gtk2/docs/reference/gtk/getting_started.md

# Getting Started with GTK {#gtk-getting-started}

GTK is a [widget toolkit](http://en.wikipedia.org/wiki/Widget_toolkit).
Each user interface created by GTK consists of widgets. This is implemented
in C using [GObject](#gobject), an object-oriented framework for C. Widgets
are organized in a hierarchy. 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](https://glade.gnome.org/).

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](#gtk-compiling) section in this
reference.

## Basics

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

![A window](window-default.png)

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

``` {.c source=examples/window-default.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 (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 gtk4` -o example-0 example-0.c `pkg-config --libs gtk4`
```

For more information on how to compile a GTK application, please
refer to the [Compiling GTK Applications](#gtk-compiling)
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 othe 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 declared and then initialized using gtk_application_new().

When creating a GtkApplication, you need to pick an application identifier
(a name) and pass it to gtk_application_new() as parameter. For this example
`org.gtk.example` is used. For choosing an identifier for your application, see
[this guide](https://wiki.gnome.org/HowDoI/ChooseApplicationID). Lastly,
gtk_application_new() takes GApplicationFlags as input for your application,
if your application would have special needs.

Next the [activate signal](https://wiki.gnome.org/HowDoI/GtkApplication) is
connected to the activate() function above the main() function. The `activate`
signal will be emitted when your application is launched with g_application_run()
on the line below. The g_application_run() call also takes as arguments the
command line arguments (the `argc` count and the `argv` string array).
Your application can override the command line handling, e.g. to open
files passed on the commandline.

Within g_application_run() the activate signal is sent and we then proceed
into the activate() function of the application. This is where we construct
our GTK window, so that a window is shown when the application is launched.
The call to gtk_application_window_new() will create 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](https://developer.gnome.org/gobject/stable/gtype-conventions.html).

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

When you close the window, by for example pressing the X, the g_application_run()
call returns with a number which is saved inside an integer variable named
`status`. Afterwards, the GtkApplication object is freed from memory with
g_object_unref(). Finally the status integer is returned and the 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.

## Hello, World

In the long tradition of programming languages and libraries,
this example is called *Hello, World*.

![Hello, world](hello-world.png)

### Hello World in C {#gtk-getting-started-hello-world}

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

``` {.c source=examples/hello-world.c }
#include <gtk/gtk.h>

static void
print_hello (GtkWidget *widget,
             gpointer   data)
{
  g_print ("Hello World\n");
}

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

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

  box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
  gtk_window_set_child (GTK_WINDOW (window), box);

  button = gtk_button_new_with_label ("Hello World");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);
  g_signal_connect_swapped (button, "clicked", G_CALLBACK (gtk_window_destroy), window);
  gtk_box_append (GTK_BOX (box), button);

  gtk_widget_show (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 gtk4` -o example-1 example-1.c `pkg-config --libs gtk4`
```

As seen above, example-1.c builds further upon example-0.c by adding a
button to our window, with the label "Hello World". Two new GtkWidget
pointers are declared to accomplish this, `button` and `box`. The box
variable is created to store a GtkBox, which is GTK's way of controlling
the size and layout of buttons.

The GtkBox is created with gtk_box_new() which takes a GtkOrientation
enum as parameter. The buttons which this box will contain can either be layed
out horizontally or vertically. This does not matter in this particular case,
as we are dealing with only one button. After initializing box with the newly
created GtkBox, the code adds the box widget to the window widget using
gtk_window_set_child().

Next the `button` variable is initialized in similar manner.
gtk_button_new_with_label() is called which returns a GtkButton to be
stored in `button`. Afterwards `button` is added to our `box`.

Using g_signal_connect(), the button is connected to a function in our app called
print_hello(), so that when the button is clicked, GTK will call this function.
As the print_hello() function does not use any data as input, NULL is passed
to it. print_hello() calls g_print() with the string "Hello World" which will
print Hello World in a terminal if the GTK application was started from one.

After connecting print_hello(), another signal is connected to the "clicked"
state of the button using g_signal_connect_swapped(). This functions is similar
to a g_signal_connect() with the difference lying in how the callback function
is treated. g_signal_connect_swapped() allows you to specify what the callback
function should take as parameter by letting you pass it as data. In this case
the function being called back is gtk_window_destroy() and the `window` pointer
is passed to it. This has the effect that when the button is clicked, the whole
GTK window is destroyed. In contrast if a normal g_signal_connect() were used
to connect the "clicked" signal with gtk_window_destroy(), then the function
would be called on `button` (which would not go well, since the function expects
a GtkWindow as argument).

More information about creating buttons can be found
[here](https://wiki.gnome.org/HowDoI/Buttons).

The rest of the code in `example-1.c` is identical to `example-0.c`. The next
section will elaborate further on how to add several GtkWidgets to your GTK
application.

## Packing

When creating an application, you'll want to put more than one widget inside
a window. When you do so, it becomes important to control how each widget is
positioned and sized. This is where packing comes in.

GTK comes with a large variety of _layout containers_ whose purpose it
is to control the layout of the child widgets that are added to them.
See [Layout containers](#LayoutContainers) for an overview.

