js 捕获鼠标事件

function drag(overFlow,title){
        title.onmousedown = function(evt){
            var doc = document;
            var evt = evt || window.event;
            var x = evt.offsetX?evt.offsetX:evt.layerX;
            var y = evt.offsetY?evt.offsetY:evt.layerY;
            if(overFlow.setCapture){
                overFlow.setCapture();
            }else if(window.captureEvents){
                window.captureEvents(Event.MOUSEMOVE|Event.MOUSEUP);
            }

            doc.onmousemove = function(evt){
                evt = evt || window.event;
                var xPosition = evt.pageX || evt.clientX;
                var yPosition = evt.pageY || evt.clientY;
                var newX = xPosition - x;
                var newY = yPosition - y;
                overFlow.style.left = newX;
                overFlow.style.top =  newY;
            };

            doc.onmouseup = function(){
                if(overFlow.releaseCapture){
                    overFlow.releaseCapture();
                }else if(window.captureEvents){
                    window.captureEvents(Event.MOUSEMOVE|Event.MOUSEUP);
                }
                doc.onmousemove=null;
                doc.onmouseup=null;
            };
        };
  }