feat: Add function numberOfInstances()

This function returns the number of running instances, by add a new variable in InstancesInfo.

This feature is asked in itay-grudev#89.

README.md

@@ -206,6 +206,14 @@ qint64 SingleApplication::primaryPid()
 
 Returns the process ID (PID) of the primary instance.
 
+---
+
+```cpp
+quint32 numberOfInstances()
+```
+
+Returns the number of running instances.
+
 ### Signals
 
 ```cpp

singleapplication.cpp

@@ -187,3 +187,10 @@ bool SingleApplication::sendMessage( QByteArray message, int timeout )
     d->socket->flush();
     return dataWritten;
 }
+
+quint32 SingleApplication::numberOfInstances()
+{
+    Q_D(SingleApplication);
+    InstancesInfo* inst = static_cast<InstancesInfo*>( d->memory->data() );
+    return inst->numberOfInstances;
+}

singleapplication.h

@@ -121,6 +121,12 @@ class SingleApplication : public QAPPLICATION_CLASS
      */
     bool sendMessage( QByteArray message, int timeout = 100 );
 
+    /**
+     * @brief Returns the number of running instances
+     * @returns {quint32}
+     */
+    quint32 numberOfInstances();
+
 Q_SIGNALS:
     void instanceStarted();
     void receivedMessage( quint32 instanceId, QByteArray message );

singleapplication_p.cpp

@@ -71,6 +71,7 @@ SingleApplicationPrivate::~SingleApplicationPrivate()
 
     memory->lock();
     InstancesInfo* inst = static_cast<InstancesInfo*>(memory->data());
+    inst->numberOfInstances -= 1;
     if( server != nullptr ) {
         server->close();
         delete server;
@@ -141,6 +142,7 @@ void SingleApplicationPrivate::initializeMemoryBlock()
     inst->secondary = 0;
     inst->primaryPid = -1;
     inst->checksum = blockChecksum();
+    inst->numberOfInstances = 0;
 }
 
 void SingleApplicationPrivate::startPrimary()
@@ -174,12 +176,15 @@ void SingleApplicationPrivate::startPrimary()
     inst->primary = true;
     inst->primaryPid = q->applicationPid();
     inst->checksum = blockChecksum();
+    inst->numberOfInstances += 1;
 
     instanceNumber = 0;
 }
 
 void SingleApplicationPrivate::startSecondary()
 {
+    InstancesInfo* inst = static_cast<InstancesInfo*>( memory->data() );
+    inst->numberOfInstances += 1;
 }
 
 void SingleApplicationPrivate::connectToPrimary( int msecs, ConnectionType connectionType )

singleapplication_p.h

@@ -42,6 +42,7 @@ struct InstancesInfo {
     quint32 secondary;
     qint64 primaryPid;
     quint16 checksum;
+    quint32 numberOfInstances;
 };
 
 struct ConnectionInfo {