The following example shows how the GtkGrid container lets you
arrange several buttons:

![Grid packing](grid-packing.png)

### Packing buttons {#gtk-getting-started-grid-packing}

Create a new file with the following content named `example-2.c`.

``` {.c source=examples/grid-packing.c }
#include <gtk/gtk.h>

static void
print_hello (GtkWidget *widget,
             gpointer   data)
{
  g_print ("Hello World\n");
}

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

  /* create a new window, and set its title */
  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");

  /* Here we construct the container that is going pack our buttons */
  grid = gtk_grid_new ();

  /* Pack the container in the window */
  gtk_window_set_child (GTK_WINDOW (window), grid);

  button = gtk_button_new_with_label ("Button 1");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  /* Place the first button in the grid cell (0, 0), and make it fill
   * just 1 cell horizontally and vertically (ie no spanning)
   */
  gtk_grid_attach (GTK_GRID (grid), button, 0, 0, 1, 1);

  button = gtk_button_new_with_label ("Button 2");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  /* Place the second button in the grid cell (1, 0), and make it fill
   * just 1 cell horizontally and vertically (ie no spanning)
   */
  gtk_grid_attach (GTK_GRID (grid), button, 1, 0, 1, 1);

  button = gtk_button_new_with_label ("Quit");
  g_signal_connect_swapped (button, "clicked", G_CALLBACK (gtk_window_destroy), window);

  /* Place the Quit button in the grid cell (0, 1), and make it
   * span 2 columns.
   */
  gtk_grid_attach (GTK_GRID (grid), button, 0, 1, 2, 1);

  gtk_widget_show (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 gtk4` -o example-2 example-2.c `pkg-config --libs gtk4`
```

## Custom Drawing

Many widgets, like buttons, do all their drawing themselves. You just tell
them the label you want to see, and they figure out what font to use, draw
the button outline and focus rectangle, etc. Sometimes, it is necessary to
do some custom drawing. In that case, a GtkDrawingArea might be the right
widget to use. It offers a canvas on which you can draw by connecting to
the ::draw signal.

The contents of a widget often need to be partially or fully redrawn,
e.g. when another window is moved and uncovers part of the widget, or
when the window containing it is resized. It is also possible to explicitly
cause part or all of the widget to be redrawn, by calling
gtk_widget_queue_draw() or its variants. GTK takes care of most of the
details by providing a ready-to-use cairo context to the ::draw signal
handler.

The following example shows a ::draw signal handler. It is a bit more
complicated than the previous examples, since it also demonstrates
input event handling by means of event controllers.

![Drawing](drawing.png)

### Drawing in response to input {#gtk-getting-started-drawing}

Create a new file with the following content named `example-4.c`.

``` {.c source=examples/drawing.c }
#include <gtk/gtk.h>

/* Surface to store current scribbles */
static cairo_surface_t *surface = NULL;

static void
clear_surface (void)
{
  cairo_t *cr;

  cr = cairo_create (surface);

  cairo_set_source_rgb (cr, 1, 1, 1);
  cairo_paint (cr);

  cairo_destroy (cr);
}

/* Create a new surface of the appropriate size to store our scribbles */
static void
resize_cb (GtkWidget *widget,
           int        width,
           int        height,
           gpointer   data)
{
  if (surface)
    {
      cairo_surface_destroy (surface);
      surface = NULL;
    }

  if (gtk_native_get_surface (gtk_widget_get_native (widget)))
    {
      surface = gdk_surface_create_similar_surface (gtk_native_get_surface (gtk_widget_get_native (widget)),
                                                   CAIRO_CONTENT_COLOR,
                                                   gtk_widget_get_width (widget),
                                                   gtk_widget_get_height (widget));

      /* Initialize the surface to white */
      clear_surface ();
    }
}

/* Redraw the screen from the surface. Note that the draw
 * callback receives a ready-to-be-used cairo_t that is already
 * clipped to only draw the exposed areas of the widget
 */
static void
draw_cb (GtkDrawingArea *drawing_area,
         cairo_t        *cr,
         int             width,
         int             height,
         gpointer        data)
{
  cairo_set_source_surface (cr, surface, 0, 0);
  cairo_paint (cr);
}

/* Draw a rectangle on the surface at the given position */
static void
draw_brush (GtkWidget *widget,
            double     x,
            double     y)
{
  cairo_t *cr;

  /* Paint to the surface, where we store our state */
  cr = cairo_create (surface);

  cairo_rectangle (cr, x - 3, y - 3, 6, 6);
  cairo_fill (cr);

  cairo_destroy (cr);

  /* Now invalidate the drawing area. */
  gtk_widget_queue_draw (widget);
}

static double start_x;
static double start_y;

static void
drag_begin (GtkGestureDrag *gesture,
            double          x,
            double          y,
            GtkWidget      *area)
{
  start_x = x;
  start_y = y;

  draw_brush (area, x, y);
}

static void
drag_update (GtkGestureDrag *gesture,
             double          x,
             double          y,
             GtkWidget      *area)
{
  draw_brush (area, start_x + x, start_y + y);
}

static void
drag_end (GtkGestureDrag *gesture,
          double          x,
          double          y,
          GtkWidget      *area)
{
  draw_brush (area, start_x + x, start_y + y);
}

static void
pressed (GtkGestureClick *gesture,
         int              n_press,
         double           x,
         double           y,
         GtkWidget       *area)
{
  clear_surface ();
  gtk_widget_queue_draw (area);
}

static void
close_window (void)
{
  if (surface)
    cairo_surface_destroy (surface);
}

static void
activate (GtkApplication *app,
          gpointer        user_data)
{
  GtkWidget *window;
  GtkWidget *frame;
  GtkWidget *drawing_area;
  GtkGesture *drag;
  GtkGesture *press;

  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Drawing Area");

  g_signal_connect (window, "destroy", G_CALLBACK (close_window), NULL);

  frame = gtk_frame_new (NULL);
  gtk_window_set_child (GTK_WINDOW (window), frame);

  drawing_area = gtk_drawing_area_new ();
  /* set a minimum size */
  gtk_widget_set_size_request (drawing_area, 100, 100);

  gtk_frame_set_child (GTK_FRAME (frame), drawing_area);

  gtk_drawing_area_set_draw_func (GTK_DRAWING_AREA (drawing_area), draw_cb, NULL, NULL);

  g_signal_connect_after (drawing_area, "resize", G_CALLBACK (resize_cb), NULL);

  drag = gtk_gesture_drag_new ();
  gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (drag), GDK_BUTTON_PRIMARY);
  gtk_widget_add_controller (drawing_area, GTK_EVENT_CONTROLLER (drag));
  g_signal_connect (drag, "drag-begin", G_CALLBACK (drag_begin), drawing_area);
  g_signal_connect (drag, "drag-update", G_CALLBACK (drag_update), drawing_area);
  g_signal_connect (drag, "drag-end", G_CALLBACK (drag_end), drawing_area);

  press = gtk_gesture_click_new ();
  gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (press), GDK_BUTTON_SECONDARY);
  gtk_widget_add_controller (drawing_area, GTK_EVENT_CONTROLLER (press));

  g_signal_connect (press, "pressed", G_CALLBACK (pressed), drawing_area);

  gtk_widget_show (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 gtk4` -o example-4 example-4.c `pkg-config --libs gtk4`
```

