GNOME.org

Message board

In this tutorial, you will learn:

  • How to display a web page with WebKit.

  • How to manipulate the contents of a web page using WebKit's DOM functions.

This tutorial assumes you are familiar with the C programming language and have a basic understanding of GTK+, including how to create and place widgets and how to connect callback functions to signals. See Image viewer to learn the basics of GTK+.

Create a project in Anjuta

The GNOME platform includes WebKitGTK+, built on top of the powerful WebKit HTML framework. WebKit is used throughout GNOME, not just to view web pages on the Internet, but also to create rich user interfaces that can be easily styled with CSS.

In this tutorial, you will create a simple message board using WebKit. The message board will allow you to enter some text and have it added to a list of messages in HTML. Before you begin, you need to set up a project in Anjuta.

  1. In Anjuta, click File ▸ New ▸ Project to open the new project assistant.

  2. Select GTK+ (simple) on the C tab, and click Continue.

  3. Fill out your details on the Basic information page. Use message-board for the project name. Click Continue.

  4. Disable the Use GtkBuilder for user interface option as this tutorial builds the user-interface manually.

  5. You need to tell Anjuta you're using WebKitGTK+ on this project. On the Project options page, select Configure external packages. Click Continue. On the Configure external packages page, check webkitgtk-3.0.

After you finish the new project assistant, open the file src/main.c from either the Project or the File tab. Anjuta will have filled this in with some basic GTK+ code from its templates. Since you are creating a WebKit project, you first need to include the WebKit headers. After the line that includes gtk/gtk.h, add the following line:

#include <webkit/webkit.h>

Verify that everything works by building what you have so far. Click Build ▸ Build Project or just press Shift+F7. The first time you build, you will be asked for some configure options. Just accept the defaults and click Execute.

You should now be able to run the program. Click Run ▸ Execute or just press F3. You should see an empty window appear.

Lay out your window and web view

Now that you can show a window, it's time to start working with WebKit. For this tutorial, you'll create a text entry and a web view and pack them both into a window. Find the function create_window and replace it with the following:

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static GtkWidget*
create_window (void)
{
    GtkWidget *window, *box, *scroll, *view, *entry;

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_window_set_default_size (GTK_WINDOW (window), 400, 400);
    gtk_window_set_title (GTK_WINDOW (window), "Message Board");
    g_signal_connect (window, "delete-event", G_CALLBACK (gtk_main_quit), NULL);

    box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 6);
    gtk_container_set_border_width (GTK_CONTAINER (box), 6);
    gtk_container_add (GTK_CONTAINER (window), box);

    entry = gtk_entry_new ();
    gtk_box_pack_start (GTK_BOX (box), entry, FALSE, FALSE, 0);

    scroll = gtk_scrolled_window_new (NULL, NULL);
    g_object_set (scroll, "shadow-type", GTK_SHADOW_IN, NULL);
    gtk_box_pack_start (GTK_BOX (box), scroll, TRUE, TRUE, 0);

    view = webkit_web_view_new ();
    gtk_container_add (GTK_CONTAINER (scroll), view);
    webkit_web_view_load_string (WEBKIT_WEB_VIEW (view),
                                 "<html><body></body></html>",
                                 "text/html",
                                 "UTF-8",
                                 NULL);

    gtk_widget_show_all (GTK_WIDGET (box));
    return window;
}

You first create a GtkWindow object and set its title and default size. You also connect the gtk_main_quit function to the delete-event signal. The delete-event signal is emitted when the window is closed. The gtk_main_quit function is part of GTK, and it quits the application.

You then create a vertical box and add it to the window. A window can only hold a single child widget, so you need to use a box to add multiple widgets. The second argument to gtk_box_new sets the amount of padding (in pixels) between each child, and the next line puts a six-pixel border around the entire thing.

You next create a GtkEntry object and pack it into the box. The third and fourth arguments to gtk_box_pack_start specify that the entry shouldn't take up any extra space the box has available. The fourth argument is the amount of padding you want around the entry. In this case, you set the padding to zero, because you're allowing the box to handle all the padding.

Before you add a web view, you have to create a scrolled window to put it inside of. The scrolled window will place scrollbars on the right and bottom when necessary, and prevent your web view from filling your entire screen. This time, you pass TRUE and TRUE to gtk_box_pack_start to allow the scrolled window (and thus, the web view) to use any extra space available in the box.

Finally, you create a WebKitWebView and add it to the scrolled window. Then load a very basic HTML page into the web view by calling webkit_web_view_load_string with the following arguments:

WEBKIT_WEB_VIEW (view)

The view itself. Because view is typed as a GtkWidget*, you have to use WEBKIT_WEB_VIEW to safely cast the object.

"<html><body></body></html>"

The simplest HTML file you could possibly write.

"text/html"

The MIME type of the content you provided. In this case, you're using plain HTML.

"UTF-8"

The character encoding of the content you provided. Although you only used ASCII characters, it's a good idea to specify UTF-8. UTF-8 is used as the default encoding throughout the GNOME platform.

NULL

The base URI. You don't need it in this simple example, but you might want to provide a file: URI if you add images or other features where you want to use relative URI references.

Every time you add a widget, you have to call gtk_widget_show on it for it to be visible. If you call gtk_widget_show_all on a container widget like a GtkBox, GTK+ will automatically show all the widgets inside the container, to any depth. Sometimes you don't want to call gtk_widget_show_all, such as when you want to dynamically hide and show some widgets in response to events.

Finally, you have to call gtk_widget_show_all on the box. Otherwise, none of the widgets you created will be visible. (The window is shown in the main function with gtk_widget_show.)

Build and run the message board again. You should see a window with a text entry and a web view. It doesn't do anything yet because the text entry and the web view don't know anything about each other.

Hook up signals

Now you want to make the message board actually do something when you enter text into the text entry. To do this, connect a callback function to the activate signal of entry. GTK+ emits the activate signal whenever the user presses Enter in the entry. Add the following into create_window, anywhere after both entry and view have been defined:

g_signal_connect (entry, "activate", G_CALLBACK (entry_activate_cb), view);

You then have to actually define entry_activate_cb. Define it as follows, anywhere above create_window:

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static void
entry_activate_cb (GtkEntry *entry, WebKitWebView *view)
{
    WebKitDOMDocument *document;
    WebKitDOMElement *body, *div;

    document = webkit_web_view_get_dom_document (view);
    body = webkit_dom_document_query_selector (document, "body", NULL);
    div = webkit_dom_document_create_element (document, "div", NULL);
    webkit_dom_node_set_text_content (WEBKIT_DOM_NODE (div),
                                      gtk_entry_get_text (entry),
                                      NULL);
    webkit_dom_node_append_child (WEBKIT_DOM_NODE (body),
                                  WEBKIT_DOM_NODE (div),
                                  NULL);
    gtk_entry_set_text (entry, "");
}

The first thing you do is get a WebKitDOMDocument object that represents the HTML document displayed in view. The DOM classes and methods in WebKit allow you to inspect and manipulate the HTML document, and work very similarly to the DOM APIs you might already know from JavaScript.

Once you have the document, you want to get the body element so that you can add div elements to it. The webkit_dom_document_query_selector function lets you find an element in the document using CSS selectors. This keeps you from having to write tedious loops to traverse the document.

Next, you create a new div element to hold the message. Every element you create has to be attached to a document, so the function to create an element takes the WebKitDOMDocument as its first arguments. You then set the text content of the element to the contents of the text entry. Because gtk_entry_get_text returns a const gchar*, you don't have to free the result.

Finally, you append the new div element to the body and clear out the text entry so you can type something new. Build and run the program again and test it for yourself.

Make it look better with CSS

At this point, your program is completely functional, but not very pretty. You can style the message display with CSS, just like you can with any other HTML page. There are many ways you could attach some CSS to the page: You could add it in the initial HTML document. You could inline it in the style attribute of the div elements. You could even programmatically construct it using the DOM APIs.

In this tutorial, you'll attach the CSS using the user-stylesheet-uri property of the WebKitWebSetting object attached to your web view. In a more complete application, you would want to save and load your HTML file. Keeping the style information outside the actual HTML means that you can change the styling completely within your application, without having to change users' files. You would normally just install a file along with your application, but just to keep everything in one file for this demo, we'll use a trick called a data URI. First, define the CSS as a static string near the top of your file.

static const guchar CSS[] =
"body { margin: 0; padding: 0; }\n"
"div { "
" -webkit-border-radius: 2px;"
" background: -webkit-gradient(linear, 0% 100%, 0% 0%,"
" from(#f1f1f1), to(white));"
" border: solid 1px #c6c6c6;"
" -webkit-box-shadow: 0px 0px 2px #c6c6c6;"
" margin: 12px; padding: 6px;"
"}";

All you have in this example are div elements inside a body element. If you created more complicated HTML, you could use whatever CSS is necessary. In fact, if you're comfortable with CSS, you should trying changing this to something you like better.

To apply the CSS, you set the user-stylesheet-uri in the create_window function, anywhere after view has already been defined.

tmp = g_base64_encode (CSS, strlen((gchar *) CSS));
css = g_strconcat ("data:text/css;charset=utf-8;base64,",
                   tmp, NULL);
g_object_set (webkit_web_view_get_settings (WEBKIT_WEB_VIEW (view)),
              "user-stylesheet-uri", css, NULL);
g_free (css);
g_free (tmp);

Also, make sure to add variable declarations for tmp and css to the top of create_window.

gchar *tmp, *css;

A data URI starts with data: and some information about the content type and how the data is encoded. The actual data follows after a comma, in this case encoded in Base64. Unlike other URI schemes like http:, ftp:, and file:, the data: URI scheme doesn't specify where to find a file to load. Rather, it gives the entire contents of the file.

The code above first encodes your CSS definitions in Base64, then combines that with a fixed string to create a data URI. The g_strconcat function can take any number of string arguments and concatenate them all together, so you have to pass NULL as the final argument so it knows when to stop. And don't forget to free those temporary strings after you set the stylesheet property.

Build and run the program again. It should now work exactly the same as at the end of the last section, except the messages will be nicely styled with a border and a subtle background gradient.

Learn more

This tutorial showed you how to create a basic application using GTK+ and WebKit, including showing a document and manipulating its contents. To create a real application, you probably want to do a little bit more. Try adding features on your own. Here are a few ideas:

  • If you're comfortable with CSS, try changing the style of the message display. CSS is easy to get started with, but increasingly more powerful. There is a wealth of CSS tutorials on the Internet, and just about everything you can do on the web, you can do in this application.

  • Right now, you lose all your messages whenever you close the message board. Try saving the HTML contents after each post, and loading the saved file (if it exists) on startup.

  • If you keep your messages around for a long time, you'll start wondering when you posted them. Add a timestamp to each message when it's posted. You'll probably want to create some additional child div elements with different classes that you can style in the CSS.

  • This program keeps messages around forever. Think about ways you could allow the user to delete messages. Perhaps you want messages to disappear automatically after they're too old, or after there are a certain number of messages before them. Or you could add a link in each message to delete it. You could even override the context menu when you right-click on a message. These features involve exploring WebKit's DOM API more.