I need to refresh my screen when JsonStore has loaded all the records, is there an event for this event?
The load event will work:
store.on('load', function(store, recs, opt){ //update your display here }, this);
The "load" event is what you are looking for. See online documentation:
http://dev.sencha.com/deploy/ext-3.3.0/docs/?class=Ext.data.JsonStore