static-cms/packages/netlify-cms-core/src/reducers/cursors.js

import { fromJS, Map } from 'immutable';
import { Cursor } from 'netlify-cms-lib-util';
import {
  ENTRIES_SUCCESS,
} from 'Actions/entries';

// Since pagination can be used for a variety of views (collections
// and searches are the most common examples), we namespace cursors by
// their type before storing them in the state.
export const selectCollectionEntriesCursor = (state, collectionName) =>
  new Cursor(state.getIn(["cursorsByType", "collectionEntries", collectionName]));

const cursors = (state = fromJS({ cursorsByType: { collectionEntries: {} } }), action) => {
  switch (action.type) {
    case ENTRIES_SUCCESS: {
      return state.setIn(
        ["cursorsByType", "collectionEntries", action.payload.collection],
        Cursor.create(action.payload.cursor).store
      );
    }

    default:
      return state;
  }
};

export default cursors;