How to make a trasparent Flutter appBar

appBar in Flutter has three main things to consider: 

backgroundColor

set backgroundColor transparent using Colors.transparent

elevation

set elevation to 0 (the elevation value is between 1 and 0 (where 0 means transparent)

example

backgroundColor: Colors.transparent,
elevation: 0,

optional:

shadowColor 

it can be setted transparent using Colors.transparent too

shadowColor: Colors.transparent,

=)