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<title>The Chickadee Game Toolkit: Kernel</title>
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<a name="Kernel"></a>
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<p>
Next: <a href="Input.html#Input" accesskey="n" rel="next">Input</a>, Up: <a href="API-Reference.html#API-Reference" accesskey="u" rel="up">API Reference</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Kernel-1"></a>
<h3 class="section">2.1 Kernel</h3>
<p>At the very core of Chickadee, in the <code>(chickadee)</code> module, lies
an event loop. This loop, or “kernel”, is responsible for creating
and managing the game window, dispatching input events, ensuring that
the game is updated at the desired interval, and rendering graphics.
The kernel implements what is known as a “fixed timestep” game loop,
meaning that the game simulation will be advanced by a fixed interval
of time and will never vary from frame to frame, unlike some other
styles of game loops. The appropriately named <code>run-game</code> and
<code>abort-game</code> procedures are the entry and exit points to the
Chickadee kernel.
</p>
<p>On its own, the kernel does not do very much at all. In order to
actually respond to input events, update game state, or draw something
to the game window, a hacker with a penchant for game development must
latch onto extension points built into the kernel, called “hooks”,
and specify what action ought to be taken for any given event. For
example, the <code>key-press-hook</code> can be used to respond to the
<code>a</code> key being pressed by swinging the player’s mighty sword.
There are many hooks available, so read on to learn about all of them.
For information about using Guile’s hook API, see See <a href="http://www.gnu.org/software/guile/manual/html_node/Hooks.html#Hooks">Hooks</a> in <cite>GNU Guile Reference Manual</cite>.
</p>
<dl>
<dt><a name="index-run_002dgame"></a>Scheme Procedure: <strong>run-game</strong> <em>[#:window-title "Chickadee!"]</em></dt>
<dd><p>[#:window-width 640] [#:window-height 480] [#:window-fullscreen? #f]
[#:update-hz 60]
Start the event loop. This procedure will not return until
<code>abort-game</code> is called.
</p>
<p>The <code>update-hook</code> will be run <var>update-hz</var> times per second.
</p>
<p>A new graphical window will be opened with <var>window-width</var> x
<var>window-height</var> as its dimensions, <var>window-title</var> as its
title, and in fullscreen mode if <var>window-fullscreen?</var> is
<code>#t</code>.
</p></dd></dl>
<dl>
<dt><a name="index-abort_002dgame"></a>Scheme Procedure: <strong>abort-game</strong></dt>
<dd><p>Stop the currently running Chickadee event loop.
</p></dd></dl>
<dl>
<dt><a name="index-time"></a>Scheme Procedure: <strong>time</strong></dt>
<dd><p>Return the current game time in milliseconds.
</p></dd></dl>
<dl>
<dt><a name="index-load_002dhook"></a>Scheme Variable: <strong>load-hook</strong></dt>
<dd><p>A hook that is run once when the event loop boots, before any other
hook is run. This hook is run with zero arguments.
</p>
<div class="example">
<pre class="example">(add-hook! load-hook (lambda () (display "hello!\n")))
</pre></div>
</dd></dl>
<dl>
<dt><a name="index-update_002dhook"></a>Scheme Variable: <strong>update-hook</strong></dt>
<dd><p>A hook that is run every time the game simulation should be advanced.
This hook is run with a single argument <var>dt</var>, the fixed timestep
that was configured when the event loop was started, in milliseconds.
</p>
<div class="example">
<pre class="example">(add-hook! update-hook (lambda (dt) (display "tick!\n")))
</pre></div>
</dd></dl>
<dl>
<dt><a name="index-before_002ddraw_002dhook"></a>Scheme Variable: <strong>before-draw-hook</strong></dt>
<dd><p>A hook that is run before a frame is rendered. This hook is run with
zero arguments.
</p>
<div class="example">
<pre class="example">(add-hook! before-draw-hook (lambda () (display "about to draw!\n")))
</pre></div>
</dd></dl>
<dl>
<dt><a name="index-after_002ddraw_002dhook"></a>Scheme Variable: <strong>after-draw-hook</strong></dt>
<dd><p>A hook that is run after a frame is rendered. This hook is run with
zero arguments.
