time-travel.js

'use strict';

module.exports = TimeTravel;

function TimeTravel(state) {
    var history = [state()];

    // Tracks the current position in history.
    var cursor = 0;

    var isRedoOrUndo = false;

    state(function recordState(newState) {

        // This function gets called whenever there is a state change.
        // State changes happen due to events being handled, or due to
        // undo/redo.

        // If we are replaying items in the history,
        // we don't want to re-add them to the end of the history.
        // Just quit.
        if (isRedoOrUndo) {
            return;
        }

        // If we've made it this far, `newState` is due to a new action,
        // not due to undo/redo.

        // If we've called `undo` a bunch of times,
        // the cursor won't be at the end.
        // Any states past the cursor should be cut off.
        history.splice(cursor + 1);

        // Add the new item to the history
        history.push(newState);

        cursor = history.length - 1;
    });

    return { undo: undo, redo: redo };

    function undo() {
        if (cursor < 1) {
            // Don't move before the beginning of time
            return undefined;
        }

        cursor--;
        isRedoOrUndo = true;
        state.set(history[cursor]);
        isRedoOrUndo = false;
        return history[cursor];
    }

    function redo() {
        if (cursor + 1 >= history.length) {
            // Don't move past the end of time
            return undefined;
        }

        cursor++;
        isRedoOrUndo = true;
        state.set(history[cursor]);
        isRedoOrUndo = false;
        return history[cursor];
    }
}