return event valid event valid element chang event valid determin valid element method overriden subclass determin valid event just check chang element param event valid valid event properti chang event valid chang element collect todo consid delet event valid like go caus lot unecessari work problem valu old valu don test chang element delet valid element chang element valid tri remov valu collect col collect chang element iter col iter col valid object valid element valid old valu collect collect old valu valid valid 