terminate event?

> 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".

>From: Bert Freudenberg <[hidden email]>
>Reply-To: "A friendly place to get answers to even the most basic
>questionsabout Squeak." <[hidden email]>
>To: "A friendly place to get answers to even the most basic questions
>aboutSqueak." <[hidden email]>
>Subject: Re: [Newbies] terminate event?
>Date: Sun, 21 Jan 2007 15:48:08 +0100 (MET)
>
>
>Am Jan 21, 2007 um 15:21  schrieb Bert Freudenberg:
>
>>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".
>
>Here's an example. Evaluate this in a workspace:
>
> x := Object new.
> x toFinalizeSend: #show: to: Transcript with: 'He''s dead, Jim!\'  withCRs
>
>Nothing should happen. Then do
>
> x := nil
>
>which should print "was finalized" immediately. This is because x  still
>holds onto a relatively "new" object, which gets freed very fast.
>
>However, once an object gets "old" it takes until the next full  garbage
>collection (GC)! Create your object again, but this time, do  this:
>
> Smalltalk garbageCollect.
> x := nil.
>
>Nothing will be printed, because the GC reclaims all space, but also  marks
>all surviving objects as "old". So even though after assigning  nil to x
>your object is dead, the finalizer does not know it, yet.  Only if you
>trigger a full GC again, the object's space is reclaimed,  and the
>finalizer is activated.
>
>- Bert -
>
>
>_______________________________________________
>Beginners mailing list
>[hidden email]
>http://lists.squeakfoundation.org/mailman/listinfo/beginners

>From: Bert Freudenberg <[hidden email]>
>Reply-To: "A friendly place to get answers to even the most basic
>questionsabout Squeak." <[hidden email]>
>To: "A friendly place to get answers to even the most basic questions
>aboutSqueak." <[hidden email]>
>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
>[hidden email]
>http://lists.squeakfoundation.org/mailman/listinfo/beginners

> Or
>
> x := Object new.
> x toFinalizeSend: #show: to: Transcript with: 'He''s dead, Jim!\'  
> withCRs.
> Utilities finalize.
> x:=nil.
>
> will display the message to the transcript immediately.
>
> ----- Original Message ----
> From: David Urquhart <[hidden email]>
> To: [hidden email]
> Sent: Sunday, January 21, 2007 9:42:16 PM
> Subject: Re: [Newbies] terminate event?
>
>
> Thanks for a fantastic answer and sample code.
>
> -Dave
>
>
> >From: Bert Freudenberg <[hidden email]>
> >Reply-To: "A friendly place to get answers to even the most basic
> >questionsabout Squeak." <[hidden email]>
> >To: "A friendly place to get answers to even the most basic questions
> >aboutSqueak." <[hidden email]>
> >Subject: Re: [Newbies] terminate event?
> >Date: Sun, 21 Jan 2007 15:48:08 +0100 (MET)
> >
> >
> >Am Jan 21, 2007 um 15:21  schrieb Bert Freudenberg:
> >
> >>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".
> >
> >Here's an example. Evaluate this in a workspace:
> >
> >    x := Object new.
> >    x toFinalizeSend: #show: to: Transcript with: 'He''s dead, Jim!
> \'  withCRs
> >
> >Nothing should happen. Then do
> >
> >    x := nil
> >
> >which should print "was finalized" immediately. This is because x  
> still
> >holds onto a relatively "new" object, which gets freed very fast.
> >
> >However, once an object gets "old" it takes until the next full  
> garbage
> >collection (GC)! Create your object again, but this time, do  this:
> >
> >    Smalltalk garbageCollect.
> >    x := nil.
> >
> >Nothing will be printed, because the GC reclaims all space, but  
> also  marks
> >all surviving objects as "old". So even though after assigning  
> nil to x
> >your object is dead, the finalizer does not know it, yet.  Only if  
> you
> >trigger a full GC again, the object's space is reclaimed,  and the
> >finalizer is activated.
> >
> >- Bert -
>