Signals And Slots Across Threads Qt
How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections. Put it all together and read through the code of queuedactivate, which is called by QMetaObject::activate to prepare a Qt::QueuedConnection slot call. The code showed here has been slightly simplified and commented. The thread that called the signal will. When the signal/slot is actually executed it is done in the receiver object's thread. Qt::AutoConnection (the default parameter) is a bit smarter. When a signal is emitted Qt checks the connection type, if it's an auto connection it checks the sender and receiver's thread affinity (the threads they live in). I wanted to cite this mailing list question from me about models and views on different threads in Qt (along with the ensuing answers). The qt-interest mailing list entries from 2009 seem to have all but disappeared from the web, but I found this one in an Internet Archive cache off of 'gmane'. Problem with signal-slot connection across threads SOLVED Problem with signal-slot connection across threads SOLVED This topic has been deleted. Only users with topic management privileges can see it. So basically once I want cross thread signals and slots, I need an event loop on the thread with the slots?/quoteCorrect. The Qt documentation on Signals and Slots Across Threads suggests the right connection will be automatically picked – that’ll be a queued connection in the case of multithreading to keep it all safe. The connection is made from the main Qt thread. 2 Signals and Slots in PySide.
- Qt Signal Slot Parameter
- Qt Signals And Slots Tutorial
- Qt Signal Slot With 2 Arguments
- Qt Connect Signal Slot
One of the key features of Qt is its use of signals and slots to communicatebetween objects. Their use encourages the development of reusable components.
A signal is emitted when something of potential interest happens. A slot is aPython callable. If a signal is connected to a slot then the slot is calledwhen the signal is emitted. If a signal isn't connected then nothing happens.The code (or component) that emits the signal does not know or care if thesignal is being used.
The signal/slot mechanism has the following features.
- A signal may be connected to many slots.
- A signal may also be connected to another signal.
- Signal arguments may be any Python type.
- A slot may be connected to many signals.
- Connections may be direct (ie. synchronous) or queued (ie. asynchronous).
- Connections may be made across threads.
- Signals may be disconnected.
Unbound and Bound Signals¶
A signal (specifically an unbound signal) is a class attribute. When a signalis referenced as an attribute of an instance of the class then PyQt5automatically binds the instance to the signal in order to create a boundsignal. This is the same mechanism that Python itself uses to create boundmethods from class functions.
A bound signal has connect(), disconnect() and emit() methods thatimplement the associated functionality. It also has a signal attributethat is the signature of the signal that would be returned by Qt's SIGNAL()macro.
A signal may be overloaded, ie. a signal with a particular name may supportmore than one signature. A signal may be indexed with a signature in order toselect the one required. A signature is a sequence of types. A type is eithera Python type object or a string that is the name of a C++ type. The name of aC++ type is automatically normalised so that, for example, QVariant can beused instead of the non-normalised constQVariant&.
If a signal is overloaded then it will have a default that will be used if noindex is given.
When a signal is emitted then any arguments are converted to C++ types ifpossible. If an argument doesn't have a corresponding C++ type then it iswrapped in a special C++ type that allows it to be passed around Qt's meta-typesystem while ensuring that its reference count is properly maintained.
Defining New Signals with pyqtSignal()¶
PyQt5 automatically defines signals for all Qt's built-in signals. New signalscan be defined as class attributes using the pyqtSignal()factory.
PyQt5.QtCore.pyqtSignal(types[, name[, revision=0[, arguments=[]]]])¶Create one or more overloaded unbound signals as a class attribute.
| Parameters: |
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| Return type: | an unbound signal |
The following example shows the definition of a number of new signals:
New signals should only be defined in sub-classes ofQObject. They must be part of the class definition andcannot be dynamically added as class attributes after the class has beendefined.
New signals defined in this way will be automatically added to the class'sQMetaObject. This means that they will appear in QtDesigner and can be introspected using the QMetaObjectAPI.
Overloaded signals should be used with care when an argument has a Python typethat has no corresponding C++ type. PyQt5 uses the same internal C++ class torepresent such objects and so it is possible to have overloaded signals withdifferent Python signatures that are implemented with identical C++ signatureswith unexpected results. The following is an example of this:
Connecting, Disconnecting and Emitting Signals¶
Qt Signal Slot Parameter
Signals are connected to slots using the connect() method of a boundsignal.
connect(slot[, type=PyQt5.QtCore.Qt.AutoConnection[, no_receiver_check=False]])¶Connect a signal to a slot. An exception will be raised if the connectionfailed.
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Signals are disconnected from slots using the disconnect() method of abound signal.
disconnect([slot])¶Disconnect one or more slots from a signal. An exception will be raised ifthe slot is not connected to the signal or if the signal has no connectionsat all.
| Parameters: | slot -- the optional slot to disconnect from, either a Python callable oranother bound signal. If it is omitted then all slots connected to thesignal are disconnected. |
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Signals are emitted from using the emit() method of a bound signal.
emit(*args)¶Emit a signal.
| Parameters: | args -- the optional sequence of arguments to pass to any connected slots. |
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The following code demonstrates the definition, connection and emit of asignal without arguments:
The following code demonstrates the connection of overloaded signals:
Connecting Signals Using Keyword Arguments¶
It is also possible to connect signals by passing a slot as a keyword argumentcorresponding to the name of the signal when creating an object, or using thepyqtConfigure() method. For example the followingthree fragments are equivalent:
The pyqtSlot() Decorator¶
Although PyQt5 allows any Python callable to be used as a slot when connectingsignals, it is sometimes necessary to explicitly mark a Python method as beinga Qt slot and to provide a C++ signature for it. PyQt5 provides thepyqtSlot() function decorator to do this.
