[Squeakfoundation]Ad-hoc final harvesting for 3.4

[Daniel Vainsencher]

Cool!

I think many more things in Squeak should be happening asynchronously.
I've been trying to figure out a place to hook that up that wouldn't
require people to be always forking things explicitly, and the event
handling mechanism sounds like a good solution to that problem...

Daniel

Andreas Raab <andreas.raab@gmx.de> wrote:
> Hi Rob,
> 
> > 3) Weak message sends and conversion of the NuBlu events to use them.
> 
> Ever looked at EventMessageSet?! I did this for probably the exact same
> reasons that you are using weak messages sends now. And yet, neither is
> not going to solve your problems ;-(
> 
> Seriously, did you ever find the time to look closely at how this is
> handled in the event stuff that I did a while back (it is now in
> Croquet)?! The problem with the NuBlue (and any other event) framework
> is that it does not allow the subscriber to reflect about its
> subscriptions. In other words, if I have a window and a button and say:
> 
> 	button on: #click send: #close to: window.
> 
> then what I say is that the button should tell the window to close when
> I click on it. And of course, the button has to know the window in order
> to do this. And while this is a nice first-order theory it is
> problematic if we close that window by any other means (in which case we
> may want to stop the subscription) and naturally, the button will hold
> on to the window for the rest of its live. This is the problem which you
> are trying to solve with weak references but it is conceptually wrong.
> 
> The problem is that #click is really a trigger for a method (script) in
> the window which may happen asynchronously. And this script is clearly
> owned by the window and not by the button and that's what all of these
> event frameworks never take into account. With the model that I built
> this is different (and not only in this respect ;-) Here, the subscriber
> always remembers its registrations (messages) *strongly* and the
> publisher always remembers the subscriptions *weakly* (by default). This
> has various advantages, an obvious one being that if the subscriber goes
> away so does its subscription (this is what you are trying to do with
> weak refs) and the publisher is not bothered by it [*1]. However, what's
> even more important is that in a case like the above, the window can
> reflect about its subscriptions and cancel them if it gets closed
> without having to know any "external subscriptions" (e.g., added by
> someone else).
> 
> [*1] I find it _extremely_ questionable to raise an error by default
> here. There may be a few applications which need to know about it but by
> and large I would expect people to get really confused and say "yeah, of
> course it's gone that's what it *ought* to do". For those who do it is
> utterly trivial to provide a different subscription (MessageSend
> subclass).
> 
> In Croquet, the above example would look like:
> 
> 	window startScript: #close when: {button. #click}.
> 
> which tells the window to start the #close script when the button is
> clicked. [Slight side note here: #startScript denotes an asynchronous
> signal; #runScript a synchronous one. So the exact equivalent would be
> "window runScript: #close when: {button. #click}" but I haven't used
> synchronous signals in about a year except for debugging]. The magic is
> now that the window could do something like
> 
> Window>>close
> 	super close. "to get rid of it"
> 	self stopScript: #close.
> 
> in order to cancel its "subscription" (I don't like the term in this
> context but it's useful to show the similarities). Or even more
> radically (and that's code I am really using):
> 
> Window>>destroy
> 	"I shall be destroyed. Stop all scripts"
> 	self scripts do:[:script| self stopScript: script].
> 
> This latter makes it impossible to have any pending references if the
> window should be destroyed. But there is even more fun stuff you can do
> with this. Consider that if we close the window, we want to put into the
> trash and be able to get it back. The button should of course work if we
> get it back. How can we solve this problem?! Well, here's one solution:
> 
> Window>>initialize
> 	"Set me up in a way that I will only run #close 
> 	scripts while I am in the world"
> 	"pause close script when I get out of the world"
> 	self pauseScript: #close when: {self. #outOfWorld}.
> 	"resume close when I get back in"
> 	self resumeScript: #close when: {self. #intoWorld}.
> 
> And this will work for any number of buttons who want to send close to
> the window. All of this is only possible because the *window* knows
> about the scripts (subscriptions) and not the button.
> 
> There are even more advantages to this (such as for serialization) but I
> think this will get you an idea why I think that any kind of
> subscription mechanism that doesn't give the subscriber access to its
> subscriptions is ultimately doomed to fail - you simply cannot write
> fault-tolerant code if you cannot reflect about them.
> 
> Cheers,
>   - Andreas
> 
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