup), NULL);
  g_signal_connect (app, "activate", G_CALLBACK (app_activate), NULL);
  g_signal_connect (app, "open", G_CALLBACK (app_open), NULL);

  stat =g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);
  return stat;
}

4-11: Activate signal handler. It uses tfe_window_notebook_page_new instead of notebook_page_new.
13-20: Open signal handler. Thanks to tfe_window_notebook_page_new_with_files, this handler becomes very simple.
22-44: Startup signal handler. Most of the tasks are moved to TfeWindow, the remaining tasks are creating a window and setting accelerations.
48-64: A function main.

Other files

Resource XML file.

<?xml version="1.0" encoding="UTF-8"?>
<gresources>
  <gresource prefix="/com/github/ToshioCP/tfe">
    <file>tfewindow.ui</file>
    <file>tfepref.ui</file>
    <file>tfealert.ui</file>
    <file>menu.ui</file>
  </gresource>
</gresources>

GSchema XML file

<?xml version="1.0" encoding="UTF-8"?>
<schemalist>
  <schema path="/com/github/ToshioCP/tfe/" id="com.github.ToshioCP.tfe">
    <key name="font" type="s">
      <default>'Monospace 12'</default>
      <summary>Font</summary>
      <description>The font to be used for textview.</description>
    </key>
  </schema>
</schemalist>

Meson.build

project('tfe', 'c')

gtkdep = dependency('gtk4')

gnome=import('gnome')
resources = gnome.compile_resources('resources','tfe.gresource.xml')
gnome.compile_schemas(build_by_default: true, depend_files: 'com.github.ToshioCP.tfe.gschema.xml')

sourcefiles=files('tfeapplication.c', 'tfewindow.c', 'tfenotebook.c', 'tfepref.c', 'tfealert.c', 'css.c', '../tfetextview/tfetextview.c')

executable('tfe', sourcefiles, resources, dependencies: gtkdep, export_dynamic: true, install: true)

schema_dir = get_option('prefix') / get_option('datadir') / 'glib-2.0/schemas/'
install_data('com.github.ToshioCP.tfe.gschema.xml', install_dir: schema_dir)
meson.add_install_script('glib-compile-schemas', schema_dir)

Compilation and installation.

If you build GTK 4 from the source, use --prefix option.

$ meson --prefix=$HOME/local _build
$ ninja -C _build
$ ninja -C _build install

If you install GTK 4 from the distribution packages, you don’t need the prefix option. Maybe you need root privilege to install it.

$ meson _build
$ ninja -C _build
$ ninja -C _build install  # or 'sudo ninja -C _build install'

Source files are in src/tfe7 directory.