</p>
<div class="example">
<pre class="example">(add-hook! after-draw-hook (lambda () (display "done drawing!\n")))
</pre></div>
<p>Combined with <code>before-draw-hook</code>, one can perform a frames per
second calculation to monitor game performance and stability.
</p>
</dd></dl>
<dl>
<dt><a name="index-draw_002dhook"></a>Scheme Variable: <strong>draw-hook</strong></dt>
<dd><p>A hook that is run each time a frame should be rendered. This hook is
run with a single argument <var>alpha</var>, a value in the range [0, 1]
which represents how much time has past since the last game state
update relative to the upcoming game state update, as a percentage.
Because the game state is updated independent of rendering, it is
often the case that rendering is occuring between two updates. If the
game is rendered as it was during the last update, a strange
side-effect will occur that makes animation appear rough or
“choppy”. To counter this, the <var>alpha</var> value can be used to
perfrom a linear interpolation of a moving object between its current
position and its previous position. This odd trick has the pleasing
result of making the animation look smooth again, but requires keeping
track of previous state.
</p>
<div class="example">
<pre class="example">(add-hook! draw-hook (lambda (alpha) (display "<(._.<) \n")))
</pre></div>
</dd></dl>
<dl>
<dt><a name="index-quit_002dhook"></a>Scheme Variable: <strong>quit-hook</strong></dt>
<dd><p>A hook that is run when the user clicks the close button on the game
window. This hook is run with zero arguments.
</p>
<div class="example">
<pre class="example">(add-hook! quit-hook (lambda () (display "bye!\n")))
</pre></div>
</dd></dl>
<dl>
<dt><a name="index-key_002dpress_002dhook"></a>Scheme Variable: <strong>key-press-hook</strong></dt>
<dd><p>A hook that is run when a key is pressed on the keyboard. This hook
is run with four arguments:
</p>
<ol>
<li> <var>key</var>: The symbolic name of the “virtual” key that was pressed.
For example: <code>backspace</code>. It’s called a virtual key because the
operating system may map a physical keyboard key to another key
entirely, such as how the author binds the “caps lock” key to mean
“control”.
</li><li> <var>scancode</var>: The symbolic name of the physical key that was
pressed.
</li><li> <var>modifiers</var>: A list of the symbolic names of modifier keys that
were being held down when the key was pressed. Possible values
include <code>ctrl</code>, <code>alt</code>, and <code>shift</code>.
</li><li> <var>repeat?</var>: <code>#t</code> if this is a repeated press of the same key.
</li></ol>
<div class="example">
<pre class="example">(add-hook! key-press-hook
(lambda (key scancode modifiers repeat?)
(display "pressed key: ")
(display key)
(newline)))
</pre></div>
</dd></dl>
<dl>
<dt><a name="index-key_002drelease_002dhook"></a>Scheme Variable: <strong>key-release-hook</strong></dt>
<dd><p>A hook that is run when a key is released on the keyboard. This hook
is run with three arguments:
</p>
<ol>
<li> <var>key</var>: The symbolic name of the “virtual” key that was released.
</li><li> <var>scancode</var>: The symbolic name of the physical key that was
released.
</li><li> <var>modifiers</var>: A list of the symbolic names of modifier keys that
were being held down when the key was released.
</li></ol>
</dd></dl>
<dl>
<dt><a name="index-text_002dinput_002dhook"></a>Scheme Variable: <strong>text-input-hook</strong></dt>
<dd><p>A hook that is run when printable text is typed on the keyboard. This
hook is run with a single argument, <var>text</var>, a string containing
the text that was entered.
</p></dd></dl>
<dl>
<dt><a name="index-mouse_002dpress_002dhook"></a>Scheme Variable: <strong>mouse-press-hook</strong></dt>
<dd><p>A hook that is run when a mouse button is pressed. This hook is run
with four arguments:
</p>
<ol>
<li> <var>button</var>: The symbolic name of the button that was pressed, such
as <code>left</code>, <code>middle</code>, or <code>right</code>.
</li><li> <var>clicks</var>: The number of times the button has been clicked in a row.
</li><li> <var>x</var>: The x coordinate of the mouse cursor.