## Building user interfaces

When constructing a more complicated user interface, with dozens
or hundreds of widgets, doing all the setup work in C code is
cumbersome, and making changes becomes next to impossible.

Thankfully, GTK supports the separation of user interface
layout from your business logic, by using UI descriptions in an
XML format that can be parsed by the GtkBuilder class.</para>

### Packing buttons with GtkBuilder

Create a new file with the following content named `example-3.c`.

``` {.c source=examples/builder.c }
#include <gtk/gtk.h>
#include <glib/gstdio.h>

static void
print_hello (GtkWidget *widget,
             gpointer   data)
{
  g_print ("Hello World\n");
}

static void
quit_cb (GtkWidget *widget, gpointer data)
{
  gboolean *done = data;

  *done = TRUE;

  g_main_context_wakeup (NULL);
}

int
main (int   argc,
      char *argv[])
{
  GtkBuilder *builder;
  GObject *window;
  GObject *button;
  gboolean done = FALSE;

#ifdef GTK_SRCDIR
  g_chdir (GTK_SRCDIR);
#endif

  gtk_init ();

  /* Construct a GtkBuilder instance and load our UI description */
  builder = gtk_builder_new ();
  gtk_builder_add_from_file (builder, "builder.ui", NULL);

  /* Connect signal handlers to the constructed widgets. */
  window = gtk_builder_get_object (builder, "window");
  g_signal_connect (window, "destroy", G_CALLBACK (quit_cb), &done);

  button = gtk_builder_get_object (builder, "button1");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  button = gtk_builder_get_object (builder, "button2");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  button = gtk_builder_get_object (builder, "quit");
  g_signal_connect (button, "clicked", G_CALLBACK (quit_cb), &done);

  gtk_widget_show (GTK_WIDGET (window));

  while (!done)
    g_main_context_iteration (NULL, TRUE);

  return 0;
}
```

Create a new file with the following content named `builder.ui`.

``` {.xml source=examples/builder.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <object id="window" class="GtkWindow">
    <property name="title">Grid</property>
    <child>
      <object id="grid" class="GtkGrid">
        <child>
          <object id="button1" class="GtkButton">
            <property name="label">Button 1</property>
            <layout>
              <property name="column">0</property>
              <property name="row">0</property>
            </layout>
          </object>
        </child>
        <child>
          <object id="button2" class="GtkButton">
            <property name="label">Button 2</property>
            <layout>
              <property name="column">1</property>
              <property name="row">0</property>
            </layout>
          </object>
        </child>
        <child>
          <object id="quit" class="GtkButton">
            <property name="label">Quit</property>
            <layout>
              <property name="column">0</property>
              <property name="row">1</property>
              <property name="column-span">2</property>
            </layout>
          </object>
        </child>
      </object>
    </child>
  </object>
</interface>
```

You can compile the program above with GCC using:

```
gcc `pkg-config --cflags gtk4` -o example-3 example-3.c `pkg-config --libs gtk4`
```

Note that GtkBuilder can also be used to construct objects that are
not widgets, such as tree models, adjustments, etc. That is the reason
the method we use here is called gtk_builder_get_object() and returns
a GObject* instead of a GtkWidget*.

Normally, you would pass a full path to gtk_builder_add_from_file() to
make the execution of your program independent of the current directory.
A common location to install UI descriptions and similar data is
`/usr/share/appname`.

It is also possible to embed the UI description in the source code as a
string and use gtk_builder_add_from_string() to load it. But keeping the
UI description in a separate file has several advantages: It is then possible
to make minor adjustments to the UI without recompiling your program, and,
more importantly, graphical UI editors such as [glade](http://glade.gnome.org)
can load the file and allow you to create and modify your UI by point-and-click.

## Building applications

An application consists of a number of files:

The binary
 : This gets installed in `/usr/bin`.
A desktop file
 : The desktop file provides important information about the application to
   the desktop shell, such as its name, icon, D-Bus name, commandline to launch
   it, etc. It is installed in `/usr/share/applications`.
An icon
 : The icon gets installed in `/usr/share/icons/hicolor/48x48/apps`, where it
will be found regardless of the current theme.
A settings schema
 : If the application uses GSettings, it will install its schema in
    `/usr/share/glib-2.0/schemas`, so that tools like dconf-editor can find it.
Other resources
 : Other files, such as GtkBuilder ui files, are best loaded from
   resources stored in the application binary itself. This eliminates the
   need for most of the files that would traditionally be installed in
   an application-specific location in `/usr/share`.

