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https://github.com/itay-grudev/singleapplication

Replacement of QtSingleApplication for Qt 5 and Qt 6 with support for inter-instance communication.
https://github.com/itay-grudev/singleapplication

application c-plus-plus cpp hacktoberfest qt qt5 qt6 qtsingleapplication singleapplication singleton subsequent-instances

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Replacement of QtSingleApplication for Qt 5 and Qt 6 with support for inter-instance communication.

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README 

        
# SingleApplication



[![CI](https://github.com/itay-grudev/SingleApplication/workflows/CI:%20Build%20Test/badge.svg)](https://github.com/itay-grudev/SingleApplication/actions)



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.



# [Documentation](https://itay-grudev.github.io/SingleApplication/)



You can find the full usage reference and examples [here](https://itay-grudev.github.io/SingleApplication/classSingleApplication.html).



## 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:



```cpp

#include 

#include 



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:



```bash

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

```



**Qmake:**



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



```qmake

include(singleapplication/singleapplication.pri)

DEFINES += QAPPLICATION_CLASS=QApplication

```



**CMake:**



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



```cmake

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`.

```cmake



# 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.



## 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.



```cpp

// 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](Windows.md)

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.



```cpp

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:



```cpp

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 three examples provided in this repository:



* Basic example that prevents a secondary instance from starting [`examples/basic`](https://github.com/itay-grudev/SingleApplication/tree/master/examples/basic)

* An example of a graphical application raising it's parent window [`examples/calculator`](https://github.com/itay-grudev/SingleApplication/tree/master/examples/calculator)

* A console application sending the primary instance it's command line parameters [`examples/sending_arguments`](https://github.com/itay-grudev/SingleApplication/tree/master/examples/sending_arguments)



## 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](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:



```txt

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.

```