Qt Signal Emitted But Slot Not Called

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Qt's signal and slot mechanism makes sure that if the signal is connected to a slot, the slot is automatically called with the signal's parameters. One signal can be connected to as many slots as needed. If more than one slot is connected, they are called when the signal is emitted one after another in the same order they were connected. Qt's runtime connection of signals and slots prints out a warning to the application output when it can't perform the connection. You can also look at the return value from QObject::connect. A side note, not directly related to the question, but I've had issues with QVector and storing local objects in it. If done improperly, the objects will. 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 slots machine, often known as the “one armed bandit”, became an icon Qt Signal Emitted But Slot Not Called of modern online gaming. At Slotomania, you can start playing your favorite slot games with crazy graphics, top of the Qt Signal Emitted But Slot Not Called line sound effects, and hundreds of variations to choose from.

Introduction

Signals and slots are used for communication between objects. 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. Slots can be used for receiving signals, but they are also normal member functions.

Remarks

Official documentation on this topic can be found here.

A Small Example

Qt Signal Emitted But Slot Not Called The Most

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

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

Qt Signal Emitted But Slot Not Called

mainwindow.cpp

website.h

website.cpp

Project composition:

Consider the Uis to be composed:

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

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

Qt Signal Emitted But Slot Not Called One