This page describes the use of signals and slots in Qt for Python.The emphasis is on illustrating the use of so-called new-style signals and slots, although the traditional syntax is also given as a reference.
Before writing our first GUI app, let's discover Qt Creator. Qt Creator is yet another IDE for C, but it is very well suited for coding Qt applications. It provides a doc browser and the 'designer', which makes creation of windows easier, all wrapped in a well-designed user interface. It's also one of the fastest IDE's available. Our first window. In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Jun 29, 2012 This feature is not available right now. Please try again later. Feb 08, 2018 QObjects are one of the fundamental building blocks of Qt applications. QObjects provide memory management, advanced event handling, and signals and slots: a devious mechanism to allow communication between QObjects and modules in a thread-safe manner, while allowing your system to remain loosely coupled and flexible. Oct 17, 2014 create a signal and connect it to a slot. Create a signal and connect it to a slot. Skip navigation. Tutorial Qt Creator - signals & slots - Duration: 10:22. DuarteCorporation Tutoriales 4,362.
The receivers of signals are called Slots in Qt terminology. A number of standard slots are provided on Qt classes to allow you to wire together different parts of your application. However, you can also use any Python function as a slot, and therefore receive the message yourself. @lunaryorn thanks. Sorry but how do you create the slot 'mycustomfunction' in order to bind it to the clicked signal of the pushButton on the Signal/Slot editor. Maybe I miss something but I can not find the way in designer documentation, where I just found how to bind sender and receiver predefined signals and slots.
The main goal of this new-style is to provide a more Pythonic syntax to Python programmers.
2New syntax: Signal() and Slot()
Traditional syntax: SIGNAL () and SLOT()
QtCore.SIGNAL() and QtCore.SLOT() macros allow Python to interface with Qt signal and slot delivery mechanisms.This is the old way of using signals and slots.
The example below uses the well known clicked signal from a QPushButton.The connect method has a non python-friendly syntax.It is necessary to inform the object, its signal (via macro) and a slot to be connected to.
New syntax: Signal() and Slot()
The new-style uses a different syntax to create and to connect signals and slots.The previous example could be rewritten as:
Signals can be defined using the QtCore.Signal() class.Python types and C types can be passed as parameters to it.If you need to overload it just pass the types as tuples or lists.
In addition to that, it can receive also a named argument name that defines the signal name.If nothing is passed as name then the new signal will have the same name as the variable that it is being assigned to.
The Examples section below has a collection of examples on the use of QtCore.Signal().
Note: Signals should be defined only within classes inheriting from QObject.This way the signal information is added to the class QMetaObject structure.
Slots are assigned and overloaded using the decorator QtCore.Slot().Again, to define a signature just pass the types like the QtCore.Signal() class.Unlike the Signal() class, to overload a function, you don't pass every variation as tuple or list.Instead, you have to define a new decorator for every different signature.The examples section below will make it clearer.
Another difference is about its keywords.Slot() accepts a name and a result.The result keyword defines the type that will be returned and can be a C or Python type.name behaves the same way as in Signal().If nothing is passed as name then the new slot will have the same name as the function that is being decorated.
The examples below illustrate how to define and connect signals and slots in PySide2.Both basic connections and more complex examples are given.
Hello World example: the basic example, showing how to connect a signal to a slot without any parameters.
Next, some arguments are added. This is a modified Hello World version. Some arguments are added to the slot and a new signal is created.
Add some overloads. A small modification of the previous example, now with overloaded decorators.
An example with slot overloads and more complicated signal connections and emissions (note that when passing arguments to a signal you use ''):
An example of an object method emitting a signal:
An example of a signal emitted from another QThread:
Signals are runtime objects owned by instances, they are not class attributes:
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This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.
Differences between String-Based and Functor-Based Connections (Official documentation)
Introduction (Woboq blog)
Implementation Details (Woboq blog)
Note: This is in addition to the old string-based syntax which remains valid.
1Connecting in Qt 5
2Disconnecting in Qt 5
Connecting in Qt 5
There are several ways to connect a signal in Qt 5.
Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget)
New: connecting to QObject member
Here's Qt 5's new way to connect two QObjects and pass non-string objects:
Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
Argument can be by typedefs or with different namespace specifier, and it works.
Possibility to automatically cast the types if there is implicit conversion (e.g. from QString to QVariant)
It is possible to connect to any member function of QObject, not only slots.
More complicated syntax? (you need to specify the type of your object)
Very complicated syntax in cases of overloads? (see below)
Default arguments in slot is not supported anymore.
New: connecting to simple function
The new syntax can even connect to functions, not just QObjects:
Can be used with std::bind:
Can be used with C++11 lambda expressions:
There is no automatic disconnection when the 'receiver' is destroyed because it's a functor with no QObject. However, since 5.2 there is an overload which adds a 'context object'. When that object is destroyed, the connection is broken (the context is also used for the thread affinity: the lambda will be called in the thread of the event loop of the object used as context).
Disconnecting in Qt 5
As you might expect, there are some changes in how connections can be terminated in Qt 5, too.
You can disconnect in the old way (using SIGNAL, SLOT) but only if
You connected using the old way, or
If you want to disconnect all the slots from a given signal using wild card character
Symetric to the function pointer one
Only works if you connected with the symmetric call, with function pointers (Or you can also use 0 for wild card)In particular, does not work with static function, functors or lambda functions.
New way using QMetaObject::Connection
Works in all cases, including lambda functions or functors.
Asynchronous made easier
With C++11 it is possible to keep the code inline
Here's a QDialog without re-entering the eventloop, and keeping the code where it belongs:
Another example using QHttpServer : http://pastebin.com/pfbTMqUm
Signal And Slot In Qt
Tested with GCC.
Fortunately, IDEs like Qt Creator simplifies the function naming
Missing Q_OBJECT in class definition
Default arguments in slot
If you have code like this:
The old method allows you to connect that slot to a signal that does not have arguments.But I cannot know with template code if a function has default arguments or not.So this feature is disabled.
There was an implementation that falls back to the old method if there are more arguments in the slot than in the signal.This however is quite inconsistent, since the old method does not perform type-checking or type conversion. It was removed from the patch that has been merged.
As you might see in the example above, connecting to QAbstractSocket::error is not really beautiful since error has an overload, and taking the address of an overloaded function requires explicit casting, e.g. a connection that previously was made as follows:
cannot be simply converted to:
...because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code needs to be:
Unfortunately, using an explicit cast here allows several types of errors to slip past the compiler. Adding a temporary variable assignment preserves these compile-time checks:
Some macro could help (with C++11 or typeof extensions). A template based solution was introduced in Qt 5.7: qOverload
The best thing is probably to recommend not to overload signals or slots …
… but we have been adding overloads in past minor releases of Qt because taking the address of a function was not a use case we support. But now this would be impossible without breaking the source compatibility.
Should QMetaObject::Connection have a disconnect() function?
The other problem is that there is no automatic disconnection for some object in the closure if we use the syntax that takes a closure.One could add a list of objects in the disconnection, or a new function like QMetaObject::Connection::require
Qt Connect Signal Slot
Function such as QHostInfo::lookupHost or QTimer::singleShot or QFileDialog::open take a QObject receiver and char* slot.This does not work for the new method.If one wants to do callback C++ way, one should use std::functionBut we cannot use STL types in our ABI, so a QFunction should be done to copy std::function.In any case, this is irrelevant for QObject connections.
Qt Debug Signal And Slot
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