NSApp nextEventMatchingMask with dispatch inside dispatch - objective-c

Question

I am currently developing an application that runs its own event loop. If i use dispatch async when not enqueued from another dispatch, its block runs correctly.
Example:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(#"This should appear!");
});
for (;;)
{
NSEvent* event = [NSApp nextEventMatchingMask:NSAnyEventMask
untilDate:[NSDate distantPast]
inMode:NSDefaultRunLoopMode
dequeue:YES];
[NSApp sendEvent:event];
}
If I run this, the log will be called as expected.
But let's say I put a wrap around that dispatch, like this:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(#"This should appear!");
});
for (;;)
{
NSEvent* event = [NSApp nextEventMatchingMask:NSAnyEventMask
untilDate:[NSDate distantPast]
inMode:NSDefaultRunLoopMode
dequeue:YES];
[NSApp sendEvent:event];
}
});
If i run this code, the log will not be executed. I can imagine that since extEventMatchingMask: is being called inside a enqueued block, it will not be able to handle future blocks that are dispatched. But the weird thing is that if I use performSelector: it gets executed even inside a enqueued block.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(#"This should appear!");
});
[self performSelector:#selector(logAppear) withObject:nil afterDelay:2];
for (;;)
{
NSEvent* event = [NSApp nextEventMatchingMask:NSAnyEventMask
untilDate:[NSDate distantPast]
inMode:NSDefaultRunLoopMode
dequeue:YES];
[NSApp sendEvent:event];
}
});
-(void) logAppear {
NSLog(#"This should appear!");
}
If I run this example the method logAppear will be called as expected.
So, in short I wanted to ask why the second case doesn't work, and how (if possible) can I run a event loop inside an enqueued block that is able to run dispatch blocks.

Answer 1

After some experimenting I realized that what I needed to do was to make sure that that event loop was executed outside any enqueued block. We can do this by putting the loop inside a method and make sure it is called by a performSelector.
Example:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(#"This should appear!");
});
[self performSelector:#selector(logAppear) withObject:nil afterDelay:2];
[self performSelector:#selector(mainLoop) withObject:nil afterDelay:0];
});
-(void) mainLoop {
for (;;)
{
NSEvent* event = [NSApp nextEventMatchingMask:NSAnyEventMask
untilDate:[NSDate distantPast]
inMode:NSDefaultRunLoopMode
dequeue:YES];
[NSApp sendEvent:event];
}
}
-(void) logAppear {
NSLog(#"This should appear!");
}
By running this example we now get both logs as expected