Editing Cn/WxHaskell/Quick start (section)

==== Bouncing ====

The timer event handler uses <tt>nextBalls</tt> to advance all the balls to their next postion.

<pre>    -- advance all the balls to their next position
    nextBalls :: Var [[Point]] -&gt; Panel () -&gt; IO ()
    nextBalls vballs p
      = do varUpdate vballs (filter (not.null) . map (drop 1))
           repaint p</pre>
Updating the positions simply consists of dropping all initial positions and filtering out all empty lists. The function <tt>repaint</tt> is used to invoke the paint event handler of the panel.

When a users clicks on the panel, a new ball is created in <tt>dropBall</tt>.

<pre>    -- drop a new ball, gets mouse position as last argument
    dropBall :: Var [[Point]] -&gt; Panel () -&gt; Point -&gt; IO ()
    dropBall vballs p pt
      = do varUpdate vballs (bouncing pt:)
           repaint p

    -- calculate all future positions
    bouncing (Point x y)
      = map (\h -&gt; Point x (maxH-h)) (bounce (maxH-y) 0)

    -- calculate all future heights
    bounce h v
      | h &lt;= 0 &amp;&amp; v == 0 = replicate 20 0 -- keep still for 20 frames
      | h &lt;= 0 &amp;&amp; v&#160; &lt; 0 = bounce 0 ((-v)-2)
      | otherwise&#160;&#160;&#160;&#160;&#160;&#160;&#160; = h : bounce (h+v) (v-1)</pre>
We prepend a new list of ball positions to the existing list using the <tt>varUpdate</tt> function and we repaint the panel. The new list of positions is calculated with the <tt>bouncing</tt> function that takes the position of the mouse pointer as its argument. This function uses the <tt>bounce</tt> function to calculate all future heights given an initial height and speed. Each time the ball touches the ground, it loses 2 units of speed.

Hopefully this sample inspires you to write more interesting GUI's. Don't forget to look at the samples provided with the wxHaskell documentation.

[[Category:wxHaskell|Quick start]]
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