Pyqt Designer Signals And Slots

1. Introduction of Signal Slot Mechanism

1. Introduction of signal slots

Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. As part of our PyQt Tutorial series, we’ve gone through some basic layout management in addition to a conversation about some interface design but now when I click buttons I want things to happen! In order to achieve that goal, we’re going to have to learn about signals and slots. Let me let you in on a little secret. Signals and slots?

Signal slot is the core mechanism of Qt and also the mechanism of object communication in PyQt programming.In Qt, the QObject object and all controls in PyQt that inherit from QWidget support the slot mechanism.When the signal is transmitted, the connected slot function is automatically executed.In PyQt5, the signal and slot functions are connected by the object.signal.connect() method.
The characteristics of the signal slot are as follows:
(1) One signal can connect multiple slots
(2) One signal may connect another signal
(3) Signal parameters can be of any Python type
(4) One slot can monitor multiple signals
(5) The connection between signal and slot can be synchronous or asynchronous.
(6) Signal-slot connections can cross threads
(7) Signal can be disconnected
When writing a class, the signal and slot of the class are defined first, and the signal and slot are connected in the class to realize data transmission between objects.When an event or state changes, a signal is issued that triggers the execution of the slot function associated with the event or signal.The signal slot mechanism is illustrated as follows:

2. Define the signal

PyQt's built-in signals are automatically defined, using the PyQt5.QtCore.pyqtSignal function to create a signal for a QObject object, and using the pyqtSignal function to define the signal as a property of a class.

The types parameter denotes the type of parameters that define the signal, the name parameter denotes the name of the signal, and the class's attribute name is used by default.
Use the pyqtSignal function to create one or more overloaded unbound signals as attributes of the class, signals can only be defined in subclasses of QObject.Signals must be defined at class creation time and cannot be added dynamically as attributes of a class after it is created.When a signal is defined using the pyqtSignal function, it can pass multiple parameters and specify the type of the signal transfer parameters, which are standard Python data types, including strings, dates, Boolean types, numbers, lists, dictionaries, tuples.

3. Operational signal

The connect function binds the signal to the slot function, the disconnect function unbinds the signal to the slot function, and the emit function emits the signal.
QObject.signal.connect(self, slot, type=None, no_receiver_check=False)
Establish the connection of signal to slot function, type is connection type.
QObject.signal.disconnect(self, slot=None)
Disconnect signal from slot
emit(self, *args)
Send signal, args is variable parameter.

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2. Signal and Slot Application

1. Built-in signal and slot function

The built-in signal is the signal automatically defined by the QObject object, and the built-in slot function is the slot function automatically defined by the QObject object. The built-in signal of the QObject object can be connected to the slot function of the QObject object through the QObject.signal.connect function.

2. Built-in signal and custom slot function

When a button is clicked, the button's built-in clicked signal is triggered to execute the bound custom slot function onClose.

3. Custom signal and built-in slot function

By connecting the built-in signal clicked to the custom slot function onClose, a custom signal closeSignal is sent within the custom slot function onClose, and the custom signal closeSignal is connected to the built-in slot function close.

4. Custom Signal and Custom Slot Function

3. Advances in signal slot applications

1. Custom Signal Slot

Signal objects are usually defined through class variables, and custom signals are defined before u init_u functions.

The slot function definition of a class is the same as the general method definition of a class.

Signal and slot function can belong to the same QObject object or different QObject objects by connect ing signal and slot function.

The signal is sent by the emit method.

2. Signal slot transfer custom parameters

The number of parameters emitted by the signal in Qt must be greater than or equal to the number of parameters of the slot function. PyQt uses custom parameter transfer to solve the problem that there are more parameters in the slot function than in the signal.A Lambda expression or a functools partial function can be used to pass custom parameters to the slot function, which can be of any Python type.

3. Signal slots and ornaments

Signal and slot functions can be defined in PyQt using a Python decorator as follows:

The sender object name is the object name set for the QObject object using setObjectName, and the connectSlotsByName function connected to the slot function by the signal name is as follows:
QtCore.QMetaObject.connectSlotsByName(self, QObject)
ConneSlotsByName is used to connect certain signals of QObject descendant objects to corresponding slot functions of certain QObject objects by name.

4. Event handling mechanism

1. The difference between event mechanism and signal slot mechanism

PyQt provides a high-level signal slot mechanism and a low-level event processing mechanism for event processing, which is an advanced encapsulation of event processing mechanisms.When using a control, you don't need to consider the event handling mechanism, you only need to care about the signal slot. For a custom derived control, you must consider the event handling mechanism and re-implement the corresponding event handling function according to the behavior requirements of the control.

2. Method of event handling

PyQt provides five event handling and filtering methods:
(1) Re-implement event handling functions
Common event handling functions such as paintEvent, mouseMoveEvent, mousePressEvent, mouseReleaseEvent, and so on.
(2) Re-implement the QObject.event event distribution function
When adding new events, you need to re-implement the QObject.event method and add distribution routes for new events.
(3) Install event filters
If the installEventFilter method is called on a QObject object, an event filter is installed on the QObject object.All events of the QObject object are first passed to the event filter eventFilter function, in which certain events can be discarded or modified, custom event handling mechanisms are used for events of interest, and default event handling mechanisms are used for other events.Event filtering mechanism filters all events of QObject, so more events to filter can affect program performance.
(4) Install event filters in QApplication
Installing event filters on QApplication objects filters all events on all QObject objects and gets events first, which are sent to QApplication's event filters before sending them to any other event filters.
(5) Notfy method for QApplication
PyQt distributes events using the notify method of the QApplication object. The only way to capture events before any event filter is to re-implement the notify method of the QApplication.

3. Examples of event handling

The QDialog dialog automatically exits when the ESC key is pressed, and the ESC key is pressed using event handling and filtering.
(1) Re-implement event handling functions

(2) Re-implement the event function

(3) QObject Installation Event Filter

(4) QApplication Installation Event Filter

Posted by globalinsites on Sun, 21 Jul 2019 09:24:23 -0700

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Pyqt Designer Signals And Slots Free


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.

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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.

Using Qt Designer's Signal/Slot Editor

First design a simple form with a LineEdit control and a PushButton.

It is desired that if button is pressed, contents of text box should be erased. The QLineEdit widget has a clear() method for this purpose. Hence, the button’s clicked signal is to be connected to clear() method of the text box.

To start with, choose Edit signals/slots from Edit menu (or press F4). Then highlight the button with mouse and drag the cursor towards the textbox

As the mouse is released, a dialog showing signals of button and methods of slot will be displayed. Select clicked signal and clear() method

The Signal/Slot Editor window at bottom right will show the result −

Save ui and Build and Python code from ui file as shown in the below code −

Generated Python code will have the connection between signal and slot by the following statement −

Pyqt Designer Signals And Slots

Run signalslot.py and enter some text in the LineEdit. The text will be cleared if the button is pressed.

Building Signal-slot Connection

Instead of using Designer, you can directly establish signal-slot connection by following syntax −

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 technique −

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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.

The above code produces the following output −

Output