[Newbies] Stop a script for a second

Ned Konz ned at bike-nomad.com
Mon Dec 4 23:51:31 UTC 2006

Martin Bleichner wrote:
> Hi together,
> I am new to this list. So perhaps I should introduce myself. I am 25 
> year old german student studying  congitive neuroscience  in 
> Utrecht/Netherlands. I want to use squeak /etoys mainly for exploring 
> and visualizing some ideas in a quick way.
> Right now I want to build a prototype for an experiment and  think that 
> this is a good project to learn a bit more about squeak.
> I want to program a sequence of objects appearing on different positions 
> on the screen one after the other and later in a random order.
> A very dirty way to do so is just script doing something like.
> object1 show.
> script wait for 1000ms.
> object1 hide
> object2 show
> script wait for 1000ms.
> I know that this is a very basic question but couldn't find the answer, 
> as I don't know what it is I am looking for.

Hi Martin,

Because of the structure of Etoys, entire scripts are done all the way 
through (or not at all).

A useful idiom (even in Etoys) for this kind of event-driven behavior 
might be the idea of a "state machine".

Note that in Etoys you can "clone" an object that has scripts; all the 
clones of an object (its "siblings") will share the same scripts but 
have their own copies of variables (and of course their own position, 
appearance, etc.). So if you have a number of objects whose behavior is 
similar this can be a powerful way to save work (and to have a single 
place to change the behavior of all the "sibllng" objects).

The attached image is an example of a very simple state machine that 
provides the kinds of delays that would work in your application 
(provided you write your program correctly, of course!). The bottom two 
scripts (turnRed and turnGreen) would be user-specific and would be 
different in your application; the upper ones are generic state machine 
infrastructure. You'd change the name of the state in "initial" to the 
first state you wanted to run. The state machine shown changes the color 
of the ellipse from red to green and back, staying at a particular color 
for 2 ticks (you can change the tick rate of the "stateMachine" script 
to change the resolution of "delayForTicks:").

This structure provides the ability to change states (note that the 
current state script will be invoked every tick) and to delay between 
state transitions.

The trick is this: each of these state scripts only get run once per 
cycle. They set the delay timer to delay the execution of the *next* 
state, then do their thing.

See if you can use this idea for your needs, and tell us what you come 
up with!

Ned Konz
ned at bike-nomad.com
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