🍒 c++ - How to emit cross-thread signal in Qt? - Stack Overflow

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QThread is the central class in Qt to run code in a different thread. It's a QObject You can emit a signal from one thread, and have the slot invoked by another.


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in other words, this signal emitted from which thread? for example mysignal in below code. Qt Code: Switch view. class O2;.


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More interesting is that QObjects can be used in multiple threads, emit signals that invoke slots in other.


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Signal Slot Connections and Threads ○ Qt::DirectConnection ○ Slots Signals are placed in the event queue ○ All threads can emit signals.


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How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread the child thread might have already emitted, or is about to emit the signal.


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kittymagazine.ru › qt-5 › threads-qobject.


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kittymagazine.ru › threads › emit-singnals-from-another-thr.


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kittymagazine.ru › threads › emit-singnals-from-another-thr.


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This is also the case for lambda slots or what other tricks you could come up with. And in some cases they do not even understand threading at all. But with threading, you do have to worry about it. There are hybrid cases that lie somewhere between the two general classes, and you can use QThreadPool or the QtConcurrent. Unfortunately, this is probably the worst thing you can do, because that is a violation of the basic assumption of QThread itself. If these are the same, it does a direct signal to slot call. The first thing you have to know is what this means for signals and slots. If the answer is no, you have a choice between subclassing and worker threads. How to use QThread properly. If you have anything using slots in the subclass objects, the thread affinity will mean that it will use the original thread. If you continue to use this site we will assume that you are happy with it.{/INSERTKEYS}{/PARAGRAPH} Just instantiate a QThread object, instantiate the objects to live in this thread, call moveToThread thread on those objects and start the thread. Also, when the thread is finished and you want to delete it, this will be done in the creating thread as well, so the destructor runs in the creating thread. OTOH, there are certainly valid use cases for subclassing. There are cases where you can do this — one example could be a slot that locks a mutex, adds something to a queue, unlocks and returns. The second thing you need to understand, is the thread affinity of the QThread object. Those can be automatic, direct or queued. What happens is that the sending signal will create an event with the arguments of the signal and send this event to the receiver thread eventloop. When you look at this recommendation, there are two things that are the defining questions: Do you need slots? Each QObject has a thread that it belongs to. Do you need QObject instances in the new thread? There are basically two different ways of using QThread directly: Worker threads or subclassing. Usually, this is very bad, if the two objects live in different threads. If they are different, this is handled as a queued connection. Others will connect using the DirectConnection flag, which actually works, but is a very brittle solution that probably will break while people are working on the code in the years to come. QObject thread affinity The first of those two technologies is thread affinity. If the answer to one of those questions is yes, then you should follow the standard recommendation and use a worker thread. If you have a method on the QThread subclass, an object created in here will as usual have the current thread affinity — so if you call it from the constructor it will have the creating thread affinity, and if you call it from run it will have the new thread affinity. The fundamental problem here is that most developers do not really understand the thread model of Qt. On one hand, I actually encourage developers to follow the recommendations in this article, because I agree that this is the path that leads to fewer errors in your code. Share this blog post. This check is done every time the signal is emitted, which means the thread of the signal is irrelevant and the connect just works. The first of those two technologies is thread affinity. I have mixed feelings about this blog post and the arguments it presents. Those two are confusing to a lot of developers, and I will try to explain when you should use those. {PARAGRAPH}{INSERTKEYS}Posted In: Qt. With a direct connection, the slot will be called by the current thread that is emitting the signal. If you want an object to live in the new thread, you must instantiate it inside the run method and not set the thread object as the parent, because Qt does not like the parent-child relationship to go across from one thread to another. Now you can perhaps appreciate why subclassing QThread can be a problem. This is not necessary for the sender. For queued connections to work, the thread where the receiving object lives i. QThread thread affinity The second thing you need to understand, is the thread affinity of the QThread object. There are a lot of developers in this industry who should not touch multithreaded code at all, but still have tasks in their daily job that force them to do this. There are other ways to use QThread than those two. In the eventloop, the event is transformed to a normal slot call. Thread affinity has a bunch of subtle consequences for our objects, once you start going multithreaded. This means when you create objects in a QThread subclass method, they will have different thread affinity based on the instantiation time. This is the part that confuses a lot of developers, although the rules are actually quite simple. If you answer yes to either of those questions and still choose subclassing, you are probably going to have issues that are really hard to find. When you look at the discussion on whether or not to subclass QThread, you will find a lot of people who claim this is bad. But you have a problem if the object can be deleted by another thread while your slot is being executed, of course. Unless you set this manually, QObject chooses the current thread when the constructor is run. Automatic connection does a check in the signal whether the thread affinity of the receiver is the same as the current thread. And this is one of the simplest ways to send information from one thread to another. This may sound counterintuitive, until you actually think about this. Yes, there are a couple of others, but those are not interesting from a threading point of view. There are two Qt technologies that you need to understand before you can understand why QThread works the way it does. The next problem you have to consider is when you instantiate other objects from the thread object itself. However, in the run method of QThread, you are now in the new thread. First, they use moveToThread this on the thread object. This happens no matter if the two objects live in the same thread or if they are in different threads. Those cases will not be described here, but I might come back to those in later blog posts. The thread affinity of a QThread object is the creating thread. This is what you should do for most cases. This means the constructor will necessarily have to run in the thread that creates it. I will also explain how signals and slots work with threads and how they can help you or lead to problems. Note that the signalling object does not have to live in a thread with an event loop. If you have a task you need to run in a QThread, this is the place where you do as the QThread documentation says, and create a worker thread. If you create an object in the run method, this will have the new thread affinity. But for slots it matters a lot.