This class library works with two plugins: firebase_auth and google_sign_in to log into a Firebase backend using a Firebase account or a Google account. If you're familiar with these plugins, you will find the same functions used in this class library.
How it Works
Below are a series of screenshots depicting how to initialize and authenticate or 'sign in' an individual into your app's Firebase database using a either an email and password or a Google account. The following will sign in 'silently' (i.e. automatically if the user had already signed in in the past.). Note, settings are passed as parameters in the screenshot below. These examples have the class library called in the State object's iniState() function, but, of course, you could instead 'initialize' the class library in the initState() function and then 'sign in' elsewhere. Below, the init() function is used instead just to initialize the class library.
This is a class library is covered again in the Medium article, Auth in Flutter.