[Interest] Best way to invoke a slot after some time with QObject::sender

Bo Thorsen bo at fioniasoftware.dk
Tue Aug 28 08:25:44 CEST 2012

Den 28-08-2012 04:10, Preet skrev:
> Hiya,
> I have a project where I'm trying to asynchronously reply to a signal.
> So if ObjectA sends ObjectB a signal, ObjectB will send ObjectA a
> reply signal at some later point in time. ObjectA is guaranteed to be
> alive when ObjectB decides to send a reply.  ObjectAs and ObjectB also
> live in different QThreads. ObjectB doesn't know about ObjectA in
> advance, so I use QObject::sender() to store ObjectA's pointer in a
> queue for when ObjectB is ready to reply. There are many such
> "ObjectA"s out there that ObjectB can reply to.
> I see three ways to implement this in an application:
> When ObjectB is ready to reply,
> 1  have it connect to the right ObjectA, emit a signal, then disconnect.
> 2  use QMetaObject::invoke() to invoke the right ObjectA's corresponding slot
> 3  be connected to all ObjectA's and just emit a signal with the
> QObject sender pointer; this way the ObjectAs can filter out which
> signals are for them in the slot itself
> I'd like to know what the best method is in terms of speed/minimal
> complexity, or if there's a better method I haven't listed. ObjectB
> may be required to send hundreds of replies every second, so speed is
> important. 3 seems like its the worst, since every reply will invoke
> multiple slots. 1 and 2 seem similar; I'm kind of concerned about
> calling "connect" and "disconnect" so often in 1.

The third is the worst, if you have a bunch of ObjectA's. If you only 
have a couple of them, this is by far the best. Because this also means 
you don't have to worry about whether the object has been deleted since 
it emitted the signal.

The first actually uses the second, because those are cross thread 
signal slot connections. But by using the connect, you will 
automatically get the call moved across to the other thread. If you use 
invoke, you don't get this. If it's okay to call across threads, invoke 
is the fastest.

You have to figure out how to make sure the ObjectA still exists at the 
point where ObjectB wants to call it. This is the part that really 
worries me with the situation you have described here.

Instead of doing this, I'd probably implement some kind of multiplexer 
object that sits between the two threads and handles all the 
communication and knows the lifetime of both ObjectA and ObjectB. 
Whether you would want one big object that handles all of them, or 
create a connection object for each connect, that's up to you.

Bo Thorsen,
Fionia Software.


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