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Smalltalk Challenge: Post 7 - Turtles Ahoy!

I can remember going down to the computer lab while in grade school and using Logo on the then state-of-the-art Apple IIe computers. Supposedly Logo allowed students to explore mathematics in a tangible way, but I don't remember doing much more than drawing shapes by moving the little "turtle" around on the green screen and I still don't have much confidence in my math skills 20 years later. I don't think Logo helped kindle my interest in computers, either; games like The Oregon Trail and Where in the World is Carmen Sandiego and the Commodore 64 computer were far more influential to me. But drawing little green triangles and squares was fun. And while Logo may not have been as influential in my education as Papert would have hoped, it was extremely influential to Smalltalk. In fact, the graphics Pen object is actually very similar to Logo's turtle.

The Pen class is used for drawing. It inherits from BitBlt which allows it to perform raster drawing on any form its bound to. The World is a form itself, so a pen bound to it will appear to draw on top of other windows and elements! The following example draws an octagonal-spiral on top of anything that's visible in the environment, centered around the point of a mouse click:

pen := Pen new.
    pen roundNib: 8;
    color: Color random;
(Sensor waitButton; redButtonPressed)
    ifTrue: [
            pen up;
            goto: Sensor mousePoint;
        x := 1.
        10 timesRepeat: [
            4 timesRepeat: [pen go: 10 * x; turn: 45].
            x := x + 2]].

A new Pen instance is created, the size of the line it draws is set to 8px by the roundNib: message (what better thickenss to draw octagons with?), and the color of the pen's line is randomly set by the color: message.

Sending a down message is like placing a pen against a sheet of paper. When the pen is against the paper and is moved, a line is drawn. There is also the up message which is like lifting the pen up away from the paper so it can be repositioned without making any marks.

The pen is moved about the screen using the goto:, go: and turn: messages. Like its Logo progenitor, the pen object responds to messages relative to its current position and direction. When asked to move 10px, the pen moves forward in which ever direction it's pointing in. Turning the pen 45 degrees and moving another 10px would result in a 45° angle drawn. The direction of the pen's motion can also be set with north, east, west, south, and direction.


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