Introduction

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. How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections. Which is called by QMetaObject::activate to prepare a Qt::QueuedConnection slot call. The signals and slots mechanism is a central feature of Qt. 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. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt Jambis'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. What the code below does is to create a slot and signal with a function that will emit a signal when called (the function that is) so the signal is “fired” off to what ever is listening to it, to connect a signal to a slot you use the QObject::connect as below.

Remarks

Official documentation on this topic can be found here.

A Small Example

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.

The minimal example requires a class with one signal, one slot and one connection:

counter.h

The main sets a new value. We can check how the slot is called, printing the value.

Finally, our project file:

The new Qt5 connection syntax

The conventional connect syntax that uses SIGNAL and SLOT macros works entirely at runtime, which has two drawbacks: it has some runtime overhead (resulting also in binary size overhead), and there's no compile-time correctness checking. The new syntax addresses both issues. Before checking the syntax in an example, we'd better know what happens in particular.

Let's say we are building a house and we want to connect the cables. This is exactly what connect function does. Signals and slots are the ones needing this connection. The point is if you do one connection, you need to be careful about the further overlaping connections. Whenever you connect a signal to a slot, you are trying to tell the compiler that whenever the signal was emitted, simply invoke the slot function. This is what exactly happens.

Here's a sample main.cpp:

Hint: the old syntax (SIGNAL/SLOT macros) requires that the Qt metacompiler (MOC) is run for any class that has either slots or signals. From the coding standpoint that means that such classes need to have the Q_OBJECT macro (which indicates the necessity to run MOC on this class).

The new syntax, on the other hand, still requires MOC for signals to work, but not for slots. If a class only has slots and no signals, it need not have the Q_OBJECT macro and hence may not invoke the MOC, which not only reduces the final binary size but also reduces compilation time (no MOC call and no subsequent compiler call for the generated *_moc.cpp file).

Connecting overloaded signals/slots

While being better in many regards, the new connection syntax in Qt5 has one big weakness: Connecting overloaded signals and slots. In order to let the compiler resolve the overloads we need to use static_casts to member function pointers, or (starting in Qt 5.7) qOverload and friends:

Multi window signal slot connection

Qt Signal Emitted But Slot Not Called One

A simple multiwindow example using signals and slots.

There is a MainWindow class that controls the Main Window view. A second window controlled by Website class.

The two classes are connected so that when you click a button on the Website window something happens in the MainWindow (a text label is changed).

I made a simple example that is also on GitHub:

mainwindow.h

mainwindow.cpp

website.h

Qt Signal Emitted But Slot Not Called Light

website.cpp

Project composition:

Consider the Uis to be composed:

Qt Signal Emitted But Slot Not Called Two

• Main Window: a label called 'text' and a button called 'openButton'
• Website Window: a button called 'changeButton'

Qt Signal Emitted But Slot Not Called The Most

So the keypoints are the connections between signals and slots and the management of windows pointers or references.