79 Field subscriptions

onChange receives the previous and new field values after an assignment. close() stops later notifications, while the assignment itself still updates the field.

import std.observation.onChange

class Task { String title = "Draft" }

main() {
  Task task = Task()
  var subscription = task.title.onChange {
    printLine(oldValue + " -> " + newValue)
  }
  task.title = "Done"
  subscription.close()
  task.title = "Archived"
  printLine(task.title)
}

Expected output:

Draft -> Done
Archived

Try it: Assign Done twice before closing and observe equal-value filtering.

Precise rules: Reference.