added support for ActiveSensing

From: https://www.midi.org/specifications/item/table-1-summary-of-midi-message

Active Sensing. This message is intended to be sent repeatedly to tell the receiver that a connection is alive. Use of this message is optional. When initially received, the receiver will expect to receive another Active Sensing message each 300ms (max), and if it does not then it will assume that the connection has been terminated. At termination, the receiver will turn off all voices and return to normal (non- active sensing) operation.

.gitignore

@@ -5,3 +5,4 @@ build/
 .vscode/.cmaketools.json
 src/.DS_Store
 examples/.DS_Store
+.DS_Store

src/MIDI.h

@@ -86,6 +86,8 @@ public:
                                DataByte inPressure,
                                Channel inChannel);
 
+    inline void sendActiveSensing();
+    
     inline void sendSysEx(unsigned inLength,
                           const byte* inArray,
                           bool inArrayContainsBoundaries = false);
@@ -241,7 +243,8 @@ private:
     bool            mThruActivated  : 1;
     Thru::Mode      mThruFilterMode : 7;
     MidiMessage     mMessage;
-
+    unsigned long   mLastSendMessageTime;
+    bool            mActiveSensingActivated;
 
 private:
     inline StatusByte getStatus(MidiType inType,

src/MIDI.hpp

@@ -40,6 +40,8 @@ inline MidiInterface<SerialPort, Settings>::MidiInterface(SerialPort& inSerial)
     , mPendingMessageIndex(0)
     , mCurrentRpnNumber(0xffff)
     , mCurrentNrpnNumber(0xffff)
+    , mActiveSensingActivated(false)
+    , mLastSendMessageTime(0)
     , mThruActivated(true)
     , mThruFilterMode(Thru::Full)
 {
@@ -100,6 +102,9 @@ void MidiInterface<SerialPort, Settings>::begin(Channel inChannel)
     mCurrentRpnNumber  = 0xffff;
     mCurrentNrpnNumber = 0xffff;
 
+    mActiveSensingActivated = false;
+    mLastSendMessageTime = millis();
+
     mMessage.valid   = false;
     mMessage.type    = InvalidType;
     mMessage.channel = 0;
@@ -177,6 +182,9 @@ void MidiInterface<SerialPort, Settings>::send(MidiType inType,
     {
         sendRealTime(inType); // System Real-time and 1 byte.
     }
+
+    if (mActiveSensingActivated)
+        mLastSendMessageTime = millis();
 }
 
 // -----------------------------------------------------------------------------
@@ -312,6 +320,22 @@ void MidiInterface<SerialPort, Settings>::sendPitchBend(double inPitchValue,
     sendPitchBend(value, inChannel);
 }
 
+/*! \brief Send an Active Sensing message.
+ 
+  This message is intended to be sent
+  repeatedly to tell the receiver that a connection is alive. Use
+  of this message is optional. When initially received, the
+  receiver will expect to receive another Active Sensing
+  message each 300ms (max), and if it does not then it will
+  assume that the connection has been terminated.
+ */
+template<class Encoder, class Settings>
+void MidiInterface<Encoder, Settings>::sendActiveSensing()
+{
+    sendRealTime(ActiveSensing);
+}
+
+
 /*! \brief Generate and send a System Exclusive frame.
  \param inLength  The size of the array to send
  \param inArray   The byte array containing the data to send
@@ -650,6 +674,20 @@ inline bool MidiInterface<SerialPort, Settings>::read()
 template<class SerialPort, class Settings>
 inline bool MidiInterface<SerialPort, Settings>::read(Channel inChannel)
 {
+    // Active Sensing. This message is intended to be sent
+    // repeatedly to tell the receiver that a connection is alive. Use
+    // of this message is optional. When initially received, the
+    // receiver will expect to receive another Active Sensing
+    // message each 300ms (max), and if it does not then it will
+    // assume that the connection has been terminated. At
+    // termination, the receiver will turn off all voices and return to
+    // normal (non- active sensing) operation.
+    if (mActiveSensingActivated && (millis() - mLastSendMessageTime) > 250)
+    {
+      sendActiveSensing();
+      mLastSendMessageTime = millis();
+    }
+
     if (inChannel >= MIDI_CHANNEL_OFF)
         return false; // MIDI Input disabled.