-Added typedef dfunc_t for user display function

-Added void display_d(dfunc_t dfunc) which is a variant of the display() that calls a user function after each row
-changed private data to protected to more easily derive and modify.

Adafruit_SSD1306.cpp

@@ -56,6 +56,8 @@
 
 // SOME DEFINES AND STATIC VARIABLES USED INTERNALLY -----------------------
 
+typedef void (* dfunc_t)(void);//void function pointer
+
 #if defined(I2C_BUFFER_LENGTH)
 #define WIRE_MAX min(256, I2C_BUFFER_LENGTH) ///< Particle or similar Wire lib
 #elif defined(BUFFER_LENGTH)
@@ -113,6 +115,7 @@
 // so other I2C device types still work).  All of these are encapsulated
 // in the TRANSACTION_* macros.
 
+
 // Check first if Wire, then hardware SPI, then soft SPI:
 #define TRANSACTION_START                                                      \
   if (wire) {                                                                  \
@@ -970,6 +973,71 @@ void Adafruit_SSD1306::display(void) {
 #endif
 }
 
+/*!
+    @brief  display with added call to user function pointer
+    @param  dfunc pointer to users display function
+            
+    @return None (void).
+*/
+void Adafruit_SSD1306::display_d(dfunc_t dfunc){
+	  TRANSACTION_START
+  static const uint8_t PROGMEM dlist1[] = {
+      SSD1306_PAGEADDR,
+      0,                      // Page start address
+      0xFF,                   // Page end (not really, but works here)
+      SSD1306_COLUMNADDR, 0}; // Column start address
+  ssd1306_commandList(dlist1, sizeof(dlist1));
+  ssd1306_command1(WIDTH - 1); // Column end address
+
+#if defined(ESP8266)
+  // ESP8266 needs a periodic yield() call to avoid watchdog reset.
+  // With the limited size of SSD1306 displays, and the fast bitrate
+  // being used (1 MHz or more), I think one yield() immediately before
+  // a screen write and one immediately after should cover it.  But if
+  // not, if this becomes a problem, yields() might be added in the
+  // 32-byte transfer condition below.
+  yield();
+#endif
+  uint16_t count = WIDTH * ((HEIGHT + 7) / 8);
+  uint8_t *ptr = buffer;
+  if (wire) { // I2C
+    wire->beginTransmission(i2caddr);
+    WIRE_WRITE((uint8_t)0x40);
+    uint16_t bytesOut = 1;
+    while (count--) {
+      if (bytesOut >= WIRE_MAX) {
+        wire->endTransmission();
+        wire->beginTransmission(i2caddr);
+        WIRE_WRITE((uint8_t)0x40);
+        bytesOut = 1;
+      }
+      WIRE_WRITE(*ptr++);
+      bytesOut++;
+	  //every line give the dfunc a chance to run
+      if(dfunc !=NULL){
+        if(!(count % WIDTH)){
+          wire->endTransmission();
+          TRANSACTION_END
+            dfunc();
+          TRANSACTION_START
+          wire->beginTransmission(i2caddr);
+          WIRE_WRITE((uint8_t)0x40);
+          bytesOut = 1;
+        }
+      }
+    }
+    wire->endTransmission();
+  } else { // SPI
+    SSD1306_MODE_DATA
+    while (count--)
+      SPIwrite(*ptr++);
+  }
+  TRANSACTION_END
+#if defined(ESP8266)
+  yield();
+#endif
+}
+
 // SCROLLING FUNCTIONS -----------------------------------------------------
 
 /*!

Adafruit_SSD1306.h

@@ -40,6 +40,8 @@ typedef class HardwareSPI SPIClass;
 #include <SPI.h>
 #include <Wire.h>
 
+typedef void (* dfunc_t)(void);
+
 #if defined(__AVR__)
 typedef volatile uint8_t PortReg;
 typedef uint8_t PortMask;
@@ -145,6 +147,7 @@ public:
   bool begin(uint8_t switchvcc = SSD1306_SWITCHCAPVCC, uint8_t i2caddr = 0,
              bool reset = true, bool periphBegin = true);
   void display(void);
+  void display_d(dfunc_t dfunc);
   void clearDisplay(void);
   void invertDisplay(bool i);
   void dim(bool dim);
@@ -160,7 +163,7 @@ public:
   bool getPixel(int16_t x, int16_t y);
   uint8_t *getBuffer(void);
 
-private:
+protected:
   inline void SPIwrite(uint8_t d) __attribute__((always_inline));
   void drawFastHLineInternal(int16_t x, int16_t y, int16_t w, uint16_t color);
   void drawFastVLineInternal(int16_t x, int16_t y, int16_t h, uint16_t color);