Signals and slots are used for notifications, each object notifies interested objects about what it did or to reflect its new state. I'm suggesting changing the design as follows: No singleton signal. Each Object has its own signal and slot for a relevant event (e.g. Selection change). This makes it enter in a kind of infinite event-processing loop (as “DefWindowProc(hwnd, message, wp, lp);” gets never called) No idea how I can solve this Below the callstack (Qt5 rc) in my own app: I have attached a modified version of Qt5rc1 'qtbase examples widgets tutorials gettingStarted gsQt part4' example. Infinite loop and signal deluge. I just found about Qt:. And connect this signal in a blocking way to a slot of my main thread that will just consist in a. 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. Fix stack overflow due to signal infinite loop #444 rp- merged 1 commit into sqlitebrowser: master from unknown repository Nov 18, 2015 Conversation 2 Commits 1 Checks 0 Files changed.

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.

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:

Using QtCore.Signal()

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.

Using QtCore.Slot()

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() Top 10 casinos online. 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.

Examples

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:

Retrieved from 'https://wiki.qt.io/index.php?title=Qt_for_Python_Signals_and_Slots&oldid=35927'

Qt offers many classes and functions for working with threads. Below are four different approaches that Qt programmers can use to implement multithreaded applications.

QThread: Low-Level API with Optional Event Loops

QThread is the foundation of all thread control in Qt. Each QThread instance represents and controls one thread.

QThread can either be instantiated directly or subclassed. Instantiating a QThread provides a parallel event loop, allowing QObject slots to be invoked in a secondary thread. Subclassing a QThread allows the application to initialize the new thread before starting its event loop, or to run parallel code without an event loop.

See the QThread class reference and the threading examples for demonstrations on how to use QThread.

QThreadPool and QRunnable: Reusing Threads

Creating and destroying threads frequently can be expensive. To reduce this overhead, existing threads can be reused for new tasks. QThreadPool is a collection of reuseable QThreads.

Qt Signal Slot Infinite Loop Maker

To run code in one of a QThreadPool's threads, reimplement QRunnable::run() and instantiate the subclassed QRunnable. Use QThreadPool::start() to put the QRunnable in the QThreadPool's run queue. When a thread becomes available, the code within QRunnable::run() will execute in that thread.

Each Qt application has a global thread pool, which is accessible through QThreadPool::globalInstance(). This global thread pool automatically maintains an optimal number of threads based on the number of cores in the CPU. However, a separate QThreadPool can be created and managed explicitly.

Qt Concurrent: Using a High-level API

The Qt Concurrent module provides high-level functions that deal with some common parallel computation patterns: map, filter, and reduce. Unlike using QThread and QRunnable, these functions never require the use of low-level threading primitives such as mutexes or semaphores. Instead, they return a QFuture object which can be used to retrieve the functions' results when they are ready. QFuture can also be used to query computation progress and to pause/resume/cancel the computation. For convenience, QFutureWatcher enables interactions with QFutures via signals and slots.

Qt Concurrent's map, filter and reduce algorithms automatically distribute computation across all available processor cores, so applications written today will continue to scale when deployed later on a system with more cores.

This module also provides the QtConcurrent::run() function, which can run any function in another thread. However, QtConcurrent::run() only supports a subset of features available to the map, filter and reduce functions. The QFuture can be used to retrieve the function's return value and to check if the thread is running. However, a call to QtConcurrent::run() uses one thread only, cannot be paused/resumed/canceled, and cannot be queried for progress.

See the Qt Concurrent module documentation for details on the individual functions.

WorkerScript: Threading in QML

The WorkerScript QML type lets JavaScript code run in parallel with the GUI thread.

Each WorkerScript instance can have one .js script attached to it. When WorkerScript.sendMessage() is called, the script will run in a separate thread (and a separate QML context). When the script finishes running, it can send a reply back to the GUI thread which will invoke the WorkerScript.onMessage() signal handler.

Using a WorkerScript is similar to using a worker QObject that has been moved to another thread. Data is transferred between threads via signals.

See the WorkerScript documentation for details on how to implement the script, and for a list of data types that can be passed between threads.

Choosing an Appropriate Approach

As demonstrated above, Qt provides different solutions for developing threaded applications. The right solution for a given application