I'm reviving an old thread I posted (sorry, I can't remember who the people
were who replied, but thanks again...) because I've just read something that
rather alarmed me and would appreciate peoples comments.
An extract from the original thread is below, and here is the code I wrote
based on those suggestions:
void SendNotificationOnMainThreadWithCoalescing(const NSString
*notificationName, NSPostingStyle style, id obj)
{
NSNotification *myNotification = [NSNotification
notificationWithName:notificationName object:obj];
[[NSOperationQueue mainQueue] addOperationWithBlock:
^{
[[NSNotificationQueue defaultQueue]
enqueueNotification:myNotification
postingStyle:style
coalesceMask:NSNotificationCoalescingOnName|NSNotificationCoalescingOnSender
forModes:[NSArray
arrayWithObject:NSRunLoopCommonModes]];
}];
}
Now, what has alarmed me was reading the comments at the bottom of this article:
http://www.mikeash.com/pyblog/friday-qa-2010-01-08-nsnotificationqueue.html
which refers to the apple release notes ("Using NSNotificationQueue Warning").
If I am reading this right, it seems to be saying that there is no guarantee
that the code I have written - or indeed any code at all - will cause the
notification to execute on the main thread. As commented in the linked article,
this surely makes the class utterly useless? If it does not necessarily run the
notifications on the main thread this could lead to all sorts of concurrency
issues, including much of AppKit, Carbon, etc etc. If true, the same problem
applies to the sample code posted here:
http://www.cocoadev.com/index.pl?NotificationsAcrossThreads
So, can anybody comment on all this? It seems like pretty bad news. Is this
what the release notes are saying, and if so, is there an alternative approach
that still lets me take advantage of the coalescing features of notification
queues?
Thanks in advance
Jonny
>>>> My code works with a firewire video camera, and receives callbacks from
>>>> the camera driver when a new video frame is received. Whenever this
>>>> occurs, the UI needs to be updated to display the latest video frame. The
>>>> UI update must run on the main thread - the callback thread is not a
>>>> suitable place to do the update. Furthermore I would like to use
>>>> NSPostWhenIdle and coalescing - if several new frames arrive between
>>>> updates then we only need to draw the most recent.
>>> [...]
>>>> - I could try acquiring the NSNotificationQueue for the main thread, but
>>>> there does not appear to be a standard way of doing that. I have seen this
>>>> suggested as a strategy elsewhere, but I think I have also read that one
>>>> is not meant to post to queues other than that of the current thread (not
>>>> sure why...).
>>> [...]
>>>> I feel there must be a simple way of doing what I want - can anybody
>>>> advise?
>>>
>>> There is a way to do this. You need to set up the enqueue message in an
>>> NSInvocation, have it retain the arguments, and then call the invocation's
>>> -performSelectorOnMainThread:... method to invoke the invocation on the
>>> main thread. This works any time you need to call something on the main
>>> thread, but you need to set more than one argument, or the method has
>>> arguments that take primitives instead of objects.
>>
>> Note that if you're targeting 10.6 then you can do it MUCH more easily
>> by using blocks and either NSOperationQueue or GCD:
>>
>> // NSOpQ
>> [[NSOperationQueue mainQueue] addOperationWithBlock: ^{ /* post your
>> notification here */ }];
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