Re: libdispatch: blocks on main queue does not execute

[David Chisnall]

Hi Johannes,

Yes, this is a known limitation.  GNUstep has a runloop implementation that 
predates libdispatch and, although we can optionally use libdispatch for some 
things (e.g. array iteration).  The fix for this would be to implement a new 
GSRunLoopCtx that used libdispatch.  This is quite attractive for FreeBSD, as 
it would mean that we'd get kqueue usage as well, rather than select() or 
poll() (last time I asked about this, no one [including me] had any 
applications that had enough open event sources to worry about the bottleneck, 
so it moved down my todo list).  

In particular, this will require modifying the way NSTimer works, so that it 
just registers for the timer event.  It would be nice to abstract that a little 
bit, as Linux now has timerfd, which can work in a similar way with the 
existing poll() / select() code.  

David

P.S. Have we set you up with commit access for GNUstep yet?

On 27 Dec 2013, at 06:27, Lundberg, Johannes <johannes@brilliantservice.co.jp> 
wrote:

> Hi
> 
> I'm a big fan of dispatch and blocks. For iOS development I often use the 
> pattern
> 
> dispatch_async(dispatch_get_global_queue(0,0), ^{
>   // do some time consuming task
>   dispatch_async(dispatch_get_main_queue(), ^{
>     // update ui
>   });
> });
> 
> However, this does not work on GNUstep/FreeBSD.
> Blocks put on global queues execute fine. Blocks on main queue does not 
> execute.
> 
> Not even 
>   dispatch_async(dispatch_get_main_queue(), ^{
>     NSLog(@"Test..");
>   });
> from main thread/queue works.
> 
> Reading the documentation there seem to be a 
> dispatch_main()
> call required to execute blocks on main queue.
> If I add this call after the above the blocks on the main queue executes just 
> fine, however, the code freezes there.
> As it says:
> * This function "parks" the main thread and waits for blocks to be submitted
>  * to the main queue. This function never returns.
>  * Applications that call NSApplicationMain() or CFRunLoopRun() on the
>  * main thread do not need to call dispatch_main().
> 
> This makes me think that there is something missing in GNUstep compared to 
> Cocoa's NSRunLoop to enable this.
> 
> Anyone have any idea about this?
> 
> --
> Johannes Lundberg
> 
> 
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