js 自制滚动条

写了个js自制滚动条，首先，先看一下demo（点击这里） 先

有两个demo，右边那个黑色那个，是我一开始写的比较肤浅的代码：

var scrollself=(function(){

    var scrollblock, //滚动块
        scrollcontent,  //被滚动的内容
        scrollbar,    //滚动条
        scrollpanel,    //滚动内容的滚动区域
        cdistance,  //滚动内容要滚动的距离
        bdistance,    //滚动块要滚动的距离
        minuTop, //滚动条头尾剩下的空白
        cTop,    //滚动内容的top
        startY=0,    //滚动动作开始初鼠标的位置
        bTop=0,    //滚动动作开始初滚动块的top
        isDrag=false;  //是否拉动滚动块


    function prevent(e){

        if(e.preventDefault){
            e.preventDefault();
        }
        if(e.stopPropagation){
            e.stopPropagation();
        }
        e.cancelBubble=true;
        e.returnValue=false;
    }

    function mouseDown(event){
        isDrag=true;
        event=event||window.event;
        startY=event.clientY;
        bTop=scrollblock.offsetTop;
        cTop=scrollcontent.offsetTop;

        // prevent(event);

    }

    function mouseMove(event){
        if(isDrag){

            event=event||window.event;
            
            var newbTop=event.clientY-startY+bTop,
                newcTop=cTop-(event.clientY-startY)/bdistance*cdistance;

            if(newbTop<minuTop){
                newbTop=minuTop;
                newcTop=0;
            }else{
                if(newbTop>bdistance+minuTop){
                    newcTop=-cdistance;
                    newbTop=bdistance+minuTop;
                }
            }

            scrollblock.style.top=newbTop+'px';
            scrollcontent.style.top=newcTop+'px';
        }else{
            isDrag=false;
        }

        // prevent(event);
    }

    function mouseUp(event){

        isDrag=false;

        // prevent(event);
    }

    function addHandler(){
        scrollblock.onmousedown=mouseDown;
        scrollblock.onmousemove=mouseMove;
        scrollblock.onmouseup=mouseUp;
        document.onmouseup=mouseUp;
    }


    return{
        init:function(scrollpanel_id,scrollcontent_id,scrollbar_id,scrollblock_id){
            scrollblock=document.getElementById(scrollblock_id);
            scrollcontent=document.getElementById(scrollcontent_id);
            scrollbar=document.getElementById(scrollbar_id);
            scrollpanel=document.getElementById(scrollpanel_id);
            minuTop=scrollblock.offsetTop;
            cdistance=scrollcontent.offsetHeight-scrollpanel.offsetHeight;
            bdistance=scrollbar.offsetHeight-minuTop*2-scrollblock.offsetHeight;
        
            enclose(scrollpanel,scrollcontent,scrollbar,scrollblock,bdistance,cdistance,minuTop);
            addHandler();
        }
    }


}());

scrollself.init('scrollpanel2','scrollcontent2','scrollbar2','scrollblock2');

之所以说它肤浅，比较一下两个demo的滚动效果就知道了，右边的快速拉动滚动块时候，体验效果好差，很卡，而左边的就流畅多了。

因为很卡，我就又上网看了一下别人的代码，然后把根据别人的思路把代码改了一下，就有了左边那个绿色的那个demo，很明显，效果好了很多，代码：

var scroll=(function(){

    var scrollblock, //滚动块
        scrollcontent,  //被滚动的内容
        scrollbar,    //滚动条
        scrollpanel,    //滚动内容的滚动区域
        cdistance,  //滚动内容要滚动的距离
        bdistance,    //滚动块要滚动的距离
        minuTop, //滚动条头尾剩下的空白
        cTop,    //滚动内容的top
        startY=0,    //滚动动作开始初鼠标的位置
        bTop=0;    //滚动动作开始初滚动块的top


    
    function mouseDown(event){
        event=event||window.event;
        startY=event.clientY;
        bTop=scrollblock.offsetTop;
        cTop=scrollcontent.offsetTop;
        // if(scrollblock.setCapture){
            
