GNOME.org

Image viewer

In this tutorial, you will learn:

  • Some basic concepts of C/GObject programming

  • How to write a Gtk application in C

Create a project in Anjuta

Before you start coding, you'll need to set up a new project in Anjuta. This will create all of the files you need to build and run the code later on. It's also useful for keeping everything together.

  1. Start Anjuta and click File ▸ New ▸ Project to open the project wizard.

  2. Choose GTK+ (Simple) from the C tab, click Continue, and fill out your details on the next few pages. Use image-viewer as project name and directory.

  3. Make sure that Use GtkBuilder for user interface is disabled as we will create the UI manually in this tutorial. Check the Guitar-Tuner tutorial if you want to learn how to use the interface builder.

  4. Click Apply and the project will be created for you. Open src/main.c from the Project or File tabs. You should see some code which starts with the lines:

    #include <config.h>
#include <gtk/gtk.h>

Build the code for the first time

C is a rather verbose language, so don't be surprised that the file contains quite a lot of code. Most of it is template code. It loads an (empty) window and shows it. More details are given below; skip this list if you understand the basics:

  • The three #include lines at the top include the config (useful autoconf build defines), gtk (user interface) and gi18n (internationalization) libraries. Functions from these libraries are used in the rest of the code.

  • The create_window function creates a new (empty) window and connects a signal to exit the application when that window is closed.

    Connecting signals is how you define what happens when you push a button, or when some other event happens. Here, the destroy function is called (and quits the app) when you close the window.

  • The main function is run by default when you start a C application. It calls a few functions which set up and then run the application. The gtk_main function starts the GTK main loop, which runs the user interface and starts listening for events (like clicks and key presses).

  • The ENABLE_NLS conditional definition sets up gettext, which is a framework for translating applications. These functions specify how translation tools should handle your app when you run them.

This code is ready to be used, so you can compile it by clicking Build ▸ Build Project (or press Shift+F7).

Press Execute on the next window that appears to configure a debug build. You only need to do this once, for the first build.

Creating the user interface

Now we will bring life into the empty window. GTK organizes the user interface with GtkContainers that can contain other widgets and even other containers. Here we will use the simplest available container, a GtkBox:

static GtkWidget*
create_window (void)
{
	GtkWidget *window;
	GtkWidget *button;
	GtkWidget *image;
	GtkWidget *box;

	/* Set up the UI */
	window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
	gtk_window_set_title (GTK_WINDOW (window), "image-viewer-c");

	box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 5);
	button = gtk_button_new_with_label (_("Open image"));
	image = gtk_image_new ();

	gtk_box_pack_start (GTK_BOX (box), image, TRUE, TRUE, 0);
	gtk_box_pack_start (GTK_BOX (box), button, FALSE, FALSE, 0);

	gtk_container_add (GTK_CONTAINER (window), box);

	/* Connect signals */

	/* Show open dialog when opening a file */
	g_signal_connect (button, "clicked", G_CALLBACK (on_open_image), image);

	/* Exit when the window is closed */
	g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);

	return window;
}

  1. The first lines create the widgets we want to use: a button for opening up an image, the image view widget itself and the box we will use as a container. The macros like GTK_BOX are used for dynamic type checking and casting which is needed as C doesn't support object-orientation out-of-the-box.

  2. The calls to gtk_box_pack_start add the two widgets to the box and define their behaviour. The image will expand into any available space while the button will just be as big as needed. You will notice that we don't set explicit sizes on the widgets. In GTK this is usually not needed as it makes it much easier to have a layout that looks good in different window sizes. Next, the box is added to the window.

  3. We need to define what happens when the user clicks on the button. GTK uses the concept of signals. When the button is clicked, it fires the clicked signal, which we can connect to some action. This is done using the g_signal_connect function which tells GTK to call the on_image_open function when the button is clicked and to pass the image as an additional argument to that function. We will define the callback in the next section.

  4. The last g_signal_connect() makes sure that the application exits when the window is closed.

  5. As a last step, make sure to replace the gtk_widget_show call in the main() function by gtk_widget_show_all() to show the window and all the widgets it contains.

Showing the image

We will now define the signal handler for the clicked signal or the button we mentioned before. Add this code before the create_window() method.

static void
on_open_image (GtkButton* button, gpointer user_data)
{
	GtkWidget *image = GTK_WIDGET (user_data);
	GtkWidget *toplevel = gtk_widget_get_toplevel (image);
	GtkFileFilter *filter = gtk_file_filter_new ();
	GtkWidget *dialog = gtk_file_chooser_dialog_new (_("Open image"),
	                                                 GTK_WINDOW (toplevel),
	                                                 GTK_FILE_CHOOSER_ACTION_OPEN,
	                                                 GTK_STOCK_OK, GTK_RESPONSE_ACCEPT,
	                                                 GTK_STOCK_CANCEL, GTK_RESPONSE_CANCEL,
	                                                 NULL);

	gtk_file_filter_add_pixbuf_formats (filter);
	gtk_file_chooser_add_filter (GTK_FILE_CHOOSER (dialog),
	                             filter);

	switch (gtk_dialog_run (GTK_DIALOG (dialog)))
	{
		case GTK_RESPONSE_ACCEPT:
		{
			gchar *filename =
				gtk_file_chooser_get_filename (GTK_FILE_CHOOSER (dialog));
			gtk_image_set_from_file (GTK_IMAGE (image), filename);
			break;
		}
		default:
			break;
	}
	gtk_widget_destroy (dialog);
}

This is a bit more complicated than anything we've attempted so far, so let's break it down:

  • The first argument of the signal is always the widget that sent the signal. Sometimes other arguments related to the signal come after that, but clicked doesn't have any. Next is the user_data argument which is a pointer to the data we passed when connecting the signal. In this case it is our GtkImage object.

  • The next interesting line is where the dialog for choosing the file is created using gtk_file_chooser_dialog_new. The function takes the title of the dialog, the parent window of the dialog and several options like the number of buttons and their corresponding values.

    Notice that we are using stock button names from Gtk, instead of manually typing "Cancel" or "Open". The advantage of using stock names is that the button labels will already be translated into the user's language.

  • The next two lines restrict the Open dialog to only display files which can be opened by GtkImage. A filter object is created first; we then add all kinds of files supported by GdkPixbuf (which includes most image formats like PNG and JPEG) to the filter. Finally, we set this filter to be the Open dialog's filter.

  • gtk_dialog_run displays the Open dialog. The dialog will wait for the user to choose an image; when they do, gtk_dialog_run will return the value GTK_RESPONSE_ACCEPT (it would return GTK_RESPONSE_CANCEL if the user clicked Cancel). The switch statement tests for this.

  • Assuming that the user did click Open, the next line sets the file property of the GtkImage to the filename of the image selected by the user. The GtkImage will then load and display the chosen image.

  • In the final line of this method, we destroy the Open dialog because we don't need it any more. Destroying automatically hides the dialog.

Build and run the application

All of the code should now be ready to go. Click Build ▸ Build Project to build everything again, and then Run ▸ Execute to start the application.

If you haven't already done so, choose the Debug/src/image-viewer application in the dialog that appears. Finally, hit Run and enjoy!

Reference Implementation

If you run into problems with the tutorial, compare your code with this reference code.