Extending Ext.plugin.PullRefresh

gistfile1.js

/**
 * You were close except since you had override, it was going to try and override all Ext.plugin.PullRefresh.
 *
 * Here is an example at extending it, you are making a new class. So instead of creating Ext.plugin.PullRefresh, you would use mobuy.plugin.PullRefresh.
 *
 * You would put this in the app/plugin/PullRefresh.js file and then you can require this class and it will be loaded.
 */

Ext.define('mobuy.plugin.PullRefresh', {
    extend : 'Ext.plugin.PullRefresh',
    alias  : 'plugin.mobuy-pullrefresh',

    onLatestFetched: function(operation) {
        var store      = this.getList().getStore(),
            oldRecords = store.getData(),
            newRecords = operation.getRecords(),
            length     = newRecords.length,
            toInsert   = [],
            newRecord, oldRecord, i;

        for (i = 0; i < length; i++) {
            newRecord = newRecords[i];
            oldRecord = oldRecords.getByKey(newRecord.getId());

            if (oldRecord) {
                oldRecord.set(newRecord.getData());
            } else {
                toInsert.push(newRecord);
            }

            oldRecord = undefined;
        }

        //line edited, add instead of insert
        store.add(toInsert);
    }
});