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Qt5 Tutorial Signals and Slots - 2020





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Setting Up Signals and Slots

In this tutorial, we will learn QtGUI project with signal and slot mechanism.


File->New File or Project...

Applications->Qt Gui Application->Choose...

We keep the class as MainWindow as given by default.

QtGuiApplication.png

Next->Finish

Let's open up Forms by double-clicking the mainwindow.ui to put gui components:

CreatorDesign.png

From the Widgets, drag Horizontal Slider and Progress Bar, and place them into the main window. Then,

Slider_ProgressBar.png

GuiSlot.png

ConfigureConnection.png

Run the code. Now, if we move the slider, the progress will reflect the changes in the slider:

RunA.png

We did it via gui, but we can do it via direct programming.

Let's delete the signal and slot, and write the code for the signal and slot mechanism in the constructor of the MainWindow class as shown below:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->horizontalSlider,
            SIGNAL(valueChanged(int)),
            ui->progressBar,
            SLOT(setValue(int)));
}

MainWindow::~MainWindow()
{
    delete ui;
}
RunB.png


Signal and Slot

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called. Older toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. 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. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches. 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. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt. You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

- from Signals & Slots







Qt 5 Tutorial

  1. Hello World
  2. Signals and Slots
  3. Q_OBJECT Macro
  4. MainWindow and Action
  5. MainWindow and ImageViewer using Designer A
  6. MainWindow and ImageViewer using Designer B
  7. Layouts
  8. Layouts without Designer
  9. Grid Layouts
  10. Splitter
  11. QDir
  12. QFile (Basic)
  13. Resource Files (.qrc)
  14. QComboBox
  15. QListWidget
  16. QTreeWidget
  17. QAction and Icon Resources
  18. QStatusBar
  19. QMessageBox
  20. QTimer
  21. QList
  22. QListIterator
  23. QMutableListIterator
  24. QLinkedList
  25. QMap
  26. QHash
  27. QStringList
  28. QTextStream
  29. QMimeType and QMimeDatabase
  30. QFile (Serialization I)
  31. QFile (Serialization II - Class)
  32. Tool Tips in HTML Style and with Resource Images
  33. QPainter
  34. QBrush and QRect
  35. QPainterPath and QPolygon
  36. QPen and Cap Style
  37. QBrush and QGradient
  38. QPainter and Transformations
  39. QGraphicsView and QGraphicsScene
  40. Customizing Items by inheriting QGraphicsItem
  41. QGraphicsView Animation
  42. FFmpeg Converter using QProcess
  43. QProgress Dialog - Modal and Modeless
  44. QVariant and QMetaType
  45. QtXML - Writing to a file
  46. QtXML - QtXML DOM Reading
  47. QThreads - Introduction
  48. QThreads - Creating Threads
  49. Creating QThreads using QtConcurrent
  50. QThreads - Priority
  51. QThreads - QMutex
  52. QThreads - GuiThread
  53. QtConcurrent QProgressDialog with QFutureWatcher
  54. QSemaphores - Producer and Consumer
  55. QThreads - wait()
  56. MVC - ModelView with QListView and QStringListModel
  57. MVC - ModelView with QTreeView and QDirModel
  58. MVC - ModelView with QTreeView and QFileSystemModel
  59. MVC - ModelView with QTableView and QItemDelegate
  60. QHttp - Downloading Files
  61. QNetworkAccessManager and QNetworkRequest - Downloading Files
  62. Qt's Network Download Example - Reconstructed
  63. QNetworkAccessManager - Downloading Files with UI and QProgressDialog
  64. QUdpSocket
  65. QTcpSocket
  66. QTcpSocket with Signals and Slots
  67. QTcpServer - Client and Server
  68. QTcpServer - Loopback Dialog
  69. QTcpServer - Client and Server using MultiThreading
  70. QTcpServer - Client and Server using QThreadPool
  71. Asynchronous QTcpServer - Client and Server using QThreadPool
  72. Qt Quick2 QML Animation - A
  73. Qt Quick2 QML Animation - B
  74. Short note on Ubuntu Install
  75. OpenGL with QT5
  76. Qt5 Webkit : Web Browser with QtCreator using QWebView Part A
  77. Qt5 Webkit : Web Browser with QtCreator using QWebView Part B
  78. Video Player with HTML5 QWebView and FFmpeg Converter
  79. Qt5 Add-in and Visual Studio 2012
  80. Qt5.3 Installation on Ubuntu 14.04
  81. Qt5.5 Installation on Ubuntu 14.04
  82. Short note on deploying to Windows







