[Qt-interest] Signal delivery order?

[Jason H]

If you're on the same thread and they are directly connected, then they will be 
executed in order as local function calls.

If they are queued, the statement does not apply.
If they are across threads (which have to be queued) then the statement does not 
apply. 


In short, I would not rely on any signal/slot call order. I would use a unifying 
slot (wrapper) that would execute the functions in the correct order.  slot 
A::suff(){ if (iNeedToX) x(); if (iNeedToY) y();...}




________________________________
From: Atlant Schmidt <aschmidt at dekaresearch.com>
To: "qt-interest at trolltech.com" <qt-interest at trolltech.com>
Sent: Wed, July 14, 2010 11:20:06 AM
Subject: [Qt-interest] Signal delivery order?

 
Folks:
 
  I’m trying to understand the order in which signals are delivered to slots.
 
  When I look at:
 
    http://doc.trolltech.com/4.6.2/signalsandslots.html
 
  It first says:
 
    If several slots are connected to one signal, the slots will be executed
    one after the other, in the order they have been connected, when the
    signal is emitted.
 
  But later, it seems to take back that promise by saying:
 
    If on the other hand you want to call two different error functions when
    the number overflows, simply connect the signal to two different slots.
    Qt will call both (in arbitrary order).
 
 
  Depending on the implementation details, I could believe that, within a single 
thread,
  either statement is true (that slots are handed the signal in the order in 
which they
  were originally connected or that slots are handed the signals in an arbitrary 
order);
  clearly, the second statement subsumes the first and it’s easier for the 
trolls if it’s
  the latter. It’s also safer for me if *I BELIEVE* it’s the latter, even if 
it’s sometimes
  the former, but shouldn’t the documentation be self-consistent? Does anyone 
know
  which statement (“connection order” vs. “arbitrary order”) is more true? And 
can
  you correct the documentation? ;-)
 
  Meanwhile, I also wonder about the delivery order when multiple signals are
  emitted. Can we take it on faith that if a single thread emits the signals A, 
B,
  and C in that order, they will be delivered to a single slot in a single 
thread in
  the same order? How about if they are delivered to different (multiple) slots
  in the same thread? Still A, B, then C, no matter which slot they’re 
targetting?
 
  Note: I understand that most of the ordering promises won’t hold any time
  signals are delivered to different threads; one doesn’t know when the 
individual
  threads will actually be “runnable” so that they can receive the signals at 
their slots.
  But I would still expect that multiple signals emitted from one thread and 
targeting
  a single thread will be delivered to that thread in the order in which they 
were
  originally emitted.
 
                                                  Atlant
 
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