On 19 January 2011 15:02, Corentin Dupont <corentin.dup...@gmail.com> wrote:
> Is what you explained feasible on the user's side of happstack-server (I
> mean, if I can do it myself)?
Yes, you can do it yourself.
> If I take an empty MVar in my ServerPartTs, which are read over client's
> request,
> I think that nothing will be sent back to the browser and it will remain
> blank!
Yes, which is the intention.
Here's a short incomplete and non type-checked snapshot of an
application I'm currently developing which uses long-polling:
Somewhere on the client you have the following Javascript which uses
the JQuery library:
update = function(){
$.ajax({url: "update", success: onUpdate});
}
onUpdate = function(data){
// Update your page, possibly by making more ajax requests to the server...
update();
}
Now when you call update() the client will asynchronously request the
server for updates. Each time there's an update the onUpdate function
is run which gives you a chance to update your page. onUpdate will
finally call update() again for the next iteration.
Now on the server side you need something like this:
main :: IO ()
main = do
sessionStore <- newSessionStore
forkIO (updateLoop sessionStore)
simpleHTTP conf $ msum
[ dir "update" (update sessionStore), ... ]
update :: SessionStore -> ServerPartT IO Response
update sessionStore = do
mvar <- getSessionData sessionStore
liftIO $ takeMVar mvar
return someResponse
getSessionData will check if there's an active session and if so will
return the associated MVar. If there's no active session a new one
will be created which will have an empty MVar associated with it.
updateLoop is a loop that creates a new update and then notifies the clients:
updateLoop :: SessionStore -> IO ()
updateLoop store = forever $ do
makeNewUpdate
notifySessions store
notifySessions :: SessionStore -> IO ()
notifySessions store = do
sessionData <- getAllSessionData store
mapM_ (\mv -> tryPutMVar mv ()) sessionData
Note that it's not necessary to have a separate thread for creating
updates and notifying clients. You can just as well do this in one of
your request handlers.
I know the example is incomplete but I hope I got the essentials across.
> Is that to be combined with an HTTP refresh timer on the client side?
No, as Thu already explain, no HTTP refresh timer is needed.
> By sessions, you mean sessions that I create myself for every client
> connected?
Yes. However, I don't think sessions are required for long-polling. I
just used them in my example because I used them in a similar way in
an application I'm currently developing.
Regards,
Bas
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