SingleApplication/README.md

SingleApplication
=================

This is a replacement of the QSingleApplication 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` class:

```cpp
#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:

```bash
git submodule add git@github.com:itay-grudev/SingleApplication.git singleapplication
```

Then include the `singleapplication.pri` file in your `.pro` project file. Also
don't forget to specify which `QCoreApplication` class your app is using if it
is not `QCoreApplication`.

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

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.

```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.

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.

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.

API
---

### Members

```cpp
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.*

```cpp
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

```cpp
bool SingleApplication::isPrimary()
```

Returns if the instance is the primary instance.

```cpp
bool SingleApplication::isSecondary()
```
Returns if the instance is a secondary instance.

```cpp
quint32 SingleApplication::instanceId()
```

Returns a unique identifier for the current instance

### Signals

```cpp
void SingleApplication::instanceStarted()
```

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

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

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

### Flags

```cpp
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.
*   `SecondaryNotification` – Whether to trigger `instanceStarted()` even
    whenever secondary instances are started.

*__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
----------

The current library versions is `3.0a`.

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.

To handle an issue on `*nix` systems, where the operating system owns the shared
memory block and if the program crashes the memory remains untouched, the
library binds to the following signals and closes the program with
`error code = 128 + signum` where signum is the number representation of the
signal listed below. Handling the signal is required in order to safely delete
the `QSharedMemory` block. Each of these signals are potentially lethal and will
results in process termination.

*   `SIGHUP` - `1`, Hangup.
*   `SIGINT` - `2`, Terminal interrupt signal
*   `SIGQUIT` - `3`, Terminal quit signal.
*   `SIGILL` - `4`, Illegal instruction.
*   `SIGABRT` - `6`, Process abort signal.
*   `SIGBUS` - `7`, Access to an undefined portion of a memory object.
*   `SIGFPE` - `8`, Erroneous arithmetic operation (such as division by zero).
*   `SIGSEGV` - `11`, Invalid memory reference.
*   `SIGSYS` - `12`, Bad system call.
*   `SIGPIPE` - `13`, Write on a pipe with no one to read it.
*   `SIGALRM` - `14`, Alarm clock.
*   `SIGTERM` - `15`, Termination signal.
*   `SIGXCPU` - `24`, CPU time limit exceeded.
*   `SIGXFSZ` - `25`, File size limit exceeded.

Additionally the library can recover from being killed with uncatchable signals
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 some of the examples distributed
under the BSD license.