Qt 5 signals and slots mechanism. How signals and slots in Qt differ from the callback architecture in other widget toolkits. A Qt basics tutorial. How to add signals and slots in Qt Creator.

Python GUI Development with Qt - QtDesigner's Signal-Slot Editor, Tab Order Management - Video 12 - Duration: 13:39. PythonBo 27,727 views.

• We implement the zooming slots using the private scaleImage function. We set the scaling factors to 1.25 and 0.8, respectively. We set the scaling factors to 1.25 and 0.8, respectively. These factor values ensure that a Zoom In action and a Zoom Out action will cancel each other (since 1.25. 0.8 1), and in that way the normal image size.
• Similar to QPRIVATEPROPERTY, QPRIVATESLOT is used when you have a slot in a private class that you want to be considered part of the API of the corresponding public class, without affecting the interface (and hence binary compatibility) of that public class.
• Qt's widgets have many pre-defined signals, but we can always subclass to add our own. A slot is a function that is called in reponse to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to add your own slots so that you can handle the signals that you are interested in.

Part 9 of the Qt Creator C++ Tutorial

What are Qt 5 Signals and Slots?

Very basically, signals and slots in Qt allow communication between objects.

In Qt, a signal is emitted when an event occurs. A slot is a function that is called when a signal is emitted. For example, a push button emits a clicked signal when clicked by a user. A slot that is attached to that signal is called when the clicked signal is emitted.

Multiple signals can be connected to any slot. Signals can be connected to any number of slots.

Most of the details of signals and slots are hidden in their implementation in Qt. At this stage of the tutorial series we do not look in depth at signals and slots.

Using Signals and Slots in Qt Creator

There are several ways to use signals and slots in Qt Creator projects. This includes manually adding them in code. Here we briefly look at the easier ways to use signals and slots to respond to events. Events are generated by users interacting with widgets in an application. These events cause signals to be emitted. Corresponding slots, or functions then run.

Qt 5 Signals and Slots Demonstration

The following image shows the application built in this section using Qt Creator. It demonstrates some methods of using signals and slots.

Each section below shows a method of adding signals and slots to a Qt Creator program. Watch the video embedded near the top of this page for details.

Add a Slot to a Button for the Clicked Signal

Place a push button on the main window. Right click the push button and select Go to slot… to add code for the clicked signal.

Connect a Slider to a Progress Bar Visually

Place a Horizontal Slider and a Progress Bar on the main window.

Press F4 on the keyboard. This toggles to Edit Signals/Slots mode.

Drag to connect the slider to the progress bar.

Press F3 to change back to Edit Widgets mode.

Connect a Slider to a Progress Bar with Code

Place a second Horizontal Slider and a Progress Bar on the main window.

Right-click the Horizontal Slider. In the menu that pops up, click Go to slot…

In the dialog box that pops up, select sliderMoved(int). Click the OK button.

Add code for the sliderMoved signal.

Menu Bar Item with Action Editor

Add a File menu with Open, Close and Quit menu items.

Qt Creator must be in Design mode. Make sure that the Action Editor and Signal and Slots Editor are visible. Do this from the top menu as follows. Select Window → Views and then click the check box next to each of the desired editors.

Qt Private Slot Example Value

Add slots for the triggered() signal for the Open and Close menu items. Do this in the Action Editor as follows. Right click a menu item. Click Go to slot… on the menu that pops up. Click triggered() in the dialog box that pops up and then click the OK button.

Add code in the slot function.

Qt Private Slot Example For Real

Menu Bar Item with Signals and Slots Editor

In Design mode, select the Signals and Slots tab. Click the big green + sign to add an item. Change the following for the new item.

• Sender : actionQuit
• Signal : triggered()
• Receiver : MainWindow
• Slot : close()

Code Listing

Below is the code listing for mainwindow.cpp for the example project. Follow the video embedded near the top of this page to add the code.

Qt Private Slot Example Software

mainwindow.cpp