gnome-devel-docs 3.28.0-1 / usr / share / help / C / gnome-devel-demos / message-board.c.page

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<page xmlns="http://projectmallard.org/1.0/"
      xmlns:its="http://www.w3.org/2005/11/its"
      type="topic"
      id="message-board.c">

  <info>
    <title type="text">Message board (C)</title>
    <link type="guide" xref="c#examples"/>

    <desc>A simple program using WebKitGTK+ and the DOM.</desc>

    <revision pkgversion="0.1" version="0.1" date="2010-12-06" status="draft"/>
    <credit type="author copyright">
      <name>Shaun McCance</name>
      <email its:translate="no">shaunm@gnome.org</email>
      <years>2010</years>
    </credit>
    <credit type="editor">
      <name>Marta Maria Casetti</name>
      <email its:translate="no">mmcasetti@gmail.com</email>
      <years>2013</years>
    </credit>
  </info>

<title>Message board</title>

<synopsis>
  <p>In this tutorial, you will learn:</p>
  <list style="compact">
    <item><p>How to display a web page with WebKit.</p></item>
    <item><p>How to manipulate the contents of a web page using WebKit's DOM
    functions.</p></item>
  </list>
  <p>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 <link
  xref="image-viewer.c"/> to learn the basics of GTK+.</p>
</synopsis>

<media type="video" mime="video/ogg" src="media/message-board.ogv"/>

<links type="section"/>

<section id="create">
  <title>Create a project in Anjuta</title>

  <p>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.</p>

  <p>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.</p>

  <steps>
    <item><p>In Anjuta, click <guiseq><gui>File</gui><gui>New</gui>
    <gui>Project</gui></guiseq> to open the new project assistant.</p></item>
    <item><p>Select <gui>GTK+ (simple)</gui> on the <gui>C</gui> tab,
    and click <gui>Continue</gui>.</p></item>
    <item><p>Fill out your details on the <gui>Basic information</gui> page.
    Use <input>message-board</input> for the project name.
    Click <gui>Continue</gui>.</p></item>
    <item><p>Disable the <gui>Use GtkBuilder for user interface</gui> option as
    this tutorial builds the user-interface manually.</p>
    </item>
    <item><p>You need to tell Anjuta you're using WebKitGTK+ on this project.
    On the <gui>Project options</gui> page, select <gui>Configure external
    packages</gui>. Click <gui>Continue</gui>. On the <gui>Configure external
    packages</gui> page, check <gui>webkitgtk-3.0</gui>.</p></item>
  </steps>

  <p>After you finish the new project assistant, open the file
  <file>src/main.c</file> from either the <gui>Project</gui> or the
  <gui>File</gui> 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 <code>gtk/gtk.h</code>, add the following line:</p>

  <code>#include &lt;webkit/webkit.h></code>

  <p>Verify that everything works by building what you have so far.
  Click <guiseq><gui>Build</gui><gui>Build Project</gui></guiseq> or
  just press <keyseq><key>Shift</key><key>F7</key></keyseq>. The first
  time you build, you will be asked for some configure options. Just
  accept the defaults and click <gui>Execute</gui>.</p>

  <p>You should now be able to run the program. Click <guiseq>
  <gui>Run</gui><gui>Execute</gui></guiseq> or just press <key>F3</key>.
  You should see an empty window appear.</p>
</section>

<section id="webview">
  <title>Lay out your window and web view</title>

  <p>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 <code>create_window</code> and
  replace it with the following:</p>

<code style="numbered" mime="text/C"><![CDATA[
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;
}
]]></code>

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

  <p>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 <code>gtk_box_new</code> sets the amount of padding
  (in pixels) between each child, and the next line puts a six-pixel border
  around the entire thing.</p>

  <p>You next create a <code>GtkEntry</code> object and pack it into the box.
  The third and fourth arguments to <code>gtk_box_pack_start</code> 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.</p>

  <p>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 <code>TRUE</code> and <code>TRUE</code> to
  <code>gtk_box_pack_start</code> to allow the scrolled window (and thus, the
  web view) to use any extra space available in the box.</p>

