This library was rewritten and not committed as a fork of xreef's due to quite significant code changes, the most significant is due to how the chip handles inputs and outputs and how code should reflect that. This is explained best by a snippet of comments from the code here:

NXP's documentation of this chip (https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf p8) states that "To enter the Read mode the master (microcontroller) addresses the slave device and sets the last bit of the address byte to logic 1 (address byte read)."

We should then ensure that whenever we wish to read from the chip, we must use the address "_addr | 0x80" to ensure we set MSB. If we don't, the chip will assume we are setting all the inputs to outputs.

The chip always starts with all pins in input - i.e., the register is all 1's. NXP's documentation (https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf) states that "Ensure a logic 1 is written for any port that is being used as an input to ensure the strong external pull-down is turned off." Therefore, start with _inputmask as being all 1's, and as we set pins to output, we set the specified bit in _inputmask to 0, and push _inputmask to the chip.

Reef's original library is here and deserves full credit for setting the foundation for this library. https://github.com/xreef/PCF8574_library. This version is still under slow development; please refer to xreef's library page for documentation.