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 <[EMAIL PROTECTED]>
Reply-To: "A friendly place to get answers to even the most basic
questionsabout Squeak." <beginners@lists.squeakfoundation.org>
To: "A friendly place to get answers to even the most basic questions
aboutSqueak." <beginners@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 -
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