Introduction

1. Qt Slot Execution Order Diagram
2. Qt Signal Slot Execution Order

Qt is a cross-platform C framework for creating GUI applications. Qt uses its own build system, qmake, and also supports building with CMake starting from the version Qt4. A pure Qmake project can't be imported in CLion directly. However, when converted into CMake, it can be opened and managed as a regular CMake application. Signals and Slots. 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.

Remember old X-Window call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offer new event-handling system - signal-slot connections. Imagine alarm clock. When alarm is ringing, signal is sending (emitting). And you're handling it as a slot.

1. Every QObject class may have as many signals of slots as you want.
2. You can emit signal only from that class, where signal is.
3. You can connect signal with another signal (make chains of signals);
4. Every signal and slot can have unlimited count of connections with other.
5. ATTENTION! You can't set default value in slot attributes. e.g.void mySlot(int i = 0);

Connection

You can connect signal with this template:QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);You have to wrap const char * signal and const char * method into SIGNAL () and SLOT() macros.

And you also can disconnect signal-slot:QObject::disconnect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);

Deeper

Widgets emit signals when events occur. For example, a button will emit a 'clicked' signal when it is clicked. A developer can choose to connect to a signal by creating a function (a 'slot') and calling the connect() function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.

For example, if a Quit button's clicked() signal is connected to the application's quit() slot, a user's click on Quit makes the application terminate. In code, this is written as

connect(button, SIGNAL (clicked()), qApp, SLOT (quit()));

Connections can be added or removed at any time during the execution of a Qt application, they can be set up so that they are executed when a signal is emitted or queued for later execution, and they can be made between objects in different threads.

The signals and slots mechanism is implemented in standard C++. The implementation uses the C++ preprocessor and moc, the Meta-Object Compiler, included with Qt. Code generation is performed automatically by Qt's build system. Developers never have to edit or even look at the generated code.

• Status:Closed
• Resolution: Invalid
• Fix Version/s: None
• Labels:
• Environment:

  Ubuntu 16.04 and Windows 10 both using Qt 5.9.1
  

Summary

The order of event execution differs between Linux and Windows for mousePressEvent, mouseReleaseEvent, and contextMenuEvent.

Description

Attached is a small qt project that will reproduce the issue. It is just a QPushButton that prints when it receives the above-mentioned events.

Note: On windows I had to #undef main before the main function to get it to compile.

Expected behavior:

The events are processed in the same order on Linux and Windows.

Actual behavior:

When right clicking the widget that appears, on Linux the events will be processed in the following order:

MOUSE DOWN
CONTEXT MENU
MOUSE UP

On Windows the events will be processed in the following order:

MOUSE DOWN
MOUSE UP
CONTEXT MENU

Environment

Linux:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.2 LTS
Release: 16.04
Codename: xenial

Qt Version: 5.9.1 (installed from qt installer)

Windows:

OS: Windows 10

Qt Version: 5.9.1 (installed from qt installer)

Qt Slot Execution Order Diagram

Qt Signal Slot Execution Order