The Beginners Guide to Creating a CEGUI Window

Author

Paul D Turner

The purpose of this tutorial is to show you how to create a simple window and get it on screen. Before continuing here, it is very important that you have already read and fully understood the articles The Beginners Guide to Getting CEGUI Rendering, The Beginners Guide to resource loading with ResourceProviders and The Beginners Guide to Data Files and Defaults Initialisation because this tutorial builds upon the basic ideas introduced in those previous tutorials.

Introduction to window and widget concepts

Before we get to the the meat of this tutorial there are some essential ideas that you must first consider.

Every widget is a window

This is the most central concept that must be grasped when working with CEGUI. Every widget within the system is derived from the same Window base class; so for the purposes of this tutorial, whenever I mention a window, the same ideas could just as easily be applied to a push button or scrollbar widget.

Many settings are inherited

Many of the settings and properties available for windows in CEGUI are passed down the window hierarchy. For example, if you set the alpha transparency on a particular window to 0.5, then by default, all window and widgets attached to that window will also be affected by the change applied at the higher level; although note also that the actual setting on the child window remains unchanged - the final values and/or settings used are usually some combination of the setting values from all windows in the hierarchy down to the current window. This also applies to things such as window destruction; by default, a window will destroy attached child windows and widgets when it is destroyed. The main advantages of this arrangement are that you can easily affect a the whole GUI by making changes to the root window settings for things like alpha, visibility, enabled / disabled state, and can easily 'clean up' an entire GUI layout by simply destroying the root window. The default 'inherited' behaviours can be overridden on a per window basis where more fine grained control is needed, or where certain advanced management techniques are to be used.

Creating the windows

Enough of the waffle! Let's create a window.

There are two ways of doing this, via C++ code and XML layout files. Each approach is discussed below.

GUI Creation via C++ code

All windows in CEGUI are created by the WindowManager singleton object. You can get access to this object via the WindowManager::getSingleton function:

using namespace CEGUI;
WindowManager& wmgr = WindowManager::getSingleton();

In general, you will be using what is known as a DefaultWindow (or to use its old name, DefaultGUISheet) as the 'root' window in your GUI. This is not required, but is the accepted pattern of usage for CEGUI and, once you start adding more top-level windows, helps simplify laying things out.

So, to get the ball rolling, we'll create a DefaultWindow as set it as the root 'GUI Sheet' for the GUI:

Window* myRoot = wmgr.createWindow( "DefaultWindow", "root" );
System::getSingleton().setGUISheet( myRoot );

The WindowManager::createWindow function takes two strings as its parameters. The first one, "DefaultWindow" in this example, tells the system the type or class of the window you wish to create. Generally, the windows you have available are those which were registered when you loaded your scheme file, although some, like DefaultWindow, are global types and are always available. The second parameter, "root" in this example, is a unique name which will be assigned to the window; this name can be used to retrieve a pointer to the window from the WindowManager at a later time. Note that naming your root window "root" is not required, but is a common convention.

The System::setGUISheet function is used to specify a given window as the root of the GUI. This will replace any current sheet / root window, although do note that the previous window hierarchy is not actually destroyed - it is just detached from the display - you can easily switch between GUI 'pages' by simply flipping between them using the System::setGUISheet function.

Now you have created your first window and attached it to the GUI system, and the system will use this window as the root of the GUI when it draws the GUI. But if you were to compile a simple program using this code, you still can't see anything; what gives? There's nothing wrong with your application, the DefaultWindow which we created above is just totally invisible! This is what makes the DefaultWindow ideally suited as a root window; it serves as a blank canvas onto which other window and widgets can be attached. Lets do that now...

Here we will create a frame window; this is a window that functions in a similar manner to the windows on your desktop UI, it has a title bar and can be moved and re-sized.