GTK includes application support that is built on top of GApplication. In this
tutorial we'll build a simple application by starting from scratch, adding more
and more pieces over time. Along the way, we'll learn about GtkApplication,
templates, resources, application menus, settings, GtkHeaderBar, GtkStack,
GtkSearchBar, GtkListBox, and more.

The full, buildable sources for these examples can be found in the `examples/`
directory of the GTK source distribution, or
[online](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples in the GTK git
repository. You can build each example separately by using make with the
`Makefile.example` file. For more information, see the `README` included in the
examples directory.

### A trivial application

When using GtkApplication, the main() function can be very simple. We just call
g_application_run() and give it an instance of our application class.

``` {.c source=examples/application1/main.c }
#include <gtk/gtk.h>

#include "exampleapp.h"

int
main (int argc, char *argv[])
{
  return g_application_run (G_APPLICATION (example_app_new ()), argc, argv);
}
```

All the application logic is in the application class, which is a subclass of
GtkApplication. Our example does not yet have any interesting functionality.
All it does is open a window when it is activated without arguments, and open
the files it is given, if it is started with arguments.

To handle these two cases, we override the activate() vfunc, which gets called
when the application is launched without commandline arguments, and the open()
vfunc, which gets called when the application is launched with commandline
arguments.

To learn more about GApplication entry points, consult the GIO
[documentation](https://developer.gnome.org/gio/2.36/GApplication.html#GApplication.description).

``` {.c source=examples/application1/exampleapp.c }
#include <gtk/gtk.h>

#include "exampleapp.h"
#include "exampleappwin.h"

struct _ExampleApp
{
  GtkApplication parent;
};

G_DEFINE_TYPE(ExampleApp, example_app, GTK_TYPE_APPLICATION);

static void
example_app_init (ExampleApp *app)
{
}

static void
example_app_activate (GApplication *app)
{
  ExampleAppWindow *win;

  win = example_app_window_new (EXAMPLE_APP (app));
  gtk_window_present (GTK_WINDOW (win));
}

static void
example_app_open (GApplication  *app,
                  GFile        **files,
                  int            n_files,
                  const char    *hint)
{
  GList *windows;
  ExampleAppWindow *win;
  int i;

  windows = gtk_application_get_windows (GTK_APPLICATION (app));
  if (windows)
    win = EXAMPLE_APP_WINDOW (windows->data);
  else
    win = example_app_window_new (EXAMPLE_APP (app));

  for (i = 0; i < n_files; i++)
    example_app_window_open (win, files[i]);

  gtk_window_present (GTK_WINDOW (win));
}

static void
example_app_class_init (ExampleAppClass *class)
{
  G_APPLICATION_CLASS (class)->activate = example_app_activate;
  G_APPLICATION_CLASS (class)->open = example_app_open;
}

ExampleApp *
example_app_new (void)
{
  return g_object_new (EXAMPLE_APP_TYPE,
                       "application-id", "org.gtk.exampleapp",
                       "flags", G_APPLICATION_HANDLES_OPEN,
                       NULL);
}
```

Another important class that is part of the application support in GTK is
GtkApplicationWindow. It is typically subclassed as well. Our subclass does
not do anything yet, so we will just get an empty window.

``` {.c source=examples/application1/examplewin.c }
#include <gtk/gtk.h>

#include "exampleapp.h"
#include "exampleappwin.h"

struct _ExampleAppWindow
{
  GtkApplicationWindow parent;
};

G_DEFINE_TYPE(ExampleAppWindow, example_app_window, GTK_TYPE_APPLICATION_WINDOW);

static void
example_app_window_init (ExampleAppWindow *app)
{
}

static void
example_app_window_class_init (ExampleAppWindowClass *class)
{
}

ExampleAppWindow *
example_app_window_new (ExampleApp *app)
{
  return g_object_new (EXAMPLE_APP_WINDOW_TYPE, "application", app, NULL);
}

void
example_app_window_open (ExampleAppWindow *win,
                         GFile            *file)
{
}
```

As part of the initial setup of our application, we also
create an icon and a desktop file.

![An icon](exampleapp.png)

``` { source=examples/application1/org.gtk.exampleapp.desktop }
[Desktop Entry]
Type=Application
Name=Example
Icon=exampleapp
StartupNotify=true
Exec=@bindir@/exampleapp
```

Note that `@bindir@` needs to be replaced with the actual path to the binary
before this desktop file can be used.

Here is what we've achieved so far:

![An application](getting-started-app1.png)

This does not look very impressive yet, but our application is already
presenting itself on the session bus, it has single-instance semantics,
and it accepts files as commandline arguments.

### Populating the window

In this step, we use a GtkBuilder template to associate a
GtkBuilder ui file with our application window class.

