Lesson 39 · An undo and redo session

An undo and redo session

Undo and redo only earn trust when they compose across a real sequence of edits. Today you drive a mixed session and step backward and forward through it.

The goal

Run a sequence of edits and confirm undo and redo walk through the history correctly.

Start here - the target

TO DO

Scenario: Walking a history back and forth

Givenan editor over an empty buffer

Whenthe characters of "abc" are typed, then Enter, then the characters of "xyz", giving "abc\nxyz"

Thenone Undo gives Text() "abc\n" (cursor Row 1, Col 0), a second Undo gives Text() "abc" (cursor Row 0, Col 3)

Anda Redo then gives Text() "abc\n" again with the cursor at Row 1, Col 0

Background

This session ties the whole chapter together. Typing "abc" is one coalesced group; Enter is its own; typing "xyz" is a third. So the history has exactly three steps, and undo walks them in reverse: the first undo peels off the "xyz" run and puts the cursor back at the start of the new line, the second undoes the Enter and drops you to the end of "abc". Each step restores the cursor along with the text, so you are returned to precisely where each edit began.

Redo then retraces the path forward, re-applying the Enter and landing you back at "abc\n" with the cursor where it belonged. The coalescing, the two stacks, and the cursor-carrying snapshots all show their work at once here, exactly as they will when a person hammers undo and redo in a real edit. With the model proven end to end through its history, only one thing is left: to run the entire editor from a single scripted session - open, edit, search, save - and check it lands precisely where the design says it should.

Make it work

for _, c := range "abc" { e.InsertChar(c) } // undo group 1
e.Enter()                                    // undo group 2
for _, c := range "xyz" { e.InsertChar(c) } // undo group 3
e.Undo(); e.Undo(); e.Redo()
// assert Text() and cursor at each step

CheckpointDONE

Undo and redo compose across a real editing session. Commit and stop here.