js实现QQ面板拖拽效果（慕课网DOM事件探秘）（全）

QQ面板拖拽，效果如图

JavaScript代码如下：

function getByClass(clsName, parent) {
  var oParent = parent ? document.getElementById(parent) : document,
    eles = [],
    elements = oParent.getElementsByTagName('*');

  for (var i = 0, l = elements.length; i < l; i++) {
    if (elements[i].className == clsName) {
      eles.push(elements[i]);
    }
  }
  return eles;
}

window.onload = drag;

function drag() {
  var oTitle = getByClass("login_logo_webqq", "loginPanel")[0];
  //拖页
  oTitle.onmousedown = fnDown;
  //关闭页面
  var close = document.getElementById("ui_boxyClose");
  close.onclick = winClose;
  //切换状态
  var loginState = document.getElementById("loginState");
  var stateList = document.getElementById("loginStatePanel");
  var lis = stateList.getElementsByTagName("li");
  var stateTxt = document.getElementById("login2qq_state_txt");
  var loginStateShow = document.getElementById("loginStateShow");
  //点击显示下拉单
  loginState.onclick = function (e) {
    stateList.style.display = "block";
    //阻止事件冒泡；
    e = event || window.event;
    if(e.stopPropagation){
      e.stopPropagation();
    }else{
      e.cancelBubble = true;
    }
  };
  //鼠标滑过,背景变色
  for (var i = 0; i < lis.length; i++) {
    lis[i].onmouseover = function () {
      this.style.backgroundColor = "#888";
    };
    lis[i].onmouseout = function () {
      this.style.backgroundColor = "#fff";
    };
    //鼠标点击，txt改变，图标改变
    lis[i].onclick = function (e) {
      stateList.style.display = "none";
      //阻止事件冒泡
      e = event || window.event;
      if(typeof e.stopPropagation){
        e.stopPropagation();
      }else {
        e.cancelBubble = true;
      }
      var id = this.id;
      loginStateShow.className = "login-state-show "+id;
      var text = getByClass("stateSelect_text",id)[0].innerHTML;
      stateTxt.innerHTML = text;
    }

  }
  document.onclick = function () {
    stateList.style.display = "none";
  }
}

function winClose() {
  var box = document.getElementById("loginPanel");
  box.style.display = "none";
}

function fnDown(event) {
  var event = event || window.event;
  var oDrag = document.getElementById("loginPanel");
  //光标按下时光标和面板之间的距离；
  var disX = event.clientX - oDrag.offsetLeft;
  var disY = event.clientY - oDrag.offsetTop;
  //移动
  document.onmousemove = function (event) {
    event = event || window.event;
    fnMove(event, disX, disY);
  };
  document.onmouseup = function () {
    document.onmousemove = null;
    document.onmouseup = null;
  }
}

function fnMove(event, posX, posY) {
  var oDrag = document.getElementById("loginPanel");
  var l = event.clientX - posX;
  var t = event.clientY - posY;
  var winW = document.documentElement.clientWidth;
  var winH = document.documentElement.clientHeight;
  var maxW = winW - oDrag.offsetWidth - 10;
  var maxH = winH - oDrag.offsetHeight;
  //当l=0时，窗口不能继续外移
  if (l < 0) {
    l = 0;
  } else if (l > maxW) {
    l = maxW;
  }

  if (t < 10) {
    t = 10;
  } else if (t > maxH) {
    t = maxH;
  }

  oDrag.style.left = l + "px";
  oDrag.style.top = t + "px";


}

要点： 

1.阻止事件冒泡

loginState.onclick点击事件冒泡，导致下拉列表无法点开

loginState.onclick = function () {
  stateList.style.display = "block";

}

document.onclick = function () {
    stateList.style.display = "none";

}

lis[i].onclick列表项的点击事件冒泡，导致下拉列表无法隐藏 

lis[i].onclick = function () {
  stateList.style.display = "none";

}

loginState.onclick = function () {
  stateList.style.display = "block";

}

2.鼠标事件坐标获取

function fnDown(event) {
  var event = event || window.event;
  var oDrag = document.getElementById("loginPanel");
  //光标按下时光标和面板之间的距离；
  var disX = event.clientX - oDrag.offsetLeft;
  var disY = event.clientY - oDrag.offsetTop;
  //移动
  document.onmousemove = function (event) {
    event = event || window.event;
    fnMove(event, disX, disY);
  };
  document.onmouseup = function () {
    document.onmousemove = null;
    document.onmouseup = null;
  }
}
function fnMove(event, posX, posY) {
  var oDrag = document.getElementById("loginPanel");
  var l = event.clientX - posX;
  var t = event.clientY - posY;
  var winW = document.documentElement.clientWidth;
  var winH = document.documentElement.clientHeight;
  var maxW = winW - oDrag.offsetWidth - 10;
  var maxH = winH - oDrag.offsetHeight;
  //当l=0时，窗口不能继续外移
  if (l < 0) {
    l = 0;
  } else if (l > maxW) {
    l = maxW;
  }

  if (t < 10) {
    t = 10;
  } else if (t > maxH) {
    t = maxH;
  }

  oDrag.style.left = l + "px";
  oDrag.style.top = t + "px";
}

3.封装各浏览器通用的getElementsByClassName()方法

方法返回的是一个数组，切记

function getByClass(clsName, parent) {
 var oParent = parent ? document.getElementById(parent) : document,
  eles = [],
  elements = oParent.getElementsByTagName('*');

 for (var i = 0, l = elements.length; i < l; i++) {
  if (elements[i].className == clsName) {
   eles.push(elements[i]);
  }
 }
 return eles;
}