PyQt5.QtCore.pyqtSlot(types[, name[, result[, revision=0]]])¶Decorate a Python method to create a Qt slot.
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Connecting a signal to a decorated Python method also has the advantage ofreducing the amount of memory used and is slightly faster.
For example:
It is also possible to chain the decorators in order to define a Python methodseveral times with different signatures. For example:
The PyQt_PyObject Signal Argument Type¶
It is possible to pass any Python object as a signal argument by specifyingPyQt_PyObject as the type of the argument in the signature. For example:
This would normally be used for passing objects where the actual Python typeisn't known. It can also be used to pass an integer, for example, so that thenormal conversions from a Python object to a C++ integer and back again are notrequired.
The reference count of the object being passed is maintained automatically.There is no need for the emitter of a signal to keep a reference to the objectafter the call to finished.emit(), even if a connection is queued.
Connecting Slots By Name¶
PyQt5 supports the connectSlotsByName() functionthat is most commonly used by pyuic5 generated Python code toautomatically connect signals to slots that conform to a simple namingconvention. However, where a class has overloaded Qt signals (ie. with thesame name but with different arguments) PyQt5 needs additional information inorder to automatically connect the correct signal.
For example the QSpinBox class has the followingsignals:
When the value of the spin box changes both of these signals will be emitted.If you have implemented a slot called on_spinbox_valueChanged (whichassumes that you have given the QSpinBox instance thename spinbox) then it will be connected to both variations of the signal.Therefore, when the user changes the value, your slot will be called twice -once with an integer argument, and once with a string argument.
Qt Signals And Slots Tutorial
The pyqtSlot() decorator can be used to specify which ofthe signals should be connected to the slot.
For example, if you were only interested in the integer variant of the signalthen your slot definition would look like the following:
If you wanted to handle both variants of the signal, but with different Pythonmethods, then your slot definitions might look like the following:
I once debated over the circumstances during whichthreading would be preferable over forking. By the end of my researchand discussions with others, we had come to the conclusion thatthreads are ideal when you want access to some data between yourprocessing branches. But you have to be careful to avoid the pitfallsof non-thread-safe behavior.
Two-pair hands have one kicker, as do four-of-a-kind hands. Trips has two kickers. Straights, flushes, and full houses have no kickers. In Texas Hold'em, those five cards are whichever set of five cards out of the seven available make the best five-card hand. If those hands are tied at five cards, you split. – Lee Daniel Crocker Mar 30 '16 at 17:21. Texas holdem three of a kind kicker.
Remember to keep everything as small and simple aspossible. Nobody wants to maintain your clever multithreading tricks,or your gigantic god-structures; least of all your future self. Qtprovides an easy to use write locker method that automatically unlocksat the end of its method. If you are concerned about the performancehit, you need to get away from your computer and revisit the design.Keep your locks small and fast.
Wrong:
Qt Signal Slot With 2 Arguments
Lock -> long process -> mutate data -> unlock
Right:
Long process -> lock -> mutate data -> unlock
If you are looking at mutating a large data structure,which might take a long time to iterate and process, try to breakdown the problem: Copy out the necessary components to change and addthem back in. Since this isn't Haskell, copying the entire structurecan lock up your treads on its own. You need to break your problemdown into more manageable components. That means breaking down thedata structures into reasonable, logical modules as well as themethods.
LockerDeclaration
You will need some accessible memory set aside in yourworker class to use. Fist you will need to declare some memory thatthe lock will use. Then you will need a pointer to the location inmemory where you want to change the data. This data was initializedin the main thread, and the worker takes its address whenconstructed.
https://gist.github.com/erickveil/9d3688cccf099b51830eThe useLockToChange method will perform that quick lockand mutation we want.
Qt Connect Signal Slot
LockingMethod
Again, this is where we want to keep it a simple andsmall as possible. Qt's write locker will automatically unlock whenthe calling method returns. I think that if you need to specificallylock and unlock a section of a larger method, then you need torefactor: You are trying to do too much with a single method.
https://gist.github.com/erickveil/9ea942047aa8740f7042Signalsand Slots (The Unfortunate Wrong Way)
It's worth noting that at first Ithought signals and slots would be an excellent means of transferringdata across threads. However, you will find that the timing ofemitting a signal might not be what you expect, at least in a typicalcommand line application.
https://gist.github.com/erickveil/7f5c28f35fe7cc2ef91a
Add the member variable to be changedon the MainClass in your main thread. Also define your slot here. Weare going to emit a matching signal, with a string passed as anargument from the worker thread back to here. We make the actualchange of the value from the slot on the main thread, so you wouldthink it would be safe to assume that there is no need to lock tomake the change.
Running the application now will revealthat no matter where you emit the signal, the slot does not run untilthe emitting thread finishes. You will see the initial value for thisnew variable printed the same as it started, at the same time thealtered locked variable is printed. Only when the thread declares itis finished do we see the output from the slot method, announcing thedata change.