Consider improvements for interaction between UI code and file operation code

I think I am new to these sorts of issues. Maybe the code for the submit button, and even refresh, etc, should be decoupled. I think they're a bit tangled at the moment. I am considering it may be cognitively easier to represent the file operations with Dart's Sink/Stream objects. This way the various screens can listen for updates. For example, an indicator for, "writing to file", could be represented independently of page navigation, and considering where to add it in the UI code may become easier with the Sink/Stream model, with subscriptions, etc.

No files, tests, or entry points are named. Start by locating the submit and refresh UI code and the file-operation code, then assess how Dart Sink/Stream objects could separate them. Done would mean file operations can notify multiple screens independently, including a writing-to-file indicator without coupling it to page navigation.

Written by the indexing model from the issue text.