That and the notion that global variables or state are not a thing in modern software explains most failed UI projects built by juniors initializing things as global variables from controllers that also contain logic, layout, and other stuff that does not belong there (aka. an untestable & disgraceful mess).
The same pattern applies to MOVE. Events are data and operations should do nothing else than hand off to a service layer; if it's more than 1 or 2 lines of code, it probably is mixing concerns and should therefore be refactored. Otherwise you just moved the problem (bad pun, sorry) and made it even harder to test because now your business logic is a side effect of an event that is processed asynchronously.
Service layers are easy to test because it's just easily mockable constructor/initializer injected dependencies + a bunch of stateless functions that take input and produce output (and side effects). Classic mistakes here are to make the dependencies global variables and/or making the service layer initialization actually initialize dependencies.
The same pattern applies to MOVE. Events are data and operations should do nothing else than hand off to a service layer; if it's more than 1 or 2 lines of code, it probably is mixing concerns and should therefore be refactored. Otherwise you just moved the problem (bad pun, sorry) and made it even harder to test because now your business logic is a side effect of an event that is processed asynchronously.
Service layers are easy to test because it's just easily mockable constructor/initializer injected dependencies + a bunch of stateless functions that take input and produce output (and side effects). Classic mistakes here are to make the dependencies global variables and/or making the service layer initialization actually initialize dependencies.