FrameWindow* fWnd = static_cast<FrameWindow*>(
    wmgr.createWindow( "TaharezLook/FrameWindow", "testWindow" ));

here we are creating a "TaharezLook/FrameWindow" window. This name uses another convention seen throughout the system, whereby a window type is prefixed by the name of the widget set (if you were to load the WindowsLook scheme, you could create a "WindowsLook/FrameWindow" object instead). We have given our new window the simple test name of "testWindow". One final thing to note is the use of the cast, this is required since the WindowManager::createWindow function always returns a the Window type. In this - and many other - cases a basic Window pointer will suffice, but there are times when you'll want to access functions introduced in the sub-classes, so the use of the cast as shown is common when using CEGUI.

In order for the system to be able to do something useful with our new window, we must perform a few additional steps.

First, we must attach the window to the root window we established above:

myRoot->addChildWindow( fWnd );

Now we can set an initial position and size for our window. CEGUI uses a 'unified' co-ordinate system enabling the use of relative (scale) and absolute (offset) components at the same time - this is why each co-ordinate you will see has two parts. For a slightly extended introduction of this concept see The Unified Co-ordinate System which is part of the Falagard skinning system for CEGUI. Back to the example:

// position a quarter of the way in from the top-left of parent.
fWnd->setPosition( UVector2( UDim( 0.25f, 0 ), UDim( 0.25f, 0 ) ) );

// set size to be half the size of the parent
fWnd->setSize( UVector2( UDim( 0.5f, 0 ), UDim( 0.5f, 0 ) ) );

Finally, we set a caption for the frame window's title bar:

fWnd->setText( "Hello World!" );

And that's it! When compiled into an application, you will now see a simple frame window in the middle of the display.

XML layouts

All of the above is very nice, but there is one major drawback; any time you wish to adjust the GUI layout, you need to edit your code and recompile. This will get old pretty quick, so what you really want is to be able to specify your GUI layout externally, and have your code load the layout via a file. This is the purpose of the CEGUI layout XML files.

The system supports XML layout files, which can be loaded via the WindowManager::loadWindowLayout function. This function creates all the windows for you and returns a pointer to the root window of the loaded hierarchy - which is ideal for assigning as the root of the GUI!

So, first of all we need a layout file. The following XML saved as a text file, is a layout file equivalent to the code we discussed above:

<?xml version="1.0" ?>
<GUILayout>
    <Window Type="DefaultWindow" Name="root">
        <Window Type="TaharezLook/FrameWindow" Name="testWindow">
            <Property Name="UnifiedPosition" Value="{ {0.25, 0}, {0.25, 0} }" />
            <Property Name="UnifiedSize" Value="{ {0.5, 0}, {0.5, 0} }" />
            <Property Name="Text" Value="Hello World!" />
        </Window>
    </Window>
</GUILayout>

The Window elements show an obvious mapping to the WindowManager::createWindow function - they take a type and a name which directly correspond to the parameters discussed previously.

Nesting of the Window elements is used to attach certain windows to others. Note that you may only have one 'root' level window in a layout file, which is another reason you'll usually see the DefaultWindow used as a canvas on which other windows and widgets are placed.

The Property elements are used to set properties on the Window being defined. There are many properties available for each window/widget class, and each class also inherits all properties from it's parent class. See the Properties namespaces in the API reference for documentation on the available hard-coded properties and their value string formats. Since 'Falagard' skins can create their own properties, it is likely that the windows you are using contain many more properties than listed in the previous link - for these 'soft' properties, you need to consult whichever documentation is provided with the skin you are using (for example, see the CEGUI Wiki: TaharezLook Properties and WindowsLook Properties.

If saved as a file called "test.layout", you could load this layout and set it as the GUI root as follows:

using namespace CEGUI;
Window* myRoot = WindowManager::getSingleton().loadWindowLayout( "test.layout" );
System::getSingleton().setGUISheet( myRoot );

The end result is exactly the same as what was done in C++ code earlier, except that now you can modify and enhance the GUI layout without the need for constant editing and recompilation of the application code.

Conclusion

Here you have seen how to perform basic window creation, how to create a simple window hierarchy, and how modify some window settings. You have seen how this can be accomplished using both C++ code and external XML layout files. There are many advanced possibilities available using both methods, although such possibilities are beyond the scope of this indroductory tutorial.