primefaces：如何设置动态oncomplete事件值，如oncomplete =“＃backingbean.oncomplete”

I would like to show different dialog from a single button depends on backingbean calculation. Like show a msg "Bill no has been paid already", if client enter a duplicate bill no, if the bill no is ok than shows like "Bill has been paid successfully". How can I achieve this?I would like to show different dialog from a si