</li><li> <var>y</var>: The y coordinate of the mouse cursor.
</li></ol>
</dd></dl>
<dl>
<dt><a name="index-mouse_002drelease_002dhook"></a>Scheme Variable: <strong>mouse-release-hook</strong></dt>
<dd><p>A hook that is run when a mouse button is released. This hook is run
with three arguments:
</p>
<ol>
<li> <var>button</var>: The symbolic name of the button that was released.
</li><li> <var>x</var>: The x coordinate of the mouse cursor.
</li><li> <var>y</var>: The y coordinate of the mouse cursor.
</li></ol>
</dd></dl>
<dl>
<dt><a name="index-mouse_002dmove_002dhook"></a>Scheme Variable: <strong>mouse-move-hook</strong></dt>
<dd><p>A hook that is run when the mouse is moved. This hook is run with
five arguments:
</p>
<ol>
<li> <var>x</var>: The x coordinate of the mouse cursor.
</li><li> <var>y</var>: The y coordinate of the mouse cursor.
</li><li> <var>dx</var>: The amount the mouse has moved along the x axis since the
last mouse move event.
</li><li> <var>dy</var>: The amount the mouse has moved along the y axis since the
last mouse move event.
</li><li> <var>buttons</var>: A list of the buttons that were pressed down when the
mouse was moved.
</li></ol>
</dd></dl>
<dl>
<dt><a name="index-controller_002dadd_002dhook"></a>Scheme Variable: <strong>controller-add-hook</strong></dt>
<dd><p>A hook that is run when a game controller is connected. This hook is
run with a single argument, <var>controller</var>, the controller that was
connected.
</p></dd></dl>
<dl>
<dt><a name="index-controller_002dremove_002dhook"></a>Scheme Variable: <strong>controller-remove-hook</strong></dt>
<dd><p>A hook that is run when a game controller is disconnected. This hook
is run with a single argument, <var>controller</var>, the controller that
was disconnected.
</p></dd></dl>
<dl>
<dt><a name="index-controller_002dpress_002dhook"></a>Scheme Variable: <strong>controller-press-hook</strong></dt>
<dd><p>A hook that is run when a button on a game controller is pressed.
This hook is run with two arguments:
</p>
<ol>
<li> <var>controller</var>: The controller that triggered the event.
</li><li> <var>button</var>: The symbolic name of the button that was pressed.
Possible buttons are:
<ul>
<li> <code>a</code>
</li><li> <code>b</code>
</li><li> <code>x</code>
</li><li> <code>y</code>
</li><li> <code>back</code>
</li><li> <code>guide</code>
</li><li> <code>start</code>
</li><li> <code>left-stick</code>
</li><li> <code>right-stick</code>
</li><li> <code>left-shoulder</code>
</li><li> <code>right-shoulder</code>
</li><li> <code>dpad-up</code>
</li><li> <code>dpad-down</code>
</li><li> <code>dpad-left</code>
</li><li> <code>dpad-right</code>
</li></ul>
</li></ol>
</dd></dl>
<dl>
<dt><a name="index-controller_002drelease_002dhook"></a>Scheme Variable: <strong>controller-release-hook</strong></dt>
<dd><p>A hook that is run when a button on a game controller is released.
</p>
<p>This hook is run with two arguments:
</p>
<ol>
<li> <var>controller</var>: The controller that triggered the event.
</li><li> <var>button</var>: The symbolic name of the button that was released.
</li></ol>
</dd></dl>
<dl>
<dt><a name="index-controller_002dmove_002dhook"></a>Scheme Variable: <strong>controller-move-hook</strong></dt>
<dd><p>A hook that is run when an analog stick or trigger on a game
controller is moved. This hook is run with three arguments
</p>
<ol>
<li> <var>controller</var>: The controller that triggered the event.
</li><li> <var>axis</var>: The symbolic name of the axis that was moved. Possible
values are:
<ul>
<li> <code>left-x</code>
</li><li> <code>left-y</code>
</li><li> <code>right-x</code>
</li><li> <code>right-y</code>
</li><li> <code>trigger-left</code>
</li><li> <code>trigger-right</code>
</li></ul>
</li></ol>
</dd></dl>
<hr>
<div class="header">
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