Andy Gimblett wrote:
Hi all,
I've been doing some GUI programming recently, using wx. To help
manage dependencies between state and UI elements, I looked for a
Haskell version of the Observer design pattern, and I found an
implementation written by Bastiaan Heeren of ou.nl [1].
Now, before I make a hackage release:
1. Does anyone have any comments, on either version?
There is no way to remove an observer, which is something I'd expect to
have available. I realise this would require assigning a key to each
observer (and thus perhaps storing them in an associative map) or some
way to filter them, but I think if you can only ever add observers, it
will get awkward.
2. In particular, is the MVar version sensible? I'm aiming for mutual
exclusion between threads. I _think_ I've got it, but I'm perhaps not
familiar enough with the semantics of MVar to be certain. Advice
appreciated. If it _is_ sensible, then is there any reason not to
just use this, and discard the IORef version?
It looks fine (and thread-safe) to me, but I'd agree that you may as
well just use the MVar version and leave out the IORef version.
The current implementation is synchronous, in that any observer
functions are called immediately and synchronously (and in the same
thread as the change of subject value). I'm pondering extending the
package with an asynchronous version where the update just trips a
flag, and the observer function picks this up later - possibly in
another thread. The idea there is to help in cases where certain
operations have to be in a particular thread. But this will mean a
change to the typeclass too, I guess - or the addition of another one
for observers themselves. Again, any thoughts?
I was a bit surprised at first that the observers were called
synchronously. Asynchronous is what I'd expect, and it's also harder to
code the asynchronous handlers wrongly. One blocking call (such as
putMVar) in a synchronous handler can screw up your whole program by
delaying the subsequent observers (and at that stage, the order in which
the observers were added begins to matter). But my idea of how
asynchronous would be implemented seems different to yours, judging by
your description. Why not just augment this function in the synchronous
version:
notifyObservers :: Subject sub val => sub -> IO ()
notifyObservers subject =
do value <- getValue subject
observers <- getObservers subject
mapM_ ($ value) observers to become:
notifyObserversAsync :: Subject sub val => sub -> IO ()
notifyObserversAsync subject =
do value <- getValue subject
observers <- getObservers subject
mapM_ (forkIO . ($ value)) observers
This is what I was expecting to happen -- all the observer actions are
spawned off into their own thread to run whatever code they want (either
communicating back to an existing thread, or performing some long
in-depth action).
Thanks,
Neil.
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