Our simple ui file gives the window a title, and puts a GtkStack
widget as the main content.

``` { .xml source=examples/application2/window.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="ExampleAppWindow" parent="GtkApplicationWindow">
    <property name="title" translatable="yes">Example Application</property>
    <property name="default-width">600</property>
    <property name="default-height">400</property>
    <child>
      <object class="GtkBox" id="content_box">
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkStack" id="stack"/>
        </child>
      </object>
    </child>
  </template>
</interface>
```

To make use of this file in our application, we revisit our
GtkApplicationWindow subclass, and call
gtk_widget_class_set_template_from_resource() from the class init
function to set the ui file as template for this class. We also
add a call to gtk_widget_init_template() in the instance init
function to instantiate the template for each instance of our
class.

```
 ...

static void
example_app_window_init (ExampleAppWindow *win)
{
  gtk_widget_init_template (GTK_WIDGET (win));
}

static void
example_app_window_class_init (ExampleAppWindowClass *class)
{
  gtk_widget_class_set_template_from_resource (GTK_WIDGET_CLASS (class),
                                               "/org/gtk/exampleapp/window.ui");
}

 ...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application2/exampleappwin.c))

You may have noticed that we used the `_from_resource()` variant of the function
that sets a template. Now we need to use
[GLib's resource functionality](https://developer.gnome.org/gio/stable/GResource.html)
to include the ui file in the binary. This is commonly done by listing all resources
in a `.gresource.xml` file, such as this:

``` { .xml source=examples/application2/exampleapp.gresource.xml }
<?xml version="1.0" encoding="UTF-8"?>
<gresources>
  <gresource prefix="/org/gtk/exampleapp">
    <file preprocess="xml-stripblanks">window.ui</file>
  </gresource>
</gresources>
```

This file has to be converted into a C source file that will be compiled and linked
into the application together with the other source files. To do so, we use the
`glib-compile-resources` utility:

```
glib-compile-resources exampleapp.gresource.xml --target=resources.c --generate-source
```

Our application now looks like this:

![The application](getting-started-app2.png)

### Opening files

In this step, we make our application show the content of all the files
that it is given on the commandline.

To this end, we add a member to the struct of our application window subclass
and keep a reference to the GtkStack there. The first member of the struct
should be the parent type from which the class is derived. Here,
ExampleAppWindow is derived from GtkApplicationWindow. The
gtk_widget_class_bind_template_child() function arranges things so that after
instantiating the template, the `stack` member of the struct will point to the
widget of the same name from the template.

```
...

struct _ExampleAppWindow
{
  GtkApplicationWindow parent;

  GtkWidget *stack;
};

G_DEFINE_TYPE (ExampleAppWindow, example_app_window, GTK_TYPE_APPLICATION_WINDOW)

...

static void
example_app_window_class_init (ExampleAppWindowClass *class)
{
  gtk_widget_class_set_template_from_resource (GTK_WIDGET_CLASS (class),
                                               "/org/gtk/exampleapp/window.ui");
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), ExampleAppWindow, stack);
}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application3/exampleappwin.c))

Now we revisit the example_app_window_open() function that is called for each
commandline argument, and construct a GtkTextView that we then add as a page
to the stack:

```
...

void
example_app_window_open (ExampleAppWindow *win,
                         GFile            *file)
{
  char *basename;
  GtkWidget *scrolled, *view;
  char *contents;
  gsize length;

  basename = g_file_get_basename (file);

  scrolled = gtk_scrolled_window_new (NULL, NULL);
  gtk_widget_set_hexpand (scrolled, TRUE);
  gtk_widget_set_vexpand (scrolled, TRUE);
  view = gtk_text_view_new ();
  gtk_text_view_set_editable (GTK_TEXT_VIEW (view), FALSE);
  gtk_text_view_set_cursor_visible (GTK_TEXT_VIEW (view), FALSE);
  gtk_scrolled_window_set_child (GTK_SCROLLED_WINDOW (scrolled), view);
  gtk_stack_add_titled (GTK_STACK (win->stack), scrolled, basename, basename);

  if (g_file_load_contents (file, NULL, &contents, &length, NULL, NULL))
    {
      GtkTextBuffer *buffer;

      buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (view));
      gtk_text_buffer_set_text (buffer, contents, length);
      g_free (contents);
    }

  g_free (basename);
}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application3/exampleappwin.c))

Lastly, we add a GtkStackSwitcher to the titlebar area in the ui file, and we
tell it to display information about our stack.

The stack switcher gets all its information it needs to display tabs from
the stack that it belongs to. Here, we are passing the label to show for
each file as the last argument to the gtk_stack_add_titled() function.

Our application is beginning to take shape:

![Application window](getting-started-app3.png)

### A menu

The menu is shown at the right side of the headerbar. It is meant to collect
infrequently used actions that affect the whole application.

Just like the window template, we specify our menu in a ui file, and add it
as a resource to our binary.

``` {.xml source=examples/application4/gears-menu.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <menu id="menu">
    <section>
      <item>
        <attribute name="label" translatable="yes">_Preferences</attribute>
        <attribute name="action">app.preferences</attribute>
      </item>
    </section>
    <section>
      <item>
        <attribute name="label" translatable="yes">_Quit</attribute>
        <attribute name="action">app.quit</attribute>
      </item>
    </section>
  </menu>
</interface>
```

To make the menu appear, we have to load the ui file and associate the
resulting menu model with the menu button that we've added to the headerbar.
Since menus work by activating GActions, we also have to add a suitable set
of actions to our application.

Adding the actions is best done in the startup() vfunc, which is guaranteed
to be called once for each primary application instance:

```
...

static void
preferences_activated (GSimpleAction *action,
                       GVariant      *parameter,
                       gpointer       app)
{
}

static void
quit_activated (GSimpleAction *action,
                GVariant      *parameter,
                gpointer       app)
{
  g_application_quit (G_APPLICATION (app));
}

static GActionEntry app_entries[] =
{
  { "preferences", preferences_activated, NULL, NULL, NULL },
  { "quit", quit_activated, NULL, NULL, NULL }
};

static void
example_app_startup (GApplication *app)
{
  GtkBuilder *builder;
  GMenuModel *app_menu;
  const char *quit_accels[2] = { "&lt;Ctrl&gt;Q", NULL };

