[mac-lisp-ide] runloops, threads, etc.

[Duncan Rose]

On Thursday, February 5, 2004, at 10:00 PM, mikel evins wrote:

>
> On Feb 5, 2004, at 1:47 PM, Gary Byers wrote:
>
>> I just subscribed to this list a little while ago and wanted to 
>> respond
>> to a few things that I saw in the archives.  I probably won't be able
>> to attribute things too accurately, and I'm sure that I've missed
>> parts of the discussion and may be replying to some things out of
>> context.
>>
>> To the best of my knowledge, it's effectively the case that only one
>> thread can fetch events from/communcate directly with the window
>> server.  Prior to Panther, this was the thread whose CFRunloop was the
>> "main" CFRunloop (there's still some code in OpenMCL's Cocoa demo that
>> tries to play around with this, and it used to be the case that
>> something called an undocumented function - something like
>> "_CFRunLoopSetMain" - to try to get around this.  CF's "main" runloop
>> ordinarily got created when CF was initialized, and it was associated
>> with the thread that did that initialization.)
>>
>> Panther put an end to all this: the thread that's allowed to talk to
>> the Window Server is the application's initial thread (the one
>> that started out by calling main(), way back when) and none other.
>> (Any other thread can certainly try to call NSApplication's
>> nextEventMatchingMaskWhateverTheOtherArgsAre, but it'll get a NULL
>> pointer back.)
>>
>> The basic model is that NSApplication's -run method loops around,
>> getting events from the window server and propagating them through
>> the responder chain.  From what I understand, lots of other things
>> go on between the time that the -run method asks for an event and
>> the time that it's delivered: raw window server events arrive;
>> some of them are passed to Carbon event handlers (which might in
>> some cases call Cocoa methods), some are related to keeping the
>> window server's model of what's on screen in synch with the
>> application's  (drawRect: methods get called at this time), etc.
>> All of this is (by default) happening on the distinguished, blessed
>> event thread.
>>
>> Some of the time of course, an NSEvent will eventually get returned
>> to the caller.  Tim Moore and I tried some experiments about a year
>> ago to see if we could intercept window-specific events and dispatch
>> them to handlers that ran in window-specific threads (the thinking
>> was that this was a lot closer to McCLIM's CLX backend's behavior.)
>> The idea sort of worked (at least halfway). One way in which I
>> remember it failing was when clicking on a window close button:
>> the (blessed) event thread was waiting for the window-specific
>> thread to unlock the view focus so it could ... do whatever it
>> does, and the window-specific thread was waiting for a mouse-up
>> event that never came.  It might have been possible to think
>> a little harder and avoid this particular deadlock situation,
>> but I got the strong impression that there were lots more deadlock
>> situations waiting to happen.
>>
>> Apple had a Tech Note or similar document whose title was something
>> like "AppKit is too thread-safe! Stop saying that it isn't !".  That
>> document enumerated a small number of reasonable-sounding guidelines;
>> I don't think that it's too hard to follow them carefully and still
>> see totally unreasonable behavior.  I was trying a few months ago
>> to build an NSTextView "manually" in some random (non-blessed) thread
>> and it kept crashing; there could have been any number of reasons
>> for that failure, but I eventually determined that as soon as I'd
>> created an NSLayoutManager the blessed (if you'll pardon the 
>> expression)
>> thread decided that that'd be a good time to do background glyph
>> layout.  (It wasn't ...).  I couldn't find a reliable way of 
>> preventing
>> that from happening; in Cocoa's implicit view of the world, an
>> NSTextView could only be constructed in the blessed event thread
>> by some event handler (why on earth would a compute-bound thread
>> want to create an NSTextView or an NSLayoutManager ?)  IIRC, I
>> eventually wound up PROCESS-INTERRUPTing the event thread and
>> asked it to make the NSTextView for me; that seemed to work fine.
>>
>> I'm not sure that I'm not just violently agreeing with points that
>> other people have made; if people are thinking about bouncing events
>> around between multiple lisp threads, I hope that they're aware of
>> these issues and have thought of things that I'm missing.  The
>> conclusion I came to is that Cocoa is really designed to have just
>> about all of the interesting UI stuff happen in its blessed thread,
>> and if any other approach is possible it may go so far against the
>> grain as to be impractical.
>>
>
>
> This all agrees with my understanding, except that you know more about 
> the details than I do. My embryonic McCLIM back-end builds a normal 
> OSX application bundle and starts a blessed event loop in the normal 
> way; my theory is that the event loop can hand events to windows and 
> views in the normal-for-Cocoa way and the event handlers on the 
> windows and views can trigger McCLIM functions, marshalling data as 
> needed.
>
> Duncan says he has some things sort of working by grabbing events 
> himself, but it's possible he's living in the sort of twilight world 
> that you describe above; I'm sure he'll weigh in on that.
>

