Lesson 27 · The status line - Build a Text Editor

The status line

Editors show a status line - the file name, a modified marker, and where you are. Today you render one, a single width-filling line that reports the editor's state at a glance.

The goal

Render a status line showing the file name, a modified marker when dirty, and the cursor's line position, padded to the given width.

Start here - the target

TO DO

Scenario: Rendering the status line

Givenan editor over "a\nb\nc" with the cursor at Row 1, the file name "notes.txt", width 30

Whenthe status line is rendered

Thenwhen not dirty it is "notes.txt" followed by spaces and then "2/3" so the whole line is exactly 30 characters ("notes.txt", 18 spaces, "2/3")

Andwhen dirty the left side is "notes.txt [modified]" (then 7 spaces and "2/3"), still exactly 30 characters

Background

The status line is the strip of information every editor keeps at the bottom of the screen: what file you are in, whether it has unsaved changes, and where the cursor sits. You build it as a single string of exactly the screen width, with the left side - the file name, plus a [modified] marker when the dirty flag is set

and the right side - the current line number over the total, shown one-based because users count lines from 1, not 0.

The layout trick is the padding: fill the gap between the left and right sides with enough spaces to push the position flush against the right edge, so the whole line lands on the exact width. That is why the dirty and clean versions have different amounts of space but the same total length - the [modified] text eats into the padding, not the width. This reuses the dirty flag directly and gives the viewport its finishing touch: a frame of text, a placed cursor, and a status line that tells you where you are.

Make it work

func (e *Editor) StatusLine(name string, cols int) string {
  left := name
  if e.Dirty { left += " [modified]" }
  right := fmt.Sprintf("%d/%d", e.Row+1, e.Buf.LineCount())
  pad := cols - len(left) - len(right) // spaces between the two sides
  return left + strings.Repeat(" ", pad) + right
}

CheckpointDONE

The editor renders an informative status line. Commit and stop here.