FallObj.js

gistfile1.js

function objtimer(input,callbackfn,thisArg){
 var current = input.start;
 var timerId = setInterval(function(){
  var step = input.step;
  var end = input.end;
  var ending;
  callbackfn.call(thisArg,{type:'init'});
  current += step;
  ending = (step > 0 && current >= end) || (step < 0 && current <= end);
  if(ending) current = end;
  if (false === callbackfn.call(thisArg,{type:'timer',relatedValue:current})
      || ending){
    callbackfn.call(thisArg,{type:'complete'});
    clearInterval(timerId);
    input = callbackfn = thisArg = current = timerId = null;
  }
 },input.interval);
}

function objtimer_S (flgment,interval,step,start,end,onInit,styleSetter,onComplete,obj){
 objtimer({interval:interval,step:step,start:start,end:end},
  function(e){
   switch(e.type){
    case "init":
     this.onInit(this.document,this.flgment,this.obj);
     return true;
    case "timer":
     this.styleSetter(this.flgment,e.relatedValue,this.obj);
     return true;
    case "complete":
     this.onComplete(this.flgment,this.onInit,this.styleSetter,this.onComplete,this.obj);
     return false;
   }
  },{
   obj:obj,
   document:document,flgment:flgment,
   styleSetter: styleSetter || new Function,
   onComplete: onComplete || new Function,
   onInit: onInit || new Function,
  });
}

   function falldownobj(target){
    this.target = target.cloneNode(true);
    this.flg = true;
   }

   falldownobj.prototype.start =function(){
    var C = document /*@cc_on @if (1) [document.compatMode == 'CSS1Compat' ?
           'documentElement' : 'body'] /*@else@*/ .defaultView /*@end@*/;
    var range = {"w":/*@if (1) C.clientWidth  @else@*/ C.innerWidth  /*@end@*/,
                 "h":/*@if (1) C.clientHeight @else@*/ C.innerHeight /*@end@*/};
    var x = Math.floor(Math.random()*range.w);
    var y = Math.floor(Math.random()*range.h);
    var step = Math.ceil(Math.random()*20);
    this.target.style.left = x + "px";
    objtimer_S(this.target,100,step,y,range.h,this.setup,this.move,this.loop,this);
   };

   falldownobj.prototype.setup =function(doc,elm,obj){
    doc.body.appendChild(elm);
   }

   falldownobj.prototype.move =function(elm,value,obj){
    if(obj.flg)
　   //elm.style.top = value +  "px";　//スクロール無視
　   //elm.style.top = value + window.scrollY + "px"; //IE,Operaだめ
     elm.style.top = value + document.documentElement.scrollTop + "px"; //Safariだめ
    else{
　   elm.style.display = "none"; 
    } 
   };

   falldownobj.prototype.loop =function(elm,onInit,styleSetter,onComplete,obj){
    var C = document /*@cc_on @if (1) [document.compatMode == 'CSS1Compat' ?
           'documentElement' : 'body'] /*@else@*/ .defaultView /*@end@*/;
    var range = {"w":/*@if (1) C.clientWidth  @else@*/ C.innerWidth  /*@end@*/,
                 "h":/*@if (1) C.clientHeight @else@*/ C.innerHeight /*@end@*/};
    var x = Math.floor(Math.random()*range.w);
    var y = Math.floor(Math.random()*range.h);
    elm.style.left = x + "px";
    var step = Math.ceil(Math.random()*20);
    if(obj.flg)
     objtimer_S(elm,100,step,y,range.h,onInit,styleSetter,onComplete,obj);
    else{
     document.body.removeChild(elm);
     obj.target = null;
    }
   };

   falldownobj.prototype.end = function(){
    this.flg = false;
   }

   function init(target){
    if (1 > arguments.length) return null;
    target.style.position = 'absolute';
    var obj = new falldownobj(target);
    obj.start();
    return obj;
   }

   function setfalldownobj(target,nummber){
    var obj=[];
    for(var i=0;i<nummber;i++){obj[i] = init(target);}
    return obj;
   }

   function stopfalldownobj(obj){
    for(var i=0;i<obj.length;i++) obj[i].end();   
   }

   function changefalldownobj(obj,target,number){
    stopfalldownobj(obj);
    obj = setfalldownobj(target,number);
    return obj;
   }

Demo
http://yoneyone.my-sv.net/FallObj.htm