Signals Slots Python
1. Introduction of Signal Slot Mechanism
1. Introduction of signal slots
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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.
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
This example was ported from the PyQt4 version by Guðjón Guðjónsson.
Introduction
In some applications it is often necessary to perform long-running tasks, such as computations or network operations, that cannot be broken up into smaller pieces and processed alongside normal application events. In such cases, we would like to be able to perform these tasks in a way that does not interfere with the normal running of the application, and ensure that the user interface continues to be updated. One way of achieving this is to perform these tasks in a separate thread to the main user interface thread, and only interact with it when we have results we need to display.
This example shows how to create a separate thread to perform a task - in this case, drawing stars for a picture - while continuing to run the main user interface thread. The worker thread draws each star onto its own individual image, and it passes each image back to the example's window which resides in the main application thread.
The User Interface
We begin by importing the modules we require. We need the math and random modules to help us draw stars.
The main window in this example is just a QWidget. We create a single Worker instance that we can reuse as required.
The user interface consists of a label, spin box and a push button that the user interacts with to configure the number of stars that the thread wil draw. The output from the thread is presented in a QLabel instance, viewer.
We connect the standard finished() and terminated() signals from the thread to the same slot in the widget. This will reset the user interface when the thread stops running. The custom output(QRect, QImage) signal is connected to the addImage() slot so that we can update the viewer label every time a new star is drawn.
The start button's clicked() signal is connected to the makePicture() slot, which is responsible for starting the worker thread.
Signal Slot Python 3
We place each of the widgets into a grid layout and set the window's title:
The makePicture() slot needs to do three things: disable the user interface widgets that are used to start a thread, clear the viewer label with a new pixmap, and start the thread with the appropriate parameters.
Since the start button is the only widget that can cause this slot to be invoked, we simply disable it before starting the thread, avoiding problems with re-entrancy.
We call a custom method in the Worker thread instance with the size of the viewer label and the number of stars, obtained from the spin box.
Whenever is star is drawn by the worker thread, it will emit a signal that is connected to the addImage() slot. This slot is called with a QRect value, indicating where the star should be placed in the pixmap held by the viewer label, and an image of the star itself:
We use a QPainter to draw the image at the appropriate place on the label's pixmap.
The updateUi() slot is called when a thread stops running. Since we usually want to let the user run the thread again, we reset the user interface to enable the start button to be pressed:
Now that we have seen how an instance of the Window class uses the worker thread, let us take a look at the thread's implementation.
The Worker Thread
The worker thread is implemented as a PyQt thread rather than a Python thread since we want to take advantage of the signals and slots mechanism to communicate with the main application.
We define size and stars attributes that store information about the work the thread is required to do, and we assign default values to them. The exiting attribute is used to tell the thread to stop processing.
Each star is drawn using a QPainterPath that we define in advance:
Before a Worker object is destroyed, we need to ensure that it stops processing. For this reason, we implement the following method in a way that indicates to the part of the object that performs the processing that it must stop, and waits until it does so.
For convenience, we define a method to set up the attributes required by the thread before starting it.
The start() method is a special method that sets up the thread and calls our implementation of the run() method. We provide the render() method instead of letting our own run() method take extra arguments because the run() method is called by PyQt itself with no arguments.
Signals Slots Python Games
The run() method is where we perform the processing that occurs in the thread provided by the Worker instance:
Slot Machine In Python
Information stored as attributes in the instance determines the number of stars to be drawn and the area over which they will be distributed.
We draw the number of stars requested as long as the exiting attribute remains False. This additional check allows us to terminate the thread on demand by setting the exiting attribute to True at any time.
The drawing code is not particularly relevant to this example. We simply draw on an appropriately-sized transparent image.
For each star drawn, we send the main thread information about where it should be placed along with the star's image by emitting our custom output() signal:
Signals Slots Python Game
Since QRect and QImage objects can be serialized for transmission via the signals and slots mechanism, they can be sent between threads in this way, making it convenient to use threads in a wide range of situations where built-in types are used.
Running the Example
We only need one more piece of code to complete the example: