Qt Signal Slot Lambda Parameter
QT lambda slots I’ve toyed with QT connections trying to create anonymous function when connecting a slot to a signal. For instance, I am trying to do the following. Nd the index of the signal and of the slot Keep in an internal map which signal is connected to what slots When emitting a signal, QMetaObject::activate is called. It calls qt metacall (generated by moc) with the slot index which call the actual slot. Here, I want to briefly discuss how the same effect can be achieved with Qt itself. C is not as dynamic as Python, so Python's approaches of using lambda or functools.partial won't work 1. Fortunately, the Qt folks provided a solution that can make passing extra arguments to slots relatively simple. #. Signal Slot에서 lambda 함수 이용하기: import sys: from PyQt5. QtWidgets import QWidget: from PyQt5. QtWidgets import QLabel: from PyQt5. QtWidgets import QSlider: from PyQt5. QtWidgets import QApplication: from PyQt5. QtWidgets import QBoxLayout: from PyQt5. QtCore import Qt: author = 'Deokyu Lim ' class Form.
I’ve toyed with QT connections trying to create anonymous function when connecting a slot to a signal. For instance, I am trying to do the following:
Qt's signal and slot mechanism makes sure that if the signal is connected to a slot, the slot is automatically called with the signal's parameters. One signal can be connected to as many slots as needed. If more than one slot is connected, they are called when the signal is emitted one after another in the same order they were connected.
So far so good, but sometime I find it cumbersome to define all the slots. Create a declaration, a definition, etc…
I would like to declare the body right there! Here’s what I would like to do using std::function or a c++ lambda:
I’ve achieved that by creating a global object named _Q which is responsible to maintained the QT meta object information about all connection that needs to be called.
Here’s the header of the class in question:
And here’s the implementation:
The idea here is that we maintain the QMetaObject needed to dispatch the call, when qt_metacall is called, to the proper anonymous functions. The magic is done in FillMetaStructs() to maintain the meta object structure each time a new connection is made when Call(...) is called.
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Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.
Widgets used to build the GUI interface act as the source of such events. Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function.
In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques −
A more convenient way to call a slot_function, when a signal is emitted by a widget is as follows −
Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following two techniques −
Qt Signal Slot Lambda Parameters
or
Example
In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.
When b1 is clicked, the clicked() signal is connected to b1_clicked() function
When b2 is clicked, the clicked() signal is connected to b2_clicked() function
Example
The above code produces the following output −