Qt Call Slot Thread
- Qt Call Slot In Different Thread
- Qt Call Slot From Another Thread
- Qt Call Slot Thread Holders
- Qt Call Slot Thread Gages
- Qt Run Slot In Thread
Example
These allow great flexibility and modularity, but how do they work in the context of threads? For example, if an event or slot is triggered in a QObject (which ultimately triggers a function), which thread is calling that function? The answer lies in thread affinity. But let’s back up a bit. Qt threads and Event loops. From cjhuitt's answer isn't necessary anymore (it was, in Qt Qt::AutoConnection which now (Qt = 4.2) does the right thing and switches between queued and direct connection mode based on QThread::currentThread and the thread affinity of the receiver QObject at emit time (instead of sender and receiver.
Some times you see a signal is emitted in sender thread but connected slot doesn't called (in other words it doesn't receive signal), you have asked about it and finaly got that the connection type Qt::DirectConnection would fix it, so the problem found and everything is ok.
But generaly this is bad idea to use Qt:DirectConnection until you really know what is this and there is no other way. Lets explain it more, Each thread created by Qt (including main thread and new threads created by QThread) have Event loop, the event loop is responsible for receiving signals and call aproporiate slots in its thread. Generaly executing a blocking operation inside an slot is bad practice, because it blocks the event loop of that threads so no other slots would be called.
If you block an event loop (by making very time consuming or blocking operation) you will not receive events on that thread until the event loop will be unblocked. If the blocking operation, blocks the event loop forever (such as busy while), the slots could never be called.
Qt Call Slot In Different Thread
In this situation you may set the connection type in connect to Qt::DirectConnection, now the slots will be called even the event loop is blocked. so how this could make broke everything? In Qt::DirectConnection Slots will be called in emiter threads, and not receiver threads and it can broke data synchronizations and ran into other problems. So never use Qt::DirectConnection unless you know what are you doing. If your problem will be solved by using Qt::DirectConnection, you have to carefull and look at your code and finding out why your event loop is blocked. Its not a good idea to block the event loop and its not recomended in Qt.
Qt Call Slot From Another Thread
Here is small example which shows the problem, as you can see the nonBlockingSlot would be called even the blockingSlot blocked event loop with while(1) which indicates bad coding