SingleApplication

This is a replacement of the QtSingleApplication for Qt5 and Qt6.

Keeps the Primary Instance of your Application and kills each subsequent instances. It can (if enabled) spawn secondary (non-related to the primary) instances and can send data to the primary instance from secondary instances.

<a href="https://itay-grudev.github.io/SingleApplication/">Documentation</a>

You can find the full usage reference and examples here.

Usage

The SingleApplication class inherits from whatever Q[Core|Gui]Application class you specify via the QAPPLICATION_CLASS macro (QCoreApplication is the default). Further usage is similar to the use of the Q[Core|Gui]Application classes.

You can use the library as if you use any other QCoreApplication derived class:

#include <QApplication>
#include <SingleApplication.h>
 
int main( int argc, char* argv[] )
{
    SingleApplication app( argc, argv );
 
    return app.exec();
}

To include the library files I would recommend that you add it as a git submodule to your project. Here is how:

git submodule add https://github.com/itay-grudev/SingleApplication.git singleapplication

Qmake:

Then include the singleapplication.pri file in your .pro project file.

include(singleapplication/singleapplication.pri)
DEFINES += QAPPLICATION_CLASS=QApplication

CMake:

Then include the subdirectory in your CMakeLists.txt project file.

set(QAPPLICATION_CLASS QApplication CACHE STRING "Inheritance class for SingleApplication")
add_subdirectory(src/third-party/singleapplication)
target_link_libraries(${PROJECT_NAME} SingleApplication::SingleApplication)

Directly including this repository as a Git submodule, or even just a shallow copy of the source code into new projects might not be ideal when using CMake. Another option is using CMake's FetchContent module, available since version 3.11.

# Define the minumun CMake version, as an example 3.24
cmake_minimum_required(VERSION 3.24)
 
# Include the module
include(FetchContent)
 
# If using Qt6, override DEFAULT_MAJOR_VERSION
set(QT_DEFAULT_MAJOR_VERSION 6 CACHE STRING "Qt version to use, defaults to 6")
 
# Set QAPPLICATION_CLASS
set(QAPPLICATION_CLASS QApplication CACHE STRING "Inheritance class for SingleApplication")
 
 
# Declare how is the source going to be obtained
FetchContent_Declare(
  SingleApplication
  GIT_REPOSITORY https://github.com/itay-grudev/SingleApplication
  GIT_TAG        master
  #GIT_TAG        e22a6bc235281152b0041ce39d4827b961b66ea6
)
 
# Fetch the repository and make it available to the build
FetchContent_MakeAvailable(SingleApplication)
 
# Then simply use find_package as usual
find_package(SingleApplication)
 
# Finally add it to the target_link_libraries() section
target_link_libraries(ClientePOS PRIVATE
    Qt${QT_VERSION_MAJOR}::Widgets
    Qt${QT_VERSION_MAJOR}::Network
    Qt${QT_VERSION_MAJOR}::Sql
 
    SingleApplication::SingleApplication
)

The library sets up a QLocalServer and a QSharedMemory block. The first instance of your Application is your Primary Instance. It would check if the shared memory block exists and if not it will start a QLocalServer and listen for connections. Each subsequent instance of your application would check if the shared memory block exists and if it does, it will connect to the QLocalServer to notify the primary instance that a new instance had been started, after which it would terminate with status code 0. In the Primary Instance SingleApplication would emit the instanceStarted() signal upon detecting that a new instance had been started.

The library uses stdlib to terminate the program with the exit() function.

Also don't forget to specify which QCoreApplication class your app is using if it is not QCoreApplication as in examples above.

Freestanding mode

Traditionally, the functionality of this library is implemented as part of the Qt application class. The base class is defined by the macro QAPPLICATION_CLASS.

In freestanding mode, SingleApplication is not derived from a Qt application class. Instead, an instance of a Qt application class is created as normal, followed by a separate instance of the SingleApplication class.

