制作焦点轮播图时,为什么图片前后还要各重复增加一个
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发布时间：2022-05-15 10:30
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时间：2022-05-15 14:52
在这篇文章打造通用的匀速运动框架(实例讲解)中，封装了一个匀速运动框架，我们在这个框架的基础之上，加上缓冲运动效果，然后用运动框架来做幻灯片（上下，左右）。
缓冲运动通常有两种常见的表现：比如让一个p从0运动到500，一种是事件触发的时候，速度很快， 一种是事件触发的时候慢，然后慢慢加快.我们来实现先块后慢的，常见的就是开车，比如刚从高速路上下来的车，就是120km/小时，然后进入匝道，变成40km/时. 或者40km/小时进入小区，最后停车，变成0km/小时. 从120km/小时->40km/小时， 或者40km->0km/小时，都是速度先块后慢，这种运动怎么用程序来表示呢？
可以用目标距离( 500 ) - 当前距离( 200 ) / 一个系数( 比如12 )，就能达到速度由块而慢的变化，当前距离在起点，分子(500 - 0 )最大，所以速度最大，如果当前距离快要接近500，分子最小，除完之后的速度也是最小。
<style>
p{
width: 200px;
height: 200px;
background:red;
position: absolute;
left: 0px;
}
</style>
<script>
window.onload = function(){
var oBtn = document.querySelector( "input" ),
oBox = document.querySelector( '#box' ),
speed = 0, timer = null;
oBtn.onclick = function(){
timer = setInterval( function(){
speed = ( 500 - oBox.offsetLeft ) / 8;
oBox.style.left = oBox.offsetLeft + speed + 'px';
}, 30 );
}
}
</script>
</head>
<body>
<input type="button" value="动起来">
<p id="box"></p>
</body>但是，p并不会乖乖地停止在500px这个目标位置，最终却是停在497.375px，只要查看当前的速度，当前的值就知道原因了
你会发现，速度永远都在0.375这里停着，获取到的当前的距离停在497px？ 这里有个问题，我们的p不是停在497.375px吗，怎么获取到的没有了后面的小数0.375呢？计算机在处理浮点数会有精度损失。我们可以单独做一个小测试:
<p id="box"></p> <script> var oBox = document.querySelector( '#box' ); alert( oBox.offsetLeft ); </script>
你会发现这段代码获取到左偏移是30px而不是行间样式中写的30.2px。因为在获取当前位置的时候，会舍去小数，所以速度永远停在0.375px, 位置也是永远停在497，所以，为了到达目标，我们就得把速度变成1，对速度向上取整( Math.ceil )，我们就能把速度变成1，p也能到达500
oBtn.onclick = function(){
timer = setInterval( function(){
speed = ( 500 - oBox.offsetLeft ) / 8;
if( speed > 0 ) {
speed = Math.ceil( speed );
}
console.log( speed, oBox.offsetLeft );
oBox.style.left = oBox.offsetLeft + speed + 'px';
}, 30 );
}第二个问题，如果p的位置是在900，也就是说从900运动到500，有没有这样的需求呢？ 肯定有啊，轮播图，从右到左就是这样的啊。
<style>
#box{
width: 200px;
height: 200px;
background:red;
position: absolute;
left: 900px;
}
</style>
<script>// <![CDATA[
window.onload = function(){
var oBtn = document.querySelector( "input" ),
oBox = document.querySelector( '#box' ),
speed = 0, timer = null;
oBtn.onclick = function(){
timer = setInterval( function(){
speed = ( 500 - oBox.offsetLeft ) / 8;
if( speed > 0 ) {
speed = Math.ceil( speed );
}
oBox.style.left = oBox.offsetLeft + speed + 'px';
}, 30 );
}
}
// ]]></script>
</head>
<body>
<input type="button" value="动起来">
<p id="box"></p>
</body>最后目标停在503.5px，速度这个时候是负值，最后速度停在-0.5，对于反方向的速度，我们就要把它变成-1，才能到达目标，所以用向下取整（Math.floor）
oBtn.onclick = function(){
timer = setInterval( function(){
speed = ( 500 - oBox.offsetLeft ) / 8;
if( speed > 0 ) {
speed = Math.ceil( speed );
}else {
speed = Math.floor( speed );
}
console.log( speed, oBox.offsetLeft );
oBox.style.left = oBox.offsetLeft + speed + 'px';
}, 30 );
}然后我们把这个缓冲运动整合到匀速运动框架，就变成：
function css(obj, attr, value) {
if (arguments.length == 3) {
obj.style[attr] = value;
} else {
if (obj.currentStyle) {
return obj.currentStyle[attr];
} else {
return getComputedStyle(obj, false)[attr];
}
}
}
function animate(obj, attr, fn) {
clearInterval(obj.timer);
var cur = 0;
var target = 0;
var speed = 0;
obj.timer = setInterval(function () {
var bFlag = true;
for (var key in attr) {
if (key == 'opacity ') {
cur = css(obj, 'opacity') * 100;
} else