What you can reasonably do on iPhone
Review model code, change strings or constants, edit simple functions, compare implementation ideas, or make a quick source adjustment before moving the file back into your normal project workflow.
Why line numbers and indentation help
Swift structure can become hard to scan on a narrow screen. Line numbers make discussion and navigation easier, while automatic indentation reduces formatting cleanup.
Editing is different from building
A mobile file editor changes source text; it does not replace Xcode's compiler, simulator, signing or full debugging environment. Treat it as a portable editing tool rather than a full iOS development workstation.
Protect important source
Use version control or keep a backup when changing important files. Saving a copy is useful when you are experimenting away from your primary development environment.