{ (define LIBNAME "A Functional Drawing Library (HtDC)")
  (include "head.tinc") }

This <code>draw</code> package provides classes and methods for a visual
world. Here is its class diagram of public fields and methods: 
<pre>
<code>
 +-----------------------------------+
 | abstract World                    |
 +-----------------------------------+       +---------------------------------------+
 | Canvas theCanvas                  |------>| Canvas                                |
 +-----------------------------------+       +---------------------------------------+
 | boolean bigBang(int,int,double)   |       +---------------------------------------+
 | boolean endOfTime()               |       | boolean show()                        |
 | World endOfWorld()                |       | boolean close()                       |
 |                                   |       | boolean drawCircle(Posn,int,Color)    |
 |                                   |       | boolean drawDisk(Posn,int,Color)      |
 | abstract World onTick()           |       | boolean drawRect(Posn,int,int,Color)  |
 | abstract World onKeyEvent(String) |       | boolean drawLine(Posn,Posn,Color)     |
 | abstract boolean draw()           |       | boolean drawString(Posn,String)       |
 | abstract boolean erase(           |       | boolean clearCircle(Posn,int,Color)   |
 +-----------------------------------+       | boolean clearDisk(Posn,int,Color)     |
                                             | boolean clearRect(Posn,int,int,Color) |
                                             | boolean clearLine(Posn,Posn,Color)    |
                                             +---------------------------------------+

                      +----------+
                      | Posn     |
                      +----------+
                      | int x    |
                      | int y    |
                      +----------+

                              +-------+
                              | Color |
                              +-------+
                                  |
                                 / \
                                 ---
                                  |
             ------------------------------------------
             |          |         |         |         |     
         +-------+  +-------+ +-------+ +-------+ +-------+  
         | Blue  |  | Green | | Red   | | White | | Yellow| 
         +-------+  +-------+ +-------+ +-------+ +-------+ 
</code>
</pre>

<a name="world" />
<p>The abstract <code>World</code> class exports the following methods: 
<ul>

<li><code>bigBang(width,height,speed)</code>, which initializes the world,
associates it with a <code>width</code> x <code>height</code> <a
href="#canvas"><code>Canvas</code></a>, displays
this canvas, and finally starts the clock at a rate of one tick per
<code>speed</code> seconds. If it succeeds with all of its actions, the method
produces <code>true</code>. 

<p>A <code>World</code> is associated with one default <code>Canvas</code>,
called <code>theCanvas</code>. If a <code>World</code> needs additional
canvases for its graphical presentation, its concrete subclasses must add
those.</p>
</li>

<li><code>endOfTime</code>, stops the clock and, if it succeeds, produces
<code>true</code>. 
</li>

<li><code>endOfWorld</code>, stops the clock and, if it succeeds, produces the
last <code>World</code>. 
</li>
</ul>
The methods may fail due to the unavailability of the physical devices,
inappropriate uses, etc. In those cases, they fail with an exception.</p>

<p>A derived concrete class must supply definitions for the following methods: 
<ul>
<li><code>onTick()</code>, which is invoked for every tick of the clock. Its
purpose is to create a <code>World</code> whose differences with
<code>this</code> one represent what happened during the amount of time it takes
the clock to tick. 
</li>

<li><code>onKeyEvent(key)</code>, which is invoked for every keyboard event. Its
purposes is to create a <code>World</code> whose differences with
<code>this</code> one represent what happens due to the user's use of the
keyboard. The latter is represented with the string-valued argument
<code>key</code>.  
</li>

<li><code>draw()</code>, which is invoked <em>after</em> one of the two event
handlers has been called. Its purpose is to present <code>this World </code>
graphically on its canvas. If it succeeds, its result is <code>true.</code>
</li>

<li><code>erase()</code>, which is invoked <em>after</em> one of the two event
handlers has been called. Its purpose is to erase <code>this World</code>'s
canvas, as much as needed. If it succeeds, its result is <code>true.</code>
</li>
</ul>
A program may, in principle, start several instances of (subclasses of)
<code>World</code>. If it does, the event handlers are called in a unpredictable
order. </p>

<a name="canvas" />
<p>To create an instance of the <code>Canvas</code> class, a program must supply
two <code>int</code> values: one for the width of the canvas and one for its
height. The canvas is a rectangle, whose borders are parallel to the computer
screen's borders. A program can use the following methods on instances of
<code>Canvas:</code>
<ul>
<li><code>show()</code>, which displays the canvas. 
If it succeeds, it produces <code>true</code>.</li>

<li><code>close()</code>, which destroys the canvas. 
If it succeeds, it produces <code>true</code>.</li>

<li><code>drawCircle(p,r,c)</code>, which draws a circle on <code>this
Canvas</code> at <code>p</code> with radius <code>r</code> and color
<code>c</code>.  If it succeeds, it produces <code>true</code>.</li>

<li><code>drawDisk(p,r,c)</code>, which draws a disk on
<code>this Canvas</code> at <code>p</code> with radius <code>r</code> and color
<code>c</code>.  If it succeeds, it produces <code>true</code>.</li>

<li><code>drawRect(p,w,h, c)</code>, which draws a solid rectangle on <code>this
Canvas</code> at <code>p</code> with width <code>w</code>, height
<code>h</code>, and color <code>c</code>.  The rectangle's lines are parallel to
the canvas's borders. If it succeeds, it produces <code>true</code>.</li>

<li><code>drawLine(p0,p1,c)</code>, which draws a line on <code>this
Canvas</code> from <code>p0</code> to <code>p1</code> using color
<code>c</code>.  If it succeeds, it produces <code>true</code>.</li>

<li><code>drawString(p,s)</code>, which draws the string <code>s</code> at
<code>p</code> on <code>this Canvas</code>.  If it succeeds, it produces
<code>true</code>.</li>

<li><code>clearCircle(p,r,c)</code>, which erases a circle from <code>this
Canvas</code> at <code>p</code> with radius <code>r</code> and color
<code>c</code>.  If it succeeds, it produces <code>true</code>.</li>

<li><code>clearDisk(p,r,c)</code>, which erases a disk from
<code>this Canvas</code> at <code>p</code> with radius <code>r</code> and color
<code>c</code>.  If it succeeds, it produces <code>true</code>.</li>

<li><code>clearRect(p,w,w,c)</code>, which draws a solid rectangle from <code>this
Canvas</code> at <code>p</code> with width <code>w</code>, height
<code>h</code>, and color <code>c</code>.  The rectangle's lines are parallel to
the canvas's borders. If it succeeds, it produces <code>true</code>.</li>

<li><code>clearLine(Posn p0, Posn p1, Color c)</code>, which erases a line from <code>this
Canvas</code> from <code>p0</code> to <code>p1</code> using color
<code>c</code>.  If it succeeds, it produces <code>true</code>.</li>

</ul>
The methods may fail due to the unavailability of the physical devices,
inappropriate uses, etc. In those cases, they fail with an exception.</p>
</p>

<p>To create an instance of the <code>Posn</code> class, a program must supply
two <code>int</code> values: one for its x coordinate of the canvas and the
second for its y coordinate. </p>

<p>The <code>Color</code> class is abstract. Its variants (subclasses) are
created with no arguments. 
</p>

{(include "foot.tinc")}