Ph.D. / Golden Gate Ave, San Francisco / Seoul National Univ / Carnegie Mellon / UC Berkeley / DevOps / Deep Learning / Visualization

YouTubeMy YouTube channel

Sponsor Open Source development activities and free contents for everyone.

Thank you.

- K Hong






Sponsor Open Source development activities and free contents for everyone.

Thank you.

- K Hong







Qt 5 Tutorial



Hello World

Signals and Slots

Q_OBJECT Macro

MainWindow and Action

MainWindow and ImageViewer using Designer A

MainWindow and ImageViewer using Designer B

Layouts

Layouts without Designer

Grid Layouts

Splitter

QDir

QFile (Basic)

Resource Files (.qrc)

QComboBox

QListWidget

QTreeWidget

QAction and Icon Resources

QStatusBar

QMessageBox

QTimer

QList

QListIterator

QMutableListIterator

QLinkedList

QMap

QHash

QStringList

QTextStream

QMimeType and QMimeDatabase

QFile (Serialization I)

QFile (Serialization II - Class)

Tool Tips in HTML Style and with Resource Images

QPainter

QBrush and QRect

QPainterPath and QPolygon

QPen and Cap Style

QBrush and QGradient

QPainter and Transformations

QGraphicsView and QGraphicsScene

Customizing Items by inheriting QGraphicsItem

QGraphicsView Animation

FFmpeg Converter using QProcess

QProgress Dialog - Modal and Modeless

QVariant and QMetaType

QtXML - Writing to a file

QtXML - QtXML DOM Reading

QThreads - Introduction

QThreads - Creating Threads

Creating QThreads using QtConcurrent

QThreads - Priority

QThreads - QMutex

QThreads - GuiThread

QtConcurrent QProgressDialog with QFutureWatcher

QSemaphores - Producer and Consumer

QThreads - wait()

MVC - ModelView with QListView and QStringListModel

MVC - ModelView with QTreeView and QDirModel

MVC - ModelView with QTreeView and QFileSystemModel

MVC - ModelView with QTableView and QItemDelegate

QHttp - Downloading Files

QNetworkAccessManager and QNetworkRequest - Downloading Files

Qt's Network Download Example - Reconstructed

QNetworkAccessManager - Downloading Files with UI and QProgressDialog

QUdpSocket

QTcpSocket

QTcpSocket with Signals and Slots

QTcpServer - Client and Server

QTcpServer - Loopback Dialog

QTcpServer - Client and Server using MultiThreading

QTcpServer - Client and Server using QThreadPool

Asynchronous QTcpServer - Client and Server using QThreadPool

Qt Quick2 QML Animation - A

Qt Quick2 QML Animation - B

Short note on Ubuntu Install

OpenGL with QT5

Qt5 Webkit : Web Browser with QtCreator using QWebView Part A

Qt5 Webkit : Web Browser with QtCreator using QWebView Part B

Video Player with HTML5 QWebView and FFmpeg Converter

Qt5 Add-in and Visual Studio 2012

Qt5.3 Installation on Ubuntu 14.04

Qt5.5 Installation on Ubuntu 14.04

Short note on deploying to Windows




Sponsor Open Source development activities and free contents for everyone.

Thank you.

- K Hong













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