function once(dom, event, callback) {
 // 這一步是為了避免修改形參
 var temp = callback;
 dom.addEventListener(event, function() {
 if(temp)
 temp();
 temp = null;
 })
}

function once(dom, event, callback) {
 var handle = function() {
 callback();
 dom.removeEventListener(event, handle);
 }
 dom.addEventListener(event, handle)
}