Crazy Eddies GUI System  0.7.0
The Beginners Guide to Injecting Inputs
Paul D Turner

Having read the previous tutorials in this series, you now have your GUI rendering set up, the files loaded and even have a window on screen - however you are probably wanting to have some user interaction too. This is the subject of this final tutorial in the series; here we will show the required tasks in order to end up with a complete functioning GUI in your application.

Introduction to input for CEGUI

First the bad news

It shocks some people to discover that CEGUI does not do any automatic collection of user input; it is the responsibility of the application itself to tell CEGUI about any events that it needs to know about. This means that you have to tell CEGUI each time a key is pressed, or the mouse moves, and so on. While this may seem strange at first, the reality is that it affords you a lot more power and flexibility; we are not tying you down to any particular system for your inputs, and you may additionally filter input before it gets to CEGUI, although these are more advanced concepts best left for another time.

Get your inputs injected

In order to tell CEGUI about the input events going on around it, we have an input injection interface. This consists of a set of functions in the CEGUI::System class - there is one member function for each type of base input:

Yes, that's quite a collection! The first thing that you might notice is that there appears to be some repetition - things like 'mouse move' and 'mouse position', 'key down' and 'char'. For the mouse, we offer the possibility of injecting a relative movement of the mouse from it's last injected location or an absolute position - which one of these you choose will largely depend upon the type of inputs that your input library provides you with for the mouse. For keys, it is generally required that both up/down strokes and also characters are injected - there are a couple of reasons for this; first, not all keys generate a character code (like shift, alt, and so on), and second, it allows you to do your own custom (or operating system supplied) key-mapping and key auto-repeat (since CEGUI does not currently offer these functions).

The other thing to notice is the boolean return value from the injection functions. This is used to relay back to your application whether or not CEGUI actually consumed the input. If this returns false, it means that CEGUI did nothing useful with the input, and that your application may like to perform some action based on it instead. Generally for this to work as described, you should have a fullscreen DefaultWindow as your layout root with the MousePassThroughEnabled property set to true.

A little more detail: What each injector is used for

Here we will offer a brief description of what each injection function is used for, the data it expects, and what, in general, is done with the input.

bool System::injectMouseMove( float delta_x, float delta_y )

This is used to inject relative mouse movements. The vales delta_x and delta_y specify the direction and number of screen pixels the mouse has moved on the x axis and y axis respectively. This causes the mouse to move by the specified amount (the actual amount moved can be changed by setting a mouse scaling factor via the System::setMouseMoveScaling function). If you use this, you generally do not need to use injectMousePosition.

bool System::injectMousePosition( float x_pos, float y_pos )

This is used to inject the current absolute position of the mouse. The values x_pos and y_pos specify the position of the mouse in pixels, where a position of (0, 0) represents the top-left hand corner of the CEGUI display (so if you're in windowed mode, it's the corner of the window and not the corner of the entire screen). The CEGUI mouse cursor will be set to the new position. If you use this, you generally do not need to use injectMouseMove.

bool System::injectMouseLeaves( void )

This function informs CEGUI that the mouse cursor has left the host window that CEGUI considers it's rendering area. This is useful if running in windowed mode to inform widgets that the mouse has actually left the CEGUI display completely (otherwise it may not get to know, since under many systems no more mouse events are generated for an OS window once the mouse has left it).

bool System::injectMouseButtonDown( MouseButton button )

This tells CEGUI that a mouse button has been pressed down. The value button is one of the CEGUI::MouseButton enumerated values, which are as follows:

If the values from your input library do not match these, you will have to perform a translation step. Also note that the value NoButton is not 0.

bool System::injectMouseButtonUp( MouseButton button )

This tells CEGUI that a mouse button has been released. As for the System::injectMouseButtonDown function, the value button is one of the CEGUI::MouseButton enumerated values.

bool System::injectKeyDown( uint key_code )

This tells CEGUI that a key has been pressed. The value key_code is the scan code for the key - note that this is not an ASCII or other text encoding value. The available scan codes are defined in the CEGUI::Key::Scan enumeration. If you are using Microsoft DirectInput, then our scan codes are the same ones output by that library, in other cases you may be required to perform some translation. Note that for current releases, and depending upon your expected use, it may not be required to inject all key down/up strokes - the most common ones that you likely will need are for backspace, delete, enter, the shift keys and the arrow keys.

At present no automatic key mapping and generation of character codes is performed, also there is no integrated key auto-repeat functionality - though these functions may appear in future releases. If you need key auto-repeat then you will need to either use an input library that offers this function, or implement something directly. Of course you will almost certainly need character input, so for that look at the System::injectChar function below.

bool System::injectKeyUp( uint key_code )

This tells CEGUI that a key has been released. As for the System::injectKeyDown function, the value key_code is a scan code for the key - and again note that this is not an ASCII or other text encoding value - see above for a more detailed description of the key scan codes.

bool System::injectChar( utf32 code_point )

This function tells CEGUI that a character key has been pressed - you will need this in order to input text into CEGUI widgets. The value code_point' is a Unicode UTF32 code point value (see the unicode website at for information about unicode). How you obtain this value is something that is dependant upon the input library that you are using. For many people, who just wish to use ASCII values, you can just pass in your ASCII codes unmodified, since Unicode values between 0 and 127 are the same as the standard ASCII codes. For other uses, you will need to consult the API documentation for your input library (it is possible, for example, to get the Microsoft Windows message pump to send you key codes in UTF32 form, though exactly how it is done is beyond the scope of this introductory tutorial).

bool System::injectMouseWheelChange( float delta )

This function is used to tell CEGUI about the use of the mouse wheel or scroll wheel (whatever you like to call it!). Use positive values for forward movement (rolling the wheel away from the user), and negative values for backwards movement (rolling the wheel towards the user).

bool System::injectTimePulse( float timeElapsed )

This function is used to keep CEGUI informed about time (CEGUI doesn't have a wristwatch, you see!). The value timeElapsed is a floating point number that indicates the number of seconds - or part seconds - that have passed since the last time the injector was called (or since CEGUI was started). The use of this function is becoming more and more important - it now controls things such as fades and timing for tool-tip widgets, menus, and also auto-repeat for mouse buttons.


Here we have seen the general idiom that CEGUI uses for obtaining externally generated input events. We have seen the methods for passing these inputs to CEGUI, and the type and format of the information to be passed.

Unlike some of the other tutorials in this series, we did not provide concrete code examples. The main reason for this was to keep the tutorial reasonably short; to prevent it becoming a jumble of code for every possible combination of input system, and in the process causing more confusion. The use of any individual input library could easily fill a tutorial of it's own.