Lesson 26 · Saving and the dirty flag

Saving and the dirty flag

Edits are worthless if you cannot write them back - and the editor has to know when there are unsaved changes. Today you save the buffer's text and track a dirty flag that every edit sets and every save clears.

The goal

Write the buffer's text to a writer, and track a dirty flag set by edits and cleared by save and load.

Start here - the target

TO DO

Scenario: Saving text and tracking unsaved changes

Givenan editor over "abc" with the cursor at Row 0, Col 1

Whenedits, a save, and a load happen in sequence

Thena new editor is not dirty; InsertChar("X") makes Text() "aXbc" and sets dirty true; Save to a writer writes exactly "aXbc" and clears dirty to false

Anda further edit sets dirty true again, and Load from a reader of "z" gives Text() "z" and clears dirty to false

Background

Saving is loading run backwards: flatten the piece table into one string with Text() and hand those bytes to a writer. The piece table did its clever work in memory - splitting and trimming pieces so no byte was ever copied while editing - but a file on disk is a flat sequence, so at save time the pieces are walked once and concatenated. Writing to a generic writer mirrors the generic reader from loading, so the whole load-edit-save cycle is exercisable without touching the file system.

Riding along with save is the dirty flag, the single boolean that answers “are there changes on screen that are not yet on disk?” Every editing operation sets it, because each one moves the buffer away from what was last saved; save and load clear it, because afterward the buffer matches a file. It is one bit, but it drives the [modified] marker on the status line next lesson and the “you have unsaved changes” prompt before quitting later. The honest work here is being thorough - every edit method must set the flag, or the editor will cheerfully claim a changed file is saved.

Make it work

func (e *Editor) Save(w io.Writer) error {
  _, err := io.WriteString(w, e.Buf.Text())
  e.Dirty = false                 // disk now matches the buffer
  return err
}
// set e.Dirty = true at the end of every edit (InsertChar, Enter,
// Backspace, DeleteForward); set it false in Load too.

CheckpointDONE

The editor saves its text and knows when it has unsaved changes. Commit and stop here.