1. Qt Signal Slot Thread
2. Qt Qtimer Signal Slot Car Bodies
3. Qt Signal Slot Not Working
4. Qt Signals And Slots Tutorial
5. Qt Signal Slot Example

Introduction

1. Next, its timeout signal is connected to the slot that will do the work, it is started with a value of 1000 milliseconds, indicating that it will time out every second. QTimer also provides a static function for single-shot timers. For example: QTimer:: singleShot(200, this, SLOT(updateCaption)).
2. I have a subclass of QObject referred to as myObject, which has a QTimer data member allocated on the heap in the constructor. MyObject also has a slot which is connected to the QTimer timeout signal in the constructor. I refer to the pointer of myObject as myObjectptr. I want to run myObject on a different thread from the main thread.

The QTimer class provides repetitive and single-shot timers. The QTimer class provides a high-level programming interface for timers. To use it, create a QTimer, connect its timeout signal to the appropriate slots, and call start. From then on it will emit the timeout signal at constant intervals.

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

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

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.

Remarks

QTimer can also be used to request a function to run as soon as the event loop has processed all the other pending events. To do this, use an interval of 0 ms.

Simple example

The following example shows how to use a QTimer to call a slot every 1 second.

In the example, we use a QProgressBar to update its value and check the timer is working properly.

main.cpp

timer.h

timer.cpp

timer.pro

Singleshot Timer with Lambda function as slot

If a singleshot timer is required, it is quiet handy to have the slot as lambda function right in the place where the timer is declared:

Due to this Bug (QTBUG-26406), this is way is only possible since Qt5.4.

In earlier Qt5 versions it has to be done with more boiler plate code:

Using QTimer to run code on main thread

This is useful when you need to update a UI element from a thread. Keep in mind lifetime of anything the callback references.

Same code could be adapted to run code on any thread that runs Qt event loop, thus implementing a simple dispatch mechanism.

Basic Usage

Qt Signal Slot Thread

QTimer add the functionality to have a specific function/slot called after a certain interval (repeatedly or just once).

The QTimer thus allows a GUI application to 'check' things regularly or handle timeouts without having to manually start an extra thread for this and be careful about race conditions, because the timer will be handled in the main-event loop.

A timer can simply be used like this:

The timer triggers the timeout signal when the time is over and this will be called in the main-event loop.

QTimer::singleShot simple usage

The QTimer::singleShot is used to call a slot/lambda asynchronously after n ms.

The basic syntax is :

with myTime the time in ms, myObject the object which contain the method and myMethodInMyObject the slot to call

Qt Qtimer Signal Slot Car Bodies

So for example if you want to have a timer who write a debug line 'hello !' every 5 seconds:

Qt Signal Slot Not Working

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Qt Signals And Slots Tutorial

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Qt Signal Slot Example