Spent a little too much time trying to figure out why my new Swift file with my new class was resulting in an "Undeclared Identifier" error from the compiler after adding the class as a member variable. Doing some searching resulted in a lot of basic "check for typos" posts as well as "...if it's a framework, don't forget to add the import..." except that this is just a class, not a framework.
Turns out that, at least as of Xcode 10.1, adding a new .swift file does not automatically include it in the list of files for the compiler to build. After manually adding the file to the list of files to compile (Project/Targets/Compile Sources) it worked fine. Okay, great. Back to work!