        //     scrollblock.onmousemove=doDrag;
        //     scrollblock.onmouseup=stopDrag;
        //     scrollblock.setCapture();
        // }else{
        //     document.addEventListener("mousemove",doDrag,true);
        //     document.addEventListener("mouseup",stopDrag,true);
        // }
        document.onmousemove=function(){
            doDrag();
        }
        document.onmouseup=function(){
            stopDrag();
        }
        document.getElementsByTagName('body')[0].onselectstart=function(){
            return false;
        }
        
    }

    function doDrag(event){
        event=event||window.event;
            
        var newbTop=event.clientY-startY+bTop,
            newcTop=cTop-(event.clientY-startY)/bdistance*cdistance;

        if(newbTop<minuTop){
            newbTop=minuTop;
            newcTop=0;
        }else if(newbTop>bdistance+minuTop){
            newcTop=-cdistance;
            newbTop=bdistance+minuTop;
        }
        scrollblock.style.top=newbTop+'px';
        scrollcontent.style.top=newcTop+'px';

    }

    function stopDrag(event){
        // if(scrollblock.releaseCapture){
        //     // scrollblock.onmousemove=doDrag;
        //     // scrollblock.onmouseup=stopDrag;
        //     scrollblock.releaseCapture();
        // }else{
        //     document.removeEventListener("mousemove",doDrag,true);
        //     document.removeEventListener("mouseup",stopDrag,true);
        // }
        document.onmousemove=null;
        document.onmouseup=null;

        document.getElementsByTagName('body')[0].onselectstart=function(){
            return true;
        };
    }


    return{
        init:function(scrollpanel_id,scrollcontent_id,scrollbar_id,scrollblock_id){
            scrollblock=document.getElementById(scrollblock_id);
            scrollcontent=document.getElementById(scrollcontent_id);
            scrollbar=document.getElementById(scrollbar_id);
            scrollpanel=document.getElementById(scrollpanel_id);
            minuTop=scrollblock.offsetTop;
            cdistance=scrollcontent.offsetHeight-scrollpanel.offsetHeight;
            bdistance=scrollbar.offsetHeight-minuTop*2-scrollblock.offsetHeight;


            scrollblock.onmousedown=mouseDown;
            enclose(scrollpanel,scrollcontent,scrollbar,scrollblock,bdistance,cdistance,minuTop);
        }
    }



}());

scroll.init('scrollpanel','scrollcontent','scrollbar','scrollblock');

 

(一开始错误的误解：比较了一下两个的代码，其实修改的不多，就有一点很大的不同，流畅的那一个（左边绿色那一个）多了这个东西——setCapture、releaseCapture。

具体是怎样的，再研究一下先。

SetCapture()函数的说明为：“该函数在属于当前线程的指定窗口里设置鼠标捕获。一旦窗口捕获了鼠标，所有鼠标输入都针对该窗口，无论光标是否在窗口的边界内。同一时刻只能有一个窗口捕获鼠标。如果鼠标光标在另一个线程创建的窗口上，只有当鼠标键按下时系统才将鼠标输入指向指定的窗口。”

绿色demo不卡是因为SetCapture()函数使得鼠标onmousemove的时候，即使鼠标离开了滚动块，滚动块仍可以捕获得到onmousemove的消息，而黑色demo卡的原因就是因为鼠标离开了滚动块，就捕获不了了。)

囧啊，以上灰色部分真是不知所谓的误解，后来再认真想一下其实没那么复杂，只要在鼠标点击滚动块的时候把mousemove跟mouseup事件注册到document里就行了，然后mouseup的时候事件解绑掉，所以把代码给改了，其实没有setCapture、releaseCapture也行的，而且这两个东西是针对ie的。。