undo.ts

export class UndoAtom<T> {
   private _atom: Atom<T>
   _undo: Array<T>
   _redo: Array<T>

   constructor(atom: Atom<T>) {
    this._atom = atom
    this._undo = []
    this._redo = []

    this._atom.subscribe(x => this._undo.push(x))
   }

   static create(atom: Atom<any>): UndoAtom<any> {
     return new UndoAtom(atom)
   }

   undo() {
    const newState = this._undo.pop()
    const currentState = this._atom.get()
    if (newState) {
      this._atom.set(newState)
      this._redo.push(currentState)
    }
   }

   redo() {
      const newState = this._undo.pop()
      const currentState = this._atom.get()
      if (newState) {
        this._atom.set(newState)
        this._undo.push(currentState)
      }
   }

   get undoCount() {
    return this._undo.length
   }

  get redoCount() {
    return this._redo.length
   }

   get atom(): Atom<T> {
     return this._atom
   }
}