<page xmlns="http://projectmallard.org/1.0/"
      type="topic"
      id="image-viewer.c">

  <info>
    <link type="guide" xref="index#c"/>
    
    <desc>A little bit more than a simple "Hello world" Gtk application.</desc>
    
    <revision pkgversion="0.1" version="0.1" date="2011-03-18" status="review"/>
    <credit type="author">
      <name>GNOME Documentation Project</name>
      <email>gnome-doc-list@gnome.org</email>
    </credit>
    <credit type="author">
      <name>Johannes Schmid</name>
      <email>jhs@gnome.org</email>
    </credit>    
  </info>

<title>Image Viewer</title>

<synopsis>
  <p>In this tutorial, you will learn:</p>
  <list>
    <item><p>Some basic concepts of C/GObject programming</p></item>
    <item><p>How to write a Gtk application in C</p></item>
  </list>
</synopsis>

<media type="image" mime="image/png" src="media/image-viewer.png"/>

<section id="anjuta">
  <title>Create a project in Anjuta</title>
  <p>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.</p>
  <steps>
    <item>
    <p>Start Anjuta and click <guiseq><gui>File</gui><gui>New</gui><gui>Project</gui></guiseq> to open the project wizard.</p>
    </item>
    <item>
    <p>Choose <gui>Gtk+ (Simple)</gui> from the <gui>C</gui> tab, click <gui>Forward</gui>, and fill out your details on the next few pages. Use <file>image-viewer</file> as project name and directory.</p>
   	</item>
    <item>
    <p>Make sure that <gui>Use GtkBuilder for user interface</gui> is disabled as we will
    create the UI manually in this tutorial. Check the <link xref="guitar-tuner.c">Guitar-Tuner</link>
    tutorial if you want to learn how to use the interface builder.</p>
    </item>
    <item>
    <p>Click <gui>Apply</gui> and the project will be created for you. Open <file>src/main.c</file> from the <gui>Project</gui> or <gui>File</gui> tabs. You should see some code which starts with the lines:</p>
    <code mime="text/x-csrc"><![CDATA[
#include <config.h>
#include <gtk/gtk.h>]]></code>
    </item>
  </steps>
</section>

<section id="build">
  <title>Build the code for the first time</title>
  <p>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:</p>
  
  <list>
  <item>
    <p>The three <code>#include</code> lines at the top include the <code>config</code> (useful autoconf build defines), <code>gtk</code> (user interface) and <code>gi18n</code> (internationalization) libraries. Functions from these libraries are used in the rest of the code.</p>
   </item>
   <item>
    <p>The <code>create_window</code> function creates a new (empty) window and connects a signal to exit the application when that window is closed.</p>
    <p>Connecting signals is how you define what happens when you push a button, or when some other event happens. Here, the <code>destroy</code> function is called (and quits the app) when you close the window.</p>
   </item>
   <item>
    <p>The <code>main</code> 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 <code>gtk_main</code> function starts the GTK main loop, which runs the user interface and starts listening for events (like clicks and key presses).</p>
   </item>
   <item>
    <p>The <code>ENABLE_NLS</code> conditional definition sets up <code>gettext</code>, which is a framework for translating applications. These functions specify how translation tools should handle your app when you run them.</p>
   </item>
  </list>

  <p>This code is ready to be used, so you can compile it by clicking <guiseq><gui>Build</gui><gui>Build Project</gui></guiseq> (or press <keyseq><key>Shift</key><key>F7</key></keyseq>).</p>
  <p>Press <gui>Execute</gui> on the next window that appears to configure a debug build. You only need to do this once, for the first build.</p>
</section>

