缓动动画（透明度）

function animate(obj, json, fn) {
clearInterval(obj.timer);
obj.timer = setInterval(function () {
var flag = true;

for (var k in json) {
if (k == "opacity") {
var target = json[k] * 100;
var leader = Math.round(getStyle(obj, k) * 100) || 100;//因为透明度本身不带有单位 所以不用parseInt了，但是在做动画的时候 会除不尽 所以要取整
//console.log(leader)
} else {
var target = json[k];
var leader = parseInt(getStyle(obj, k)) || 0;//10px 10
}

var step = (target - leader) / 10;
step = step > 0 ? Math.ceil(step) : Math.floor(step);
leader = leader + step;
if (k == "opacity") {
obj.style[k] = leader / 100;
//IE8 filter:alpha(opacity=x);
obj.style.filter = "alpha(opacity=" + leader + ")"
console.log(obj.style.filter)
} else if (k == "zIndex") {
obj.style.zIndex = target;
}
else {
obj.style[k] = leader + "px";
}

if (leader != target) {
flag = false;
}
}
if (flag) {
clearInterval(obj.timer);
if (fn) {
fn();
}
}
}, 30)

}

function getStyle(obj, attr) {
if (obj.currentStyle) {
return obj.currentStyle[attr];
} else {
return window.getComputedStyle(obj, null)[attr];
}
}