#include <QApplication>
#include <SingleApplication.h>
 
int main( int argc, char* argv[] )
{
    // The normal application class with a type of your choice
    QApplication app( argc, argv );
 
    // Separate single application object (argc and argv are discarded)
    SingleApplication single( argc, argv /*, options ...*/ );
 
    // Do your stuff
 
    return app.exec();
}

Note: With the discarded arguments and the class name that sounds like a Qt application class without being one, this looks like a workaround – it is a workaround. For 4.x, the single instance functionality could be moved to something like a SingleManager class, which would then be used to implement SingleApplication. This can't be done in 3.x, because moving SingleApplication::Mode to SingleManager::Mode would be a breaking change.

To enable the freestanding mode set QAPPLICATION_CLASS to FreeStandingSingleApplication. This is a fake base class with no additional functionality.

The standalone mode allows us to use a precompiled version of this library, because we don't need the QAPPLICATION_CLASS macro to define our Qt application class at build time. Furthermore, we can use std::optional<SingleApplication> to decide at runtime whether we want single application functionality or not.

Use the standard CMake workflow to create a precompiled static library version, including CMake config files.

cmake -DQAPPLICATION_CLASS=FreeStandingSingleApplication -DSINGLEAPPLICATION_INSTALL=ON SingleApplicationDir
cmake --build .
cmake --install

This can be used via:

find_package(SingleApplication REQUIRED)
target_link_libraries(YourTarget SingleApplication::SingleApplication)

Note: The QAPPLICATION_CLASS macro is eliminated during CMake install.

Instance started signal

The SingleApplication class implements a instanceStarted() signal. You can bind to that signal to raise your application's window when a new instance had been started, for example.

// window is a QWindow instance
QObject::connect(
    &app,
    &SingleApplication::instanceStarted,
    &window,
    &QWindow::raise
);

Using SingleApplication::instance() is a neat way to get the SingleApplication instance for binding to it's signals anywhere in your program.

Note: On Windows the ability to bring the application windows to the foreground is restricted. See Windows specific implementations for a workaround and an example implementation.

Secondary Instances

If you want to be able to launch additional Secondary Instances (not related to your Primary Instance) you have to enable that with the third parameter of the SingleApplication constructor. The default is false meaning no Secondary Instances. Here is an example of how you would start a Secondary Instance send a message with the command line arguments to the primary instance and then shut down.

int main(int argc, char *argv[])
{
    SingleApplication app( argc, argv, true );
 
    if( app.isSecondary() ) {
        app.sendMessage(  app.arguments().join(' ')).toUtf8() );
        app.exit( 0 );
    }
 
    return app.exec();
}

Note: A secondary instance won't cause the emission of the instanceStarted() signal by default. See SingleApplication::Mode for more details.*

You can check whether your instance is a primary or secondary with the following methods:

app.isPrimary();
// or
app.isSecondary();

Note: If your Primary Instance is terminated a newly launched instance will replace the Primary one even if the Secondary flag has been set.*

Examples

There are five examples provided in this repository:

• Basic example that prevents a secondary instance from starting examples/basic
• An example of a graphical application raising it's parent window examples/calculator
• A console application sending the primary instance it's command line parameters examples/sending_arguments
• A variant of sending_arguments where SingleApplicationis used in freestanding mode examples/separate_object
• A graphical application with Windows specific additions raising it's parent window examples/windows_raise_widget

Versioning

Each major version introduces either very significant changes or is not backwards compatible with the previous version. Minor versions only add additional features, bug fixes or performance improvements and are backwards compatible with the previous release. See CHANGELOG.md for more details.

Implementation

The library is implemented with a QSharedMemory block which is thread safe and guarantees a race condition will not occur. It also uses a QLocalSocket to notify the main process that a new instance had been spawned and thus invoke the instanceStarted() signal and for messaging the primary instance.

Additionally the library can recover from being forcefully killed on *nix systems and will reset the memory block given that there are no other instances running.

License

This library and it's supporting documentation, with the exception of the Qt calculator examples which is distributed under the BSD license, are released under the terms of The MIT License (MIT) with an extra condition, that:

Permission is not granted to use this software or any of the associated files
as sample data for the purposes of building machine learning models.