  G_APPLICATION_CLASS (example_app_parent_class)->startup (app);

  g_action_map_add_action_entries (G_ACTION_MAP (app),
                                   app_entries, G_N_ELEMENTS (app_entries),
                                   app);
  gtk_application_set_accels_for_action (GTK_APPLICATION (app),
                                         "app.quit",
                                         quit_accels);
}

static void
example_app_class_init (ExampleAppClass *class)
{
  G_APPLICATION_CLASS (class)->startup = example_app_startup;
  ...
}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application4/exampleapp.c))

Our preferences menu item does not do anything yet, but the Quit menu item
is fully functional. Note that it can also be activated by the usual Ctrl-Q
shortcut. The shortcut was added with gtk_application_set_accels_for_action().

The application menu looks like this:

![Application window](getting-started-app4.png)

### A preference dialog

A typical application will have a some preferences that should be remembered
from one run to the next. Even for our simple example application, we may
want to change the font that is used for the content.

We are going to use GSettings to store our preferences. GSettings requires
a schema that describes our settings:

``` {.xml source=examples/application5/org.gtk.exampleapp.gschema.xml }
<?xml version="1.0" encoding="UTF-8"?>
<schemalist>
  <schema path="/org/gtk/exampleapp/" id="org.gtk.exampleapp">
    <key name="font" type="s">
      <default>'Monospace 12'</default>
      <summary>Font</summary>
      <description>The font to be used for content.</description>
    </key>
    <key name="transition" type="s">
      <choices>
        <choice value='none'/>
        <choice value='crossfade'/>
        <choice value='slide-left-right'/>
      </choices>
      <default>'none'</default>
      <summary>Transition</summary>
      <description>The transition to use when switching tabs.</description>
    </key>
  </schema>
</schemalist>
```

Before we can make use of this schema in our application, we need to compile
it into the binary form that GSettings expects. GIO provides
[macros](https://developer.gnome.org/gio/2.36/ch31s06.html) to do this in
autotools-based projects.

Next, we need to connect our settings to the widgets that they are supposed
to control. One convenient way to do this is to use GSettings bind
functionality to bind settings keys to object properties, as we do here
for the transition setting.

```
...

static void
example_app_window_init (ExampleAppWindow *win)
{
  gtk_widget_init_template (GTK_WIDGET (win));
  win->settings = g_settings_new ("org.gtk.exampleapp");

  g_settings_bind (win->settings, "transition",
                   win->stack, "transition-type",
                   G_SETTINGS_BIND_DEFAULT);
}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application5/exampleappwin.c))

The code to connect the font setting is a little more involved, since there
is no simple object property that it corresponds to, so we are not going to
go into that here.

At this point, the application will already react if you change one of the
settings, e.g. using the gsettings commandline tool. Of course, we expect
the application to provide a preference dialog for these. So lets do that
now. Our preference dialog will be a subclass of GtkDialog, and we'll use
the same techniques that we've already seen: templates, private structs,
settings bindings.

Lets start with the template.

``` {.xml source=examples/application6/prefs.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="ExampleAppPrefs" parent="GtkDialog">
    <property name="title" translatable="yes">Preferences</property>
    <property name="resizable">0</property>
    <property name="modal">1</property>
    <child internal-child="content_area">
      <object class="GtkBox" id="content_area">
        <child>
          <object class="GtkGrid" id="grid">
            <property name="margin-start">12</property>
            <property name="margin-end">12</property>
            <property name="margin-top">12</property>
            <property name="margin-bottom">12</property>
            <property name="row-spacing">12</property>
            <property name="column-spacing">12</property>
            <child>
              <object class="GtkLabel" id="fontlabel">
                <property name="label">_Font:</property>
                <property name="use-underline">1</property>
                <property name="mnemonic-widget">font</property>
                <property name="xalign">1</property>
                <layout>
                  <property name="column">0</property>
                  <property name="row">0</property>
                </layout>
              </object>
            </child>
            <child>
              <object class="GtkFontButton" id="font">
                <layout>
                  <property name="column">1</property>
                  <property name="row">0</property>
                </layout>
              </object>
            </child>
            <child>
              <object class="GtkLabel" id="transitionlabel">
                <property name="label">_Transition:</property>
                <property name="use-underline">1</property>
                <property name="mnemonic-widget">transition</property>
                <property name="xalign">1</property>
                <layout>
                  <property name="column">0</property>
                  <property name="row">1</property>
                </layout>
              </object>
            </child>
            <child>
              <object class="GtkComboBoxText" id="transition">
                <items>
                  <item translatable="yes" id="none">None</item>
                  <item translatable="yes" id="crossfade">Fade</item>
                  <item translatable="yes" id="slide-left-right">Slide</item>
                </items>
                <layout>
                  <property name="column">1</property>
                  <property name="row">1</property>
                </layout>
              </object>
            </child>
          </object>
        </child>
      </object>
    </child>
  </template>
</interface>
```

Next comes the dialog subclass.

