Qt5 C++ Signal And Slots With Practical Examples #4
Introduction In some applications it is often necessary to perform long-running tasks, such as computations or network operations, python qt signals and slots cannot be broken up into smaller pieces and processed alongside normal go here events.
In such cases, we would like to be python qt signals and slots 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 just click for source 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 read article 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.
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.
This will reset the user interface when the thread stops running.
The custom output QRect, QImage signal is connected to python qt signals and slots addImage slot so that we can update the viewer label every time a new star is drawn.
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.
Whenever python qt signals and slots star is drawn by the worker thread, it will emit python qt signals and slots signal that is connected to the addImage slot.
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: def updateUi self : self.
The Worker Thread The worker thread is implemented as a thread rather than a Python thread since we want to take advantage of the signals and slots mechanism to communicate with the main application.
The exiting attribute is used to tell the thread to stop processing.
Each star is drawn using a QPainterPath that we define in advance: self.
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.
We provide the render method instead of letting our own run method take extra arguments because the run method is called by itself with no arguments.
The run method is where we perform the processing that occurs in the thread provided by the Worker instance: def run self : Note: This is never called directly.
It is called by Qt once the thread environment has been set up.
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.
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: self.
C++ Qt 4 - Signals and Slots
This signal does nothing, by itself; it must be connected to a slot, which is an object that acts as a recipient for a signal and, given one, acts on it. Connecting Built-In PySide/PyQt Signals. Qt widgets have a number of signals built in. For example, when a QPushButton is clicked, it emits its clicked signal.
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