# Message Board (C)

A simple program using WebKitGTK+ and the DOM.
Shaun McCance
shaunm@gnome.org 2010

# Message Board

      In diesem Tutorial lernen Sie, wie:

        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  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.

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

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

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

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

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:

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`:

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.

shaunm

FIXME: Is this true? Does query_selector take CSS, CSSish, or what?

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.

shaunm

Not passing the GError**, but we should give it a quick mention and link to somewhere that explains how GError-handling works.

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.

shaunm

g_base64_encode has bad args

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.

shaunm

Link to method to get HTML from DOM and to GIO APIs.

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.

shaunm

Link to strftime or something

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.