[Arduino IDE demo source code for KC868-A6]--#07-PCF8574-Digital input code

[tochinet]

here is PCF8574 arduino library we are using.

That is the xreef one. BTW, you also need the I2C "Wire.h" library for your project to work.
Both libraries (RobTillart and Mischianti) are maintained, and they are both using C++ approach : a chip is an object from PCF8574 class.
So the same code is used for bothchips, with only the parameters of the chip stored separately.

However, note that one PCF is for digital inputs and the other for relays, so don't expect too much synergies ;-)