<section id="ui">
<title>Creating the user interface</title>
<p>Now we will bring life into the empty window. GTK organizes the user interface
with <code>GtkContainer</code>s that can contain other widgets and even other containers. Here we
will use the simplest available container, a <code>GtkBox</code>:</p>
<code mime="text/x-csrc"><![CDATA[
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;
}
]]></code>
  <steps>
    <item>
    <p>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 <code>GTK_BOX</code> are used for dynamic type checking and casting
    which is needed as C doesn't support object-orientation out-of-the-box.</p>
    </item>
    <item>
    <p>The calls to <code>gtk_box_pack_start</code> 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.</p>
    </item>
    <item>
    <p>We need to define what happens when the user clicks on the button. GTK uses the concept of <em>signals</em>. When the button is clicked, it fires the <em>clicked</em> signal, which we can connect to some action. This is done using the <code>g_signal_connect</code>
    function which tells GTK to call the <code>on_image_open</code> function when the button is clicked and
    to pass the image as an additional argument to that function. We will define the <em>callback</em> in the next section.</p>
    </item>
    <item>
    <p>The last <code>g_signal_connect()</code> makes sure that the application exits when the window is closed.</p>
    </item>
    <item>
    <p>As a last step, make sure to replace the <code>gtk_widget_show</code> call in the <code>main()</code> function by
    <code>gtk_widget_show_all()</code> to show the window and all the widgets it contains.</p>
    </item>
  </steps>
</section>

<section id="image">
<title>Showing the image</title>
<p>We will now define the signal handler for the <em>clicked</em> signal or the 
button we mentioned before. Add this code before the <code>create_window()</code>
method.</p>
<code mime="text/x-csrc"><![CDATA[
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);
}
]]></code>
  <p>This is a bit more complicated than anything we've attempted so far, so let's break it down:</p>
  <list>
    <item><p>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 <em>clicked</em> doesn't have any. Next is
    the <code>user_data</code> argument which is a pointer to the data we passed when connecting the signal.
    In this case it is our <code>GtkImage</code> object.</p>
    </item> 
    <item>
      <p>The next interesting line is where the dialog for choosing the file is created using
      <code>gtk_file_chooser_dialog_new</code>. 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.</p>
    <p>Notice that we are using <em>stock</em> 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.</p>
    </item>
    <item>
    <p>The next two lines restrict the <gui>Open</gui> 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 <code>GdkPixbuf</code> (which includes most image formats like PNG and JPEG) to the filter. Finally, we set this filter to be the <gui>Open</gui> dialog's filter.</p>
    </item>
    <item>
    <p><code>gtk_dialog_run</code> displays the <gui>Open</gui> dialog. The dialog will wait for the user to choose an image; when they do, <code>gtk_dialog_run</code> will return the value <code>GTK_RESPONSE_ACCEPT</code> (it would return <code>GTK_RESPONSE_CANCEL</code> if the user clicked <gui>Cancel</gui>). The <code>switch</code> statement tests for this.</p>
    </item>
    <item><p>Assuming that the user did click <gui>Open</gui>, the next line sets the <code>file</code> property of the GtkImage to the filename of the image selected by the user. The GtkImage will then load and display the chosen image.</p>
    </item>
    <item>
    <p>In the final line of this method, we destroy the <gui>Open</gui> dialog because we don't need it any more.
    Destroying automatically hides the dialog.</p>
    </item>
  </list>
</section>

<section id="run">
  <title>Build and run the application</title>
  <p>All of the code should now be ready to go. Click <guiseq><gui>Build</gui><gui>Build Project</gui></guiseq> to build everything again, and then <guiseq><gui>Run</gui><gui>Execute</gui></guiseq> to start the application.</p>
  <p>If you haven't already done so, choose the <file>Debug/src/image-viewer</file> application in the dialog that appears. Finally, hit <gui>Run</gui> and enjoy!</p>
</section>

<section id="impl">
 <title>Reference Implementation</title>
 <p>If you run into problems with the tutorial, compare your code with this <link href="image-viewer/image-viewer.c">reference code</link>.</p>
</section>

<section id="next">
  <title>Next steps</title>
  <p>Here are some ideas for how you can extend this simple demonstration:</p>
  <list>
   <item>
   <p>Have the user select a directory rather than a file, and provide controls to cycle through all of the images in a directory.</p>
   </item>
   <item>
   <p>Apply random filters and effects to the image when it is loaded and allow the user to save the modified image.</p>
   <p><link href="http://www.gegl.org/api.html">GEGL</link> provides powerful image manipulation capabilities.</p>
   </item>
   <item>
   <p>Allow the user to load images from network shares, scanners, and other more complicated sources.</p>
   <p>You can use <link href="http://library.gnome.org/devel/gio/unstable/">GIO</link> to handle network file tranfers and the like, and <link href="http://library.gnome.org/devel/gnome-scan/unstable/">GNOME Scan</link> to handle scanning.</p>
   </item>
  </list>
</section>


</page>
