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## Implementing a GUI with ClanLib

Thanks to the new GUI API in ClanLib, having a GUI in your game no longer has to be an ardous boring task. ClanLib GUI comes complete with implementations of most standard widgets/components, such as inputboxes, scrollbars, comboboxes, menus, listboxes, checkboxes and buttons.

At the core of the ClanLib GUI API, lies the routing of GUI input to game code. ClanLib uses a signals/slot system for this. If you are familiar with QT, you should also be familiar with signals and slots, but if you're in doubt, you can read our signals and slots overview.

The ClanLib GUI has a default look, eg. the way each component is displayed, but the logical functionality of each implemented base-component is entirely isolated from the displaying, so it's easy to create a customized GUI "style". Styles are managed through a style-manager that makes it easy to extend the default-style with new ones. This makes it possible for people to create a new GUI style, and release it for easy use by other ClanLib-users. An overview on how to create custom style can be read here.

There are two ways to create GUI components, using standard C++ objects, or using special GUI definition script files.

TODO: Write about manually creating components

GUIs can be defined in special GUI definition script files, which provide a flexible and powerful way to describe the layout of a GUI. Components are layered in a hierarchical fashion. Each component has a set of parameters (mandatory or optional) that describe the component, and through a naming-scheme, these components can be modified and hooked to callback functions at run-time. You can read more about that in the GUI resource overview.

### GUI systems in general

This is a little introduction in what a GUI framework is and how you use them. If you are already familiar with a GUI toolkit, MFC, Gtk+ og Qt, then odds are you know this already, so you might want to skip to the next section.

When you look at a normal GUI application then it consists of a set of components (often referrered as widget or controls too). In the case of notepad (you do know notepad, right? :)), it consist of a window frame, menubar, a textbox and in newer versions there's a statusbar too:

<insert picture here pointing at the different components of notepad>

Drawn as a tree, notepad looks like this:

    + Window (titlebar, resize frame around all of notepad)
      + Menu bar
        + File
        + Edit
        + View
        + Help
      + Textbox
      + Statusbar

For each of these objects, you should construct a CL_Component that handles the user interaction in these areas of the screen:

    CL_Window *window = new CL_Window(CL_Rect(0,0,640,480), gui);
    CL_MenuBar *menubar = new CL_MenuBar(CL_Rect(0,0,640,32), window);
    CL_TextBox *textbox = new CL_TextBox(CL_Rect(0,32,640,480-64), window);
    CL_StatusBar *statusbar = new CL_StatusBar(CL_Rect(0,480-32,640,32), window);

Note: clanGUI does not work exactly like this example, but it illustrates the concept.

Whenever the user clicks on the menubar, the menubar object is notified. If he types text, the textbox object is notified, and so forth.

Only one component can have the focus at a time. All keyboard activity is directed to this component until focus is put on another component. In the notepad example, the textbox will usually have the focus, but if you eg click on the File menu, then a popup component will be created that gets the focus. Pressing the arrow keys will then be sent to the popup menu component and it will move the selected item indicator.

A GUI toolkit consist mainly of two parts: first a generic component class (CL_Component in clanGUI) which represents any object in the GUI tree. It does not do anything by itself, but its the interface in which you interact with the user in a certain space of the screen. Beside that, a GUI toolkit have a set of CL_Component inheriated objects that make it easier to do common UI tasks, this is usually inputboxes, labels, menus, frames, etc.

### Event handling

One of the keys to learning how to use clanGUI is to understand how it uses signals and slots to do event handling. If someone clicks on a button component, the CL_Button object will emit its sig_clicked signal, and any slot connected to the signal will get called.

