From: Patrick Li <patrickli_2001@...>
Date: 2008-08-23T04:19:16+09:00
Subject: Re: Saving the state of a Thread?

Thanks Robert for the input.

Yes I agree. That's a very clean and flexible approach. My code is 
currently structured using state machines, which is similar in concept 
to your operation stages.

This sort of programming is actually one of my primary motivators for 
thinking about a thread-based paradigm instead.

Here's my original idea, in it's original un-condensed and unabbreviated 
mental form. Maybe you have some ingenious ideas:

Currently, a real-time simulation loop looks something like this:

while true
  scenario.update (secondsSinceLastIteration)
  scenario.draw
end

And your scenario code would like something like this (ie. for a 
traveling ball):

def update (timePassed)
  @position += timePassed*velocity
end

You would agree that this is all very straightforward. But what if you 
want the ball to travel back and forth 3 times? You'd have to do 
something like this:

def update (timePassed)
  if @numberOfBackAndForths == 3
    return

  if @travelingForward
    @position += timePassed*velocity
    @travelingForward = false if (reached end)
  else
    @position -= timePassed*velocity
    if reached-end
      @travelingForward = false
      @numberOfBackAndForths += 1
    end
  end
end

So can see that @travelingForward essentially describes the state of the 
system. This sort of manual state bookkeeping can get really out-of-hand 
pretty quickly.

BASIC IDEA: Wouldn't it be nice to be able to write something like this?

def start
  3.times do
    #TRAVEL FORWARD
    update(5 seconds) do |timePassed|
      @position += timePassed*velocity
    end

    #TRAVEL BACKWARD
    update(5 seconds) do |timePassed|
      @position -= timePassed*velocity
    end
  end
end

Ahh... so much cleaner isn't it? This is a shift to a thread-based 
paradigm.

So far, I have everything working, EXCEPT when the simulation needs to 
be saved to disk.

I can save @position, and I can save how many iterations are left. But 
after I resume the simulation, how do I get the thread to start from the 
appropriate line of code? I never thought I'd ask for it, but a goto 
statement sounds like a blessing right now.

Thanks for all your replies.
  -Patrick
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