The set of functions allow to use non-blocking/async ADC with version of Arduino core for ESP32 upper than 1.0.6.
ALL CODE WAS "STOLEN" FROM ORIGINAL REPOSITORY https://github.com/espressif/arduino-esp32
Otherwise, fill free to use this code.
Functions adcStart/adcBusy/adcEnd was removed after 1.0.4 RELEASE of "Arduino core for the esp32". But these functions are very useful for ASYNC/NO-WAIT ADC reading. I just copied code from Realease 1.0.4 of ESP32 Arduino to use it in my sketch.
Just put both files to folder of your Arduino sketch and put #include "esp32-adc-nowait.h" in begin of your .ino file.