拖拽+碰撞+重力

window.onload=function ()
{
    var oDiv=document.getElementById('div1');
    
    var lastX=0;
    var lastY=0;
    
    oDiv.onmousedown=function (ev)
    {
        var oEvent=ev||event;
        
        var disX=oEvent.clientX-oDiv.offsetLeft;
        var disY=oEvent.clientY-oDiv.offsetTop;
        
        document.onmousemove=function (ev)
        {
            var oEvent=ev||event;
            
            var l=oEvent.clientX-disX;
            var t=oEvent.clientY-disY;
            
            oDiv.style.left=l+'px';
            oDiv.style.top=t+'px';
            
            iSpeedX=l-lastX;
            iSpeedY=t-lastY;
            
            lastX=l;
            lastY=t;
            
            document.title='x:'+iSpeedX+', y:'+iSpeedY;
        };
        
        document.onmouseup=function ()
        {
            document.onmousemove=null;
            document.onmouseup=null;
            
            startMove();
        };
        
        clearInterval(timer);
    };
};

var timer=null;

var iSpeedX=0;
var iSpeedY=0;

function startMove()
{
    clearInterval(timer);
    
    timer=setInterval(function (){
        var oDiv=document.getElementById('div1');
        
        iSpeedY+=3;
        
        var l=oDiv.offsetLeft+iSpeedX;
        var t=oDiv.offsetTop+iSpeedY;
        
        if(t>=document.documentElement.clientHeight-oDiv.offsetHeight)
        {
            iSpeedY*=-0.8;
            iSpeedX*=0.8;
            t=document.documentElement.clientHeight-oDiv.offsetHeight;
        }
        else if(t<=0)
        {
            iSpeedY*=-1;
            iSpeedX*=0.8;
            t=0;
        }
        
        if(l>=document.documentElement.clientWidth-oDiv.offsetWidth)
        {
            iSpeedX*=-0.8;
            l=document.documentElement.clientWidth-oDiv.offsetWidth;
        }
        else if(l<=0)
        {
            iSpeedX*=-0.8;
            l=0;
        }
        
        if(Math.abs(iSpeedX)<1)
        {
            iSpeedX=0;
        }
        
        if(Math.abs(iSpeedY)<1)
        {
            iSpeedY=0;
        }
        
        if(iSpeedX==0 && iSpeedY==0 && t==document.documentElement.clientHeight-oDiv.offsetHeight)
        {
            clearInterval(timer);
            alert('停止');
        }
        else
        {
            oDiv.style.left=l+'px';
            oDiv.style.top=t+'px';
        }
        
        document.title=iSpeedX;
    }, 30);
}