Qt Signal Slot Call Order

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Support for signals and slots pyqt 5.10.1 reference guide support for signals and slots one of the key features of qt is its use of signals and slots to. Signals and Slots Signals and slots are used for communication between objects. The signal/slot mechanism is a central feature of Qt and probably the part that differs most from other toolkits. In most GUI toolkits widgets have a callback for each action they can trigger. This callback is a pointer to a function. To see how all the stuff works together, download the sample code! It also demonstrates the usage of Timeout and the Combine function. Boost and QT also offer signal / slot functionality (see Part 1 of the article series). @JonB said in Can a signal call a non-slot method: @SGaist. Nothing Qt 5 specific, these macros have the same functionality since the beginning. Before Qt5 signals was protected, now it is public (to allow new connection syntax). That was why I wrote Qt5+. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in. The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot.

• 2Signals
• 3Basic Usage
• 4Example

Overview

Using QPushButton developers can create and handle buttons. This class is easy to use and customize so it is among the most useful classes in Qt. In general the button displays text but an icon can also be displayed.

QPushButton inherits QAbstractButton which in turn inherits QWidget.

Signals

Inherited from QAbstractButton

• void clicked(bool checked = false)
• void pressed()
• void released()
• void toggled(bool checked)

Qt Signal Slot Call Orders

Inherited from QWidget

• void customContextMenuRequested(const QPoint &pos)

Inherited from QObject

• void destroyed(QObject *obj = nullptr)

Basic Usage

Text

The text of QPushButton can be set upon creation or using setText(). To get the current text of the button use text().

Icon

The icon of QPushButton can also be set upon creation. After creation the icon can be changed using setIcon() To get the current icon of the button use icon()

Set Position and Size

Qt Signal Slot Call Ordering

To set the position and the size of the button use setGeometry(). If you want just to modify the size of the button use resize()

Handle Button

QPushButton emits signals if an event occurs. To handle the button connect its appropriate signal to a slot:

connect(m_button, &QPushButton::released, this, &MainWindow::handleButton);

Example

The following simple code snippet shows how to create and use QPushButton. It has been tested on Qt Symbian Simulator.

An instance of QPushButton is created. Signal released() is connected to slot handleButton() which changes the text and the size of the button.

To build and run the example:

1. Create an empty folder
2. Create a file for each of the below code snippets and add the example code to them (the name of the file should match the name above the snippet).
   • All 4 files must be in the same folder.
3. Using command line, navigate into the folder with the 4 files.
4. run qmake on the project file: qmake PushButtonExample.pro
   • If successful it will not print any output.
   • This should create a file with the name Makefile in the folder.
5. Build the application: make
   • The application should compile without any issues.
6. Run the application: ./PushButtonExample

The above steps are for linux but can easily be followed on other systems by replacing make with the correct make call for the system.

mainwindow.h

mainwindow.cpp

main.cpp

PushButtonExample.pro

EnArBgDeElEsFaFiFrHiHuItJaKnKoMsNlPlPtRuSqThTrUkZh

Threads in an operating system are a very simple thing. Write a function, maybe bundle it with some data and push it onto a newly created thread. Use a mutex or other method to safely communicate with the thread if necessary. Whether it are Win32, POSIX or other threads, they all basically work the same and are quite fool-proof.

Those who have discovered the joys of the Qt framework may assume that threads in Qt are just like this, and they would be right. However, there are several different ways to use threads in Qt, and it might not be obvious which approach to choose. The article, Multithreading Technologies in Qt, compares the different approaches.

The rest of this article demonstrates one of these methods: QThread + a worker QObject. This method is intended for use cases which involve event-driven programming and signals + slots across threads.

Usage with Worker class

The main thing in this example to keep in mind when using a QThread is that it's not a thread. It's a wrapper around a thread object. This wrapper provides the signals, slots and methods to easily use the thread object within a Qt project. To use it, prepare a QObject subclass with all your desired functionality in it. Then create a new QThread instance, push the QObject onto it using moveToThread(QThread*) of the QObject instance and call start() on the QThread instance. That's all. You set up the proper signal/slot connections to make it quit properly and such, and that's all.

Declare Worker class

For a basic example, check this class declaration for the Worker class:

class Worker : public QObject {

public:

public slots:

signals:

private:

};

We add at least one public slot which will be used to trigger the instance and make it start processing data once the thread has started. Now, let's see what the implementation for this basic class looks like.

Worker::Worker() { // Constructor

}

Worker::~Worker() { // Destructor

}

void Worker::process() { // Process. Start processing data.

}

While this Worker class doesn't do anything special, it nevertheless contains all the required elements. It starts processing when its main function, in this case process(), is called and when it is done it emits the signal finished() which will then be used to trigger the shutdown of the QThread instance it is contained in.

By the way, one extremely important thing to note here is that you should NEVER allocate heap objects (using new) in the constructor of the QObject class as this allocation is then performed on the main thread and not on the new QThread instance, meaning that the newly created object is then owned by the main thread and not the QThread instance. This will make your code fail to work. Instead, allocate such resources in the main function slot such as process() in this case as when that is called the object will be on the new thread instance and thus it will own the resource.

Create a new Worker instance

Now, let's see how to use this new construction by creating a new Worker instance and putting it on a QThread instance:

QThread* thread = new QThread;Worker* worker = new Worker();worker->moveToThread(thread);connect(worker, SIGNAL (error(QString)), this, SLOT (errorString(QString)));connect(thread, SIGNAL (started()), worker, SLOT (process()));connect(worker, SIGNAL (finished()), thread, SLOT (quit()));connect(worker, SIGNAL (finished()), worker, SLOT (deleteLater()));connect(thread, SIGNAL (finished()), thread, SLOT (deleteLater()));thread->start();

The connect() series here is the most crucial part. The first connect() line hooks up the error message signal from the worker to an error processing function in the main thread. The second connects the thread's started() signal to the processing() slot in the worker, causing it to start.

Then the clean-up: when the worker instance emits finished(), as we did in the example, it will signal the thread to quit, i.e. shut down. We then mark the worker instance using the same finished() signal for deletion. Finally, to prevent nasty crashes because the thread hasn't fully shut down yet when it is deleted, we connect the finished() of the thread (not the worker!) to its own deleteLater() slot. This will cause the thread to be deleted only after it has fully shut down.

External Links

1. Maya Posch's blog, http://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/
2. Qt Blog on subclassing QThread is wrong, [1]
3. Woboq Blog on subclassing QThread is not always wrong, [2]