[Newbies] terminate event?

[David Urquhart]

Excellent - thanks that was exactly the sort of guidance I hoped for.  Very 
much appreciated.

You'll hear from me again soon :)

- Dave



>From: Bert Freudenberg <bert at freudenbergs.de>
>Reply-To: "A friendly place to get answers to even the most basic 
>questionsabout Squeak." <beginners at lists.squeakfoundation.org>
>To: "A friendly place to get answers to even the most basic questions 
>aboutSqueak." <beginners at lists.squeakfoundation.org>
>Subject: Re: [Newbies] terminate event?
>Date: Sun, 21 Jan 2007 15:21:24 +0100 (MET)
>
>Am Jan 21, 2007 um 14:07  schrieb David Urquhart:
>
>>Hi
>>
>>I'm a Squeak beginner.  I want to write to the transcript when an  object 
>>is coming to life and when its terminating.  I have an  initialize method 
>>for the birth - what is the method called that  fires at death?
>
>There is no such method. A message can be send to an object only if  there 
>is a reference to it. As long as a reference to an object  exists, it is 
>not dead, it does only get garbage-collected when the  last reference is 
>removed.
>
>About the only thing you can do is to register a *different* object  to be 
>notified when one object is garbage-collected. This is called  
>"finalization".
>
>>And this leads to two other questions...
>>- What is the sequence of events that occur when I create a visible  
>>object like a morph - ie when does onDraw occur exactly?
>
>The world cycle is:
>
>1. process events for each hand
>2. run all step methods
>3. update screen
>4. rinse and repeat
>
>In steps 1 and 2, rectangular portions of morphs can be marked as  dirty 
>(by sending #invalidRect:). In step 3, drawing happens for  those regions.
>
>>- What's the best way for me to answer these sorts of questions for  
>>myself?
>
>By surfing the image. Smalltalk source code is like a hypertext  system, 
>you navigate it by browsing senders and implementors of  methods. Just mark 
>some chunk of source code and press Cmd-m or Cmd- n, it usually figures out 
>the selector you meant.
>
>However, this only gives you a static view of course. A well-placed  "self 
>halt" and then navigating up the call chain is rather  enlightening, too. 
>Here's a recipe:
>
>Make a subclass of, say, EllipseMorph called MyMorph. Easiest to do  if you 
>shift-right-click on EllipseMorph in a class browser (find  EllipseMorph 
>class by Cmd-F in the categories list and typing "elli")  and then select 
>"subclass template" in the shifted menu. Actually,  typing over any class 
>template might be faster ;-)
>
>Anyway, then get the shifted menu of MyMorph and select "sample  instance". 
>You'll hold an instance of your morph in the hand. Put it  somewhere.
>
>Then add a drawOn: method to MyMorph:
>
>	drawOn: aCanvas
>		self doOnlyOnce: [self halt].
>		^super drawOn: aCanvas
>
>"doOnlyOnce:" is essential for not getting a gazillion of debuggers.  Read 
>its comment to learn how to rearm it.
>
>Then click your morph, a debugger should pop up, click debug, select  full 
>stack, and scroll to the very bottom. You see the complete stack  trace for 
>the UI process, the cycle I described is in  WorldState>>doOneCycleNowFor:
>
>>By the way I want to slip in a thanks to the Squeak developers and  
>>community - tuning into Squeak and smalltalk has refired my  inspiration 
>>to create software.
>
>Glad to hear that :)
>
>- Bert -
>
>
>_______________________________________________
>Beginners mailing list
>Beginners at lists.squeakfoundation.org
>http://lists.squeakfoundation.org/mailman/listinfo/beginners