``` {.c source=examples/application6/exampleappprefs.c }
#include <gtk/gtk.h>

#include "exampleapp.h"
#include "exampleappwin.h"
#include "exampleappprefs.h"

struct _ExampleAppPrefs
{
  GtkDialog parent;

  GSettings *settings;
  GtkWidget *font;
  GtkWidget *transition;
};

G_DEFINE_TYPE (ExampleAppPrefs, example_app_prefs, GTK_TYPE_DIALOG)

static void
example_app_prefs_init (ExampleAppPrefs *prefs)
{
  gtk_widget_init_template (GTK_WIDGET (prefs));
  prefs->settings = g_settings_new ("org.gtk.exampleapp");

  g_settings_bind (prefs->settings, "font",
                   prefs->font, "font",
                   G_SETTINGS_BIND_DEFAULT);
  g_settings_bind (prefs->settings, "transition",
                   prefs->transition, "active-id",
                   G_SETTINGS_BIND_DEFAULT);
}

static void
example_app_prefs_dispose (GObject *object)
{
  ExampleAppPrefs *prefs;

  prefs = EXAMPLE_APP_PREFS (object);

  g_clear_object (&prefs->settings);

  G_OBJECT_CLASS (example_app_prefs_parent_class)->dispose (object);
}

static void
example_app_prefs_class_init (ExampleAppPrefsClass *class)
{
  G_OBJECT_CLASS (class)->dispose = example_app_prefs_dispose;

  gtk_widget_class_set_template_from_resource (GTK_WIDGET_CLASS (class),
                                               "/org/gtk/exampleapp/prefs.ui");
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), ExampleAppPrefs, font);
  gtk_widget_class_bind_template_child (GTK_WIDGET_CLASS (class), ExampleAppPrefs, transition);
}

ExampleAppPrefs *
example_app_prefs_new (ExampleAppWindow *win)
{
  return g_object_new (EXAMPLE_APP_PREFS_TYPE, "transient-for", win, "use-header-bar", TRUE, NULL);
}
```

Now we revisit the `preferences_activated()` function in our application
class, and make it open a new preference dialog.

```
...

static void
preferences_activated (GSimpleAction *action,
                       GVariant      *parameter,
                       gpointer       app)
{
  ExampleAppPrefs *prefs;
  GtkWindow *win;

  win = gtk_application_get_active_window (GTK_APPLICATION (app));
  prefs = example_app_prefs_new (EXAMPLE_APP_WINDOW (win));
  gtk_window_present (GTK_WINDOW (prefs));
}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application6/exampleapp.c))

After all this work, our application can now show a preference dialog
like this:

![Preference dialog](getting-started-app6.png)

### Adding a search bar

We continue to flesh out the functionality of our application. For now, we
add search. GTK supports this with GtkSearchEntry and GtkSearchBar. The
search bar is a widget that can slide in from the top to present a search
entry.

We add a toggle button to the header bar, which can be used to slide out
the search bar below the header bar.

``` {.xml source=examples/application7/window.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="ExampleAppWindow" parent="GtkApplicationWindow">
    <property name="title" translatable="yes">Example Application</property>
    <property name="default-width">600</property>
    <property name="default-height">400</property>
    <child type="titlebar">
      <object class="GtkHeaderBar" id="header">
        <child type="title">
          <object class="GtkStackSwitcher" id="tabs">
            <property name="stack">stack</property>
          </object>
        </child>
        <child type="end">
          <object class="GtkMenuButton" id="gears">
            <property name="direction">none</property>
          </object>
        </child>
        <child type="end">
          <object class="GtkToggleButton" id="search">
            <property name="sensitive">0</property>
            <property name="icon-name">edit-find-symbolic</property>
          </object>
        </child>
      </object>
    </child>
    <child>
      <object class="GtkBox" id="content_box">
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkSearchBar" id="searchbar">
            <child>
              <object class="GtkSearchEntry" id="searchentry">
                <signal name="search-changed" handler="search_text_changed"/>
              </object>
            </child>
          </object>
        </child>
        <child>
          <object class="GtkStack" id="stack">
            <signal name="notify::visible-child" handler="visible_child_changed"/>
          </object>
        </child>
      </object>
    </child>
  </template>
</interface>
```

Implementing the search needs quite a few code changes that we are not
going to completely go over here. The central piece of the search
implementation is a signal handler that listens for text changes in
the search entry.

```
...

static void
search_text_changed (GtkEntry         *entry,
                     ExampleAppWindow *win)
{
  const char *text;
  GtkWidget *tab;
  GtkWidget *view;
  GtkTextBuffer *buffer;
  GtkTextIter start, match_start, match_end;

  text = gtk_editable_get_text (GTK_EDITABLE (entry));

  if (text[0] == '\0')
    return;

  tab = gtk_stack_get_visible_child (GTK_STACK (win->stack));
  view = gtk_scrolled_window_get_child (GTK_SCROLLED_WINDOW (tab));
  buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (view));

  /* Very simple-minded search implementation */
  gtk_text_buffer_get_start_iter (buffer, &start);
  if (gtk_text_iter_forward_search (&start, text, GTK_TEXT_SEARCH_CASE_INSENSITIVE,
                                    &match_start, &match_end, NULL))
    {
      gtk_text_buffer_select_range (buffer, &match_start, &match_end);
      gtk_text_view_scroll_to_iter (GTK_TEXT_VIEW (view), &match_start,
                                    0.0, FALSE, 0.0, 0.0);
    }
}

static void
example_app_window_init (ExampleAppWindow *win)
{

...

  gtk_widget_class_bind_template_callback (GTK_WIDGET_CLASS (class), search_text_changed);

...

}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application7/exampleappwin.c))

With the search bar, our application now looks like this:

![A search bar](getting-started-app7.png)

### Adding a side bar

As another piece of functionality, we are adding a sidebar, which demonstrates
GtkMenuButton, GtkRevealer and GtkListBox.

