Add user context to library

The current overwrite method of weak pointer is a global overwrite. As class
is not polymorphic type (aka the overwrite enabled methods are weak instead of
virtual) and there is no vtable as none of the methods are virtual, there is
no way to have instance specific overwrite.

A user context will provide a method to call back into a context of a parent
class and resolve the issue without making the class polymorphic.

For example:

    class MY_SFE_UBLOX_GNSS: public SFE_UBLOX_GNSS {
    public:
        MY_SFE_UBLOX_GNSS() {
            setUserContext(this);
        }
        void myProcessRTCM(uint8_t incoming) {}
    };

    void SFE_UBLOX_GNSS::processRTCM(uint8_t incoming) {
        static_cast<
            MY_SFE_UBLOX_GNSS *
        >(getUserContext())->myProcessRTCM(incoming);
    }

Signed-off-by: Alon Bar-Lev <[email protected]>

Author: alonbl

Diff for: src/SparkFun_u-blox_GNSS_Arduino_Library.cpp

@@ -633,6 +633,18 @@ size_t SFE_UBLOX_GNSS::getPacketCfgSpaceRemaining()
   return (packetCfgPayloadSize - packetCfg.len);
 }
 
+// Sets user context
+void SFE_UBLOX_GNSS::setUserContext(void *userContext)
+{
+  _userContext = userContext;
+}
+
+// Retrive user context
+void *SFE_UBLOX_GNSS::getUserContext(void)
+{
+  return _userContext;
+}
+
 // Initialize the I2C port
 bool SFE_UBLOX_GNSS::begin(TwoWire &wirePort, uint8_t deviceAddress, uint16_t maxWait, bool assumeSuccess)
 {

Diff for: src/SparkFun_u-blox_GNSS_Arduino_Library.h

@@ -682,6 +682,10 @@ class SFE_UBLOX_GNSS
   bool setPacketCfgPayloadSize(size_t payloadSize); // Set packetCfgPayloadSize
   size_t getPacketCfgSpaceRemaining();              // Returns the number of free bytes remaining in packetCfgPayload
 
+  // User context
+  void setUserContext(void *userContext);          // Sets user context
+  void *getUserContext(void);                      // Retrive user context
+
   // Begin communication with the GNSS. Advanced users can assume success if required. Useful if the port is already outputting messages at high navigation rate.
   // Begin will then return true if "signs of life" have been seen: reception of _any_ valid UBX packet or _any_ valid NMEA header.
   // By default use the default I2C address, and use Wire port
@@ -1708,6 +1712,7 @@ class SFE_UBLOX_GNSS
   SPIClass *_spiPort; // The instance of SPIClass
   uint8_t _csPin;     // The chip select pin
   uint32_t _spiSpeed; // The speed to use for SPI (Hz)
+  void *_userContext; // Custom user context as callback context
 
   uint8_t _gpsI2Caddress = 0x42; // Default 7-bit unshifted address of the ublox 6/7/8/M8/F9 series
   // This can be changed using the ublox configuration software