How to get the result of a future within the builder method of a FutureBuilder widget?

You can access a FutureBuilder's future value with snapshot.data.

Don't forget to check if snapshot.data is defined using snapshot.hasData :

FutureBuilder<bool>(
  future: isAuth,
  builder: (context, snapshot) {
    if(connectionState == connectionState.done){
      if (snapshot.hasData){
        if (snapshot.data) return TrueWidget();
        else return FalseWidget();
        // Or with a one-liner :
        // return snapshot.data ? TrueWidget() : FalseWidget();
      }
    }

    if(connectionState == ConnectionState.waiting){
      return CircularProgressIndicator(); // Don't forget to return a widget in a builder method
    }
  }
)