Using Dynamic with UpdateInterval and EventHandler Together

Question

I've been having some difficulty with the aforementioned use of these three functions/options and would greatly appreciate some help.

I know that they're is probably an easy way of making a disk move across a square, but, as you may be able to guess, this is an abstraction of my real problem and thus I have to work with an object-oriented system (i.e., currying).

Let's say I wanted to make a disk move from the bottom left of a square to the top right, with enter as my play/pause control:

My code:

makeobject[name_String] := object[name]

(* Default values for position and velocity *)  
object[name_]["pos"] = {0, 0};  
object[name_]["vel"] = {.05, .05};

(* Return a graphic method *)  
(this : object[name_])["pic"] := {EdgeForm[Pink], Blue,  
Disk[this["pos"], .2]}

(* update *)  
tick[obj_] := ((obj["pos"] = obj["pos"] + obj["vel"] (.1)); obj)

(* Dynamic pictures *)
DynamicModule[{play = False, myobj = makeobject["myobj"]},
    EventHandler[
        Dynamic[
            EventHandler[
                If[play,
                    myobj = tick[myobj];
                    Graphics[{Black, Rectangle[{0, 0}, {50, 50}], myobj["pic"]}, 
                        PlotRange -> {{0, 1}, {0, 1}}],
                    Graphics[{Black, Rectangle[{0, 0}, {50, 50}], myobj["pic"]}, 
                        PlotRange -> {{0, 1}, {0, 1}}]
                  ],
                {"ReturnKeyDown" :> (play = False)}],
           UpdateInterval -> .01],
       {"ReturnKeyDown" :> (play = True)}
   ],
   Initialization :> ( myobj["pos"] = {0, 0})
]  

The site's not letting me put up any pictures of what's happening, but when I evaluate, you get an image of black rectangle with the quarter circle at the bottom. Pressing enter makes the disk move up and right.

But, while it's updating, if I press enter, nothing happens, and the disk keeps moving. This means that it's still calling tick[myobj] and therefore the "ReturnKeyDown":>(play=False) never worked.

Thanks in advance for any help.

Sorry if I am missing any kind of information, this is my first question. Please let me know if so.

MMA 8.0.1.0 for students
OS Windows 7 64-bit

Answer 1

I haven't looked in depth to see what's wrong in your code yet, but making the enter key toggle play works

makeobject[name_String] := object[name]

(* Default values for position and velocity *)  
object[name_]["pos"] = {0, 0};  
object[name_]["vel"] = {.05, .05};


(*Return a graphic method*)
(this : object[name_])["pic"] := {EdgeForm[Pink], Blue, 
  Disk[this["pos"], .2]}

(*update*)
tick[obj_] := ((obj["pos"] = obj["pos"] + obj["vel"] (.1)); obj)

(*Dynamic pictures*)
DynamicModule[{play = False, myobj = makeobject["myobj"]}, 
 EventHandler[Dynamic[If[play, myobj = tick[myobj];
    Graphics[{Black, Rectangle[{0, 0}, {50, 50}], myobj["pic"]}, 
     PlotRange -> {{0, 1}, {0, 1}}], 
     Graphics[{Black, Rectangle[{0, 0}, {50, 50}], myobj["pic"]}, 
      PlotRange -> {{0, 1}, {0, 1}}]], 
   UpdateInterval -> .01], {"ReturnKeyDown" :> (play = ! play)}], 
 Initialization :> (myobj["pos"] = {0, 0})]