Indeed - I went off on a bit of a tangent for a couple of weeks 8-) 
investigating  events and run-loops and all that stuff. The only way I 
got it to work in the end was to get rid of all that stuff and 
implement a simple solution.
Basically what I do is subclass NSView with Lisp objects using 
def-objc-class and override the event methods. Each of these event 
methods looks like:

(ccl::define-objc-method ((:void :mouse-up event) lisp-view)
   (add-event-to-queue self event))

The add-event-to-queue method converts the Cocoa event into a CLIM 
event and appends it to a list; the McCLIM event processing pops events 
off the list when it wants them and does its stuff. This provides a 
mechanism allowing Cocoa to force events on the back end code, and for 
CLIM to request them as needed.

It might be easier to implement this in send-event rather than 
implementing each event method individually, but I'm only at 
pre-pre-alpha stage currently.

There is definitely mention in the Apple docs that all threads have a 
run-loop created for them, and that any thread that *starts* the 
run-loop can process events. So I don't think all is lost for handling 
events safely in a multi-threaded application. It may be the case that 
multiple run-loops can only be run if no inputs are shared; I didn't 
get far enough in my experimentation to say either way.

That said, I still get the occasional Cocoa error, "Unable to unlock 
topmost reader" but that may be the way the back end is structured at 
the moment. Because CLIM keeps track of what's been drawn and what 
needs redrawing, I don't make use of the draw-rect method at all - so 
every drawing operation currently does a lock-focus and a focus 
release. This means (for drawing something like a scroll-pane) there 
are dozens of lock - draw - unlock operations in a very short period of 
time, which I suspect may cause the problems (it seems to happen mostly 
when there's lots of debug being output on a separate window). 
Hopefully this won't be (such) a problem when native panes are 
implemented (and I suspect there are ways around the problem anyway if 
necessary - but it's not a priority at the moment).

So I'm not sure at the moment whether I have problems with events or 
not - there's still quite a way to go however.

> He and I have been discussing a merge; if debugging event-handling is 
> holding him up, and it turns out to be the same sort of problem you 
> described, then it's possible that in the forthcoming merge he and I 
> can fix things by switching to the Cocoa-event-loop-centric model.
>

I'm actually getting very close to feeling ready to do a "release" 
(i.e. let somebody else see the code 8-). I'm waiting on implementing 
mouse moved events (hopefully will be done tonight), and getting 
user-input displayed in the window (key-up / key-down events are 
already received but seem to have no effect). The user-input display 
"problem" for me at the moment is I'm not clear on whether CLIM 
responds to a key event by drawing a glyph, or if I should draw the 
glyph on my own and then just report the key event to CLIM - so it may 
be quite straight-forward to resolve. I'll be looking at that tomorrow.

Interestingly (or maybe not) CLIM and Cocoa provide very similar 
facilities for much of their windowing functionality. Unfortunately its 
done differently enough to be a bit of a pain.

-Duncan

> --me
>
>
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