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

  <terms>
    <item>
      <title><code>WEBKIT_WEB_VIEW (view)</code></title>
      <p>The view itself. Because <code>view</code> is typed as a
      <code>GtkWidget*</code>, you have to use <code>WEBKIT_WEB_VIEW</code>
      to safely cast the object.</p>
    </item>
    <item>
      <title><code>"&lt;html>&lt;body>&lt;/body>&lt;/html>"</code></title>
      <p>The simplest HTML file you could possibly write.</p>
    </item>
    <item>
      <title><code>"text/html"</code></title>
      <p>The MIME type of the content you provided. In this case, you're
      using plain HTML.</p>
    </item>
    <item>
      <title><code>"UTF-8"</code></title>
      <p>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.</p>
    </item>
    <item>
      <title><code>NULL</code></title>
      <p>The base URI. You don't need it in this simple example, but you might
      want to provide a <sys>file:</sys> URI if you add images or other features
      where you want to use relative URI references.</p>
    </item>
  </terms>

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

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

  <p>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.</p>
</section>

<section id="signals">
  <title>Hook up signals</title>

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

<code><![CDATA[
g_signal_connect (entry, "activate", G_CALLBACK (entry_activate_cb), view);
]]></code>

  <p>You then have to actually define <code>entry_activate_cb</code>. Define
  it as follows, anywhere above <code>create_window</code>:</p>

<code style="numbered"><![CDATA[
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, "");
}
]]></code>

  <p>The first thing you do is get a <code>WebKitDOMDocument</code> object
  that represents the HTML document displayed in <code>view</code>. 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.</p>

  <p>Once you have the document, you want to get the <code>body</code> element
  so that you can add <code>div</code> elements to it. The
  <code>webkit_dom_document_query_selector</code> 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.</p>

  <comment>
    <cite>shaunm</cite>
    <p>FIXME: Is this true? Does query_selector take CSS, CSSish, or what?</p>
  </comment>

  <p>Next, you create a new <code>div</code> 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 <code>WebKitDOMDocument</code> as its first arguments.
  You then set the text content of the element to the contents of the text entry.
  Because <code>gtk_entry_get_text</code> returns a <code>const gchar*</code>,
  you don't have to free the result.</p>

  <comment>
    <cite>shaunm</cite>
    <p>Not passing the GError**, but we should give it a quick mention and
    link to somewhere that explains how GError-handling works.</p>
  </comment>

  <p>Finally, you append the new <code>div</code> 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.</p>
</section>


<section id="css">
  <title>Make it look better with CSS</title>

  <p>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
  <code>style</code> attribute of the <code>div</code> elements. You could
  even programmatically construct it using the DOM APIs.</p>

  <p>In this tutorial, you'll attach the CSS using the <code>user-stylesheet-uri</code>
  property of the <code>WebKitWebSetting</code> 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.</p>

<code><![CDATA[
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;"
"}";
]]></code>

  <p>All you have in this example are <code>div</code> elements inside a
  <code>body</code> 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.</p>

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

  <comment><cite>shaunm</cite><p>g_base64_encode has bad args</p></comment>

<code><![CDATA[
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);
]]></code>

  <p>Also, make sure to add variable declarations for <code>tmp</code>
  and <code>css</code> to the top of <code>create_window</code>.</p>

<code>
gchar *tmp, *css;
</code>

 <p>A data URI starts with <sys>data:</sys> 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
 <sys>http:</sys>, <sys>ftp:</sys>, and <sys>file:</sys>, the <sys>data:</sys>
 URI scheme doesn't specify where to find a file to load. Rather, it gives
 the entire contents of the file.</p>

 <p>The code above first encodes your CSS definitions in Base64, then
 combines that with a fixed string to create a data URI. The
 <code>g_strconcat</code> function can take any number of string arguments
 and concatenate them all together, so you have to pass <code>NULL</code>
 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.</p>

 <p>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.</p>
</section>

<section id="more">
  <title>Learn more</title>

  <p>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:</p>

  <list>
    <item><p>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.</p></item>

    <item><p>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.</p>
    <comment>
      <cite>shaunm</cite><p>Link to method to get HTML from DOM and to
      GIO APIs.</p>
    </comment></item>

    <item><p>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 <code>div</code>
    elements with different classes that you can style in the CSS.</p>
    <comment><cite>shaunm</cite><p>Link to strftime or something</p></comment>
    </item>

    <item><p>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.</p></item>
  </list>
</section>
</page>