Qt Slot Thread
Example
As mentioned, each program has one thread when it is started. This thread is called the 'main thread' (also known as the 'GUI thread' in Qt applications). The Qt GUI must run in this thread. All widgets and several related classes, for example QPixmap, don't work in secondary threads. Cross Thread Signals/Slots Cross thread signals are really events The receiver needs a running event loop The sender does NOT need an event loop Signals are placed in the event queue All threads can emit signals regardless of pattern Only threads with running event loops should have in-thread slots. Qt Thread Example; Signal And Slot In Qt; Qthread Signal Slot; The main thing in this example to keep in mind when using a QThread is that it's not a thread. It's a wrapper around a thread object. This wrapper provides the signals, slots and methods to easily use the thread object within a Qt project. The started signal of the thread is connected to the doWork function of the worker which means that as soon as the thread starts, the doWork function is called. Then we connect the finished signal of the worker to the quit slot of the thread, ensuring that the thread will stop when the work is done. This connection needs to be direct.
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.
Qt Slot In Different Thread
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.
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 Run Slot In 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