``` {.xml source=examples/application8/window.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="ExampleAppWindow" parent="GtkApplicationWindow">
    <property name="title" translatable="yes">Example Application</property>
    <property name="default-width">600</property>
    <property name="default-height">400</property>
    <child type="titlebar">
      <object class="GtkHeaderBar" id="header">
        <child type="title">
          <object class="GtkStackSwitcher" id="tabs">
            <property name="stack">stack</property>
          </object>
        </child>
        <child type="end">
          <object class="GtkToggleButton" id="search">
            <property name="sensitive">0</property>
            <property name="icon-name">edit-find-symbolic</property>
          </object>
        </child>
        <child type="end">
          <object class="GtkMenuButton" id="gears">
            <property name="direction">none</property>
          </object>
        </child>
      </object>
    </child>
    <child>
      <object class="GtkBox" id="content_box">
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkSearchBar" id="searchbar">
            <child>
              <object class="GtkSearchEntry" id="searchentry">
                <signal name="search-changed" handler="search_text_changed"/>
              </object>
            </child>
          </object>
        </child>
        <child>
          <object class="GtkBox" id="hbox">
            <child>
              <object class="GtkRevealer" id="sidebar">
                <property name="transition-type">slide-right</property>
                <child>
                  <object class="GtkScrolledWindow" id="sidebar-sw">
                    <property name="hscrollbar-policy">never</property>
                    <child>
                      <object class="GtkListBox" id="words">
                        <property name="selection-mode">none</property>
                      </object>
                    </child>
                  </object>
                </child>
              </object>
            </child>
            <child>
              <object class="GtkStack" id="stack">
                <signal name="notify::visible-child" handler="visible_child_changed"/>
              </object>
            </child>
          </object>
        </child>
      </object>
    </child>
  </template>
</interface>
```

The code to populate the sidebar with buttons for the words found in each
file is a little too involved to go into here. But we'll look at the code
to add a checkbutton for the new feature to the menu.

``` {.xml source=examples/application8/gears-menu.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <menu id="menu">
    <section>
      <item>
        <attribute name="label" translatable="yes">_Words</attribute>
        <attribute name="action">win.show-words</attribute>
      </item>
      <item>
        <attribute name="label" translatable="yes">_Preferences</attribute>
        <attribute name="action">app.preferences</attribute>
      </item>
    </section>
    <section>
      <item>
        <attribute name="label" translatable="yes">_Quit</attribute>
        <attribute name="action">app.quit</attribute>
      </item>
    </section>
  </menu>
</interface>
```

To connect the menuitem to the show-words setting, we use
a GAction corresponding to the given GSettings key.

```
...

static void
example_app_window_init (ExampleAppWindow *win)
{

...

  builder = gtk_builder_new_from_resource ("/org/gtk/exampleapp/gears-menu.ui");
  menu = G_MENU_MODEL (gtk_builder_get_object (builder, "menu"));
  gtk_menu_button_set_menu_model (GTK_MENU_BUTTON (priv->gears), menu);
  g_object_unref (builder);

  action = g_settings_create_action (priv->settings, "show-words");
  g_action_map_add_action (G_ACTION_MAP (win), action);
  g_object_unref (action);
}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application8/exampleappwin.c))

What our application looks like now:

![A sidebar](getting-started-app8.png)

### Properties

Widgets and other objects have many useful properties.

Here we show some ways to use them in new and flexible ways, by wrapping
them in actions with GPropertyAction or by binding them with GBinding.

To set this up, we add two labels to the header bar in our window template,
named `lines_label` and `lines`, and bind them to struct members in the
private struct, as we've seen a couple of times by now.

We add a new "Lines" menu item to the gears menu, which triggers the
show-lines action:

``` {.xml source=examples/application9/gears-menu.ui }
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <menu id="menu">
    <section>
      <item>
        <attribute name="label" translatable="yes">_Words</attribute>
        <attribute name="action">win.show-words</attribute>
      </item>
      <item>
        <attribute name="label" translatable="yes">_Lines</attribute>
        <attribute name="action">win.show-lines</attribute>
      </item>
      <item>
        <attribute name="label" translatable="yes">_Preferences</attribute>
        <attribute name="action">app.preferences</attribute>
      </item>
    </section>
    <section>
      <item>
        <attribute name="label" translatable="yes">_Quit</attribute>
        <attribute name="action">app.quit</attribute>
      </item>
    </section>
  </menu>
</interface>
```

To make this menu item do something, we create a property action for the
visible property of the `lines` label, and add it to the actions of the
window. The effect of this is that the visibility of the label gets toggled
every time the action is activated.

Since we want both labels to appear and disappear together, we bind
the visible property of the `lines_label` widget to the same property
of the `lines` widget.

```
...

static void
example_app_window_init (ExampleAppWindow *win)
{
  ...

  action = (GAction*) g_property_action_new ("show-lines", win->lines, "visible");
  g_action_map_add_action (G_ACTION_MAP (win), action);
  g_object_unref (action);

  g_object_bind_property (win->lines, "visible",
                          win->lines_label, "visible",
                          G_BINDING_DEFAULT);
}

...
```
([full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application9/exampleappwin.c))

We also need a function that counts the lines of the currently active tab,
and updates the `lines` label. See the [full source](https://gitlab.gnome.org/GNOME/gtk/blob/master/examples/application9/exampleappwin.c)
if you are interested in the details.

This brings our example application to this appearance:

![Full application](getting-started-app9.png)