I thought I understood runloops pretty well, but I'm running into a situation
that's got me stumped. Basically it involves how to wait (block) for a future
message-send that will be queued onto the current runloop.
So I've started an asynchronous task that runs on a background thread. When the
task completes, it's going to notify the originating thread via a call to
-performSelector:onThread:....
In some circumstances the program later decides it need to block the
originating thread until the operation completes. So it runs a nested runloop
like this:
while (!taskIsComplete)
[[NSRunLoop currentRunLoop] runMode: @"myCustomMode"
beforeDate: [NSDate distantFuture]];
where taskIsComplete is some flag that will get set by the target method
invoked from the task thread.
(I use a custom runloop mode because, if I used the default mode, the
application would respond to user input and other events while waiting in the
loop, resulting in re-entrant calls to the application code, causing all sorts
of problems. In fact what I'm actually doing is fixing a bug in the current
code where it incorrectly uses the default mode, leading to a crash.)
Over on the task thread, when it completes the task it calls:
[originator performSelector: @selector(taskDone:) onThread:
originatingThread withObject: self
waitUntilDone: NO modes: @[@"myCustomMode"]];
where the -taskDone: method will set that taskIsComplete ivar that the wait
code is checking.
Unfortunately this doesn't work, because the runloop immediately bails out and
returns NO from -runMode:beforeDate:. Apparently this is because there are no
input sources registered for myCustomMode. Which is true, but that doesn't mean
that no events are going to arrive for that mode! The problem seems to be that
this part of the runloop implementation doesn't understand about
delayed-perform-in-mode calls.
What's the workaround? Should I add some sort of no-op input source to the
runloop for my custom mode? That smells very hacky.
—Jens
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