Here's an example:

    void MyApp::show_gui()
    {
    	CL_StyleManager_Silver style(....);
    	CL_GUIManager gui(&style);

    	CL_Button button_ok(CL_Rect(....), "Ok", &gui);
    	CL_Button button_cancel(CL_Rect(....), "Cancel", &gui);

    	CL_SlotContainer slots;
    	slots.connect(button_ok.sig_clicked(), this, &MyApp::on_button_ok);
    	slots.connect(button_cancel.sig_clicked(), this, &MyApp::on_button_cancel);

    	gui.run();
    }

    void MyApp::on_button_ok()
    {
    	std::cout << "Button ok was clicked!" << std::endl;
    }

    void MyApp::on_button_cancel()
    {
    	std::cout << "Button cancel was clicked!" << std::endl;
    }

Each of the components in clanGUI have their own special events that can occour, but a few signals are common for all GUI components:

- Key events (key down, key up, key repeat)
- Mouse events (mouse up, mouse down, mouse move)
- Paint events (begin_paint, paint, end_paint)

If you want to do custom drawing in your GUI, you need to hook yourself in on the sig_paint signal of the component you want to customize. Normally you should not need to do this for normal objects, but if you want to make your own objects in the GUI, then you will most likely want to use most of the common signals.

### ClanGUI System Basics

In order to be able to use any GUI toolkit properly and take fully advantage of its possibilities, it is essential to understand how the core classes operate and communicate with each others. In clanGUI there are the following core classes:

- CL_Component
- CL_ComponentStyle
- CL_StyleManager
- CL_GUIManager
- CL_ComponentManager

#### CL_Component

The component class is the core class of clanGUI. It presents any widget/control/window in the system and is responsible for drawing the components and feeding the components with user input.

As with just about any other GUI toolkit out there, components are ordered in a tree. At the top of the tree, we have the root/desktop component. It represents the entire screen area and any child components on the screen. A typical child of the root component could be a window component, and the window component then has maybe a inputbox and two button child components. Anyone ever having looked at a GUI toolkit earlier in their life should be familiar with this concept.

The root component in clanGUI is called CL_GUIManager. More on this when we examine the GUI manager more closely.

CL_Component exports a large set of signals that any subclassed component usually connects signals to. The GUI emits these events whenever interaction with the component is required. Some examples of these signals are: sig_paint, sig_key_down, sig_mouse_move and sig_got_focus. For instance, if the GUI needs the component drawn, it will emit the sig_paint signal and all hooked up painting functions will then draw the component.

#### CL_ComponentStyle

The component style class is the component styling (theme) interface. When a component is instantiated, one component style class is attached to the component.

The main purpose of the component style class is to seperate styling data and functions from the component class. This is directly compareable to a document/view relationship, where the component class is the document, and then style class is a view.

To illustrate this a bit further, imagine we are constructing an input button. At construction time, an input button specific component style is attached. The style class connects a local member function to the sig_paint signal of its CL_Button owner and stores data needed for the styling as local member variables.

When the GUI needs to draw the button, it will emit the sig_paint signal, which causes the member function in the style class to be called. This function then uses the public available attribute functions in its CL_Button owner to figure out what the text of the input button is and where its located, finally drawing the button on screen.

This two level construction of a component allows us to construct component objects without knowing what style is being used, making styles totally transparent for anything but the styling objects.

#### CL_StyleManager

Something need to attach these component styles to a component. This is what the CL_StyleManager interface does. When a component is constructed, the very first thing it does is to contact its style manager, where it asks it to attach component styles to it.

CL_StyleManager also function as a class factory. When loading components from gui definition files, the component manager asks the style manager to create an instance of a control based on a type string and a set of component options.

#### CL_GUIManager

The GUI manager is the root comopnent in clanGUI. It should always be the top-level component in any component tree. The GUI manager contain the main message pump for the GUI and is responsible for routing events from clanCore and clanDisplay into the GUI system.

#### CL_ComponentManager

The component manager is the interface used to load GUI component trees from a GUI definition file. Have a look on the login_view.cpp/h files in the CTalk example for an example of how to use it.

TODO: Add some code examples

Questions or comments, write to the ClanLib mailing list.