SingleApplication

This is a replacement of the QtSingleApplication for Qt5.

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.

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.

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.

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 and include it's contents with a .pri file. Here is
how:

git submodule add git@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)

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

The 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.

API

Members

SingleApplication::SingleApplication( int &argc, char *argv[], bool allowSecondary = false, Options options = Mode::User, int timeout = 100 )

Depending on whether allowSecondary is set, this constructor may terminate
your app if there is already a primary instance running. Additional Options
can be specified to set whether the SingleApplication block should work
user-wide or system-wide. Additionally the Mode::SecondaryNotification may be
used to notify the primary instance whenever a secondary instance had been
started (disabled by default). timeout specifies the maximum time in
milliseconds to wait for blocking operations.

Note: argc and argv may be changed as Qt removes arguments that it
recognizes.

Note: Mode::SecondaryNotification only works if set on both the primary
and the secondary instance.

Note: Operating system can restrict the shared memory blocks to the same
user, in which case the User/System modes will have no effect and the block will
be user wide.

bool SingleApplication::sendMessage( QByteArray message, int timeout = 100 )

Sends message to the Primary Instance. Uses timeout as a the maximum timeout
in milliseconds for blocking functions

bool SingleApplication::isPrimary()

Returns if the instance is the primary instance.

bool SingleApplication::isSecondary()

Returns if the instance is a secondary instance.

quint32 SingleApplication::instanceId()

Returns a unique identifier for the current instance.

qint64 SingleApplication::primaryPid()

Returns the process ID (PID) of the primary instance.

Signals

void SingleApplication::instanceStarted()

Triggered whenever a new instance had been started, except for secondary
instances if the Mode::SecondaryNotification flag is not specified.

void SingleApplication::receivedMessage( quint32 instanceId, QByteArray message )

Triggered whenever there is a message received from a secondary instance.

Flags

enum SingleApplication::Mode

• Mode::User - The SingleApplication block should apply user wide. This adds
  user specific data to the key used for the shared memory and server name.
  This is the default functionality.
• Mode::System – The SingleApplication block applies system-wide.
• Mode::SecondaryNotification – Whether to trigger instanceStarted() even
  whenever secondary instances are started.
• Mode::ExcludeAppPath – Excludes the application path from the server name
  (and memory block) hash.
• Mode::ExcludeAppVersion – Excludes the application version from the server
  name (and memory block) hash.

Note: Mode::SecondaryNotification only works if set on both the primary
and the secondary instance.

Note: Operating system can restrict the shared memory blocks to the same
user, in which case the User/System modes will have no effect and the block will
be user wide.

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 are released under
The MIT License (MIT) with the exception of the Qt calculator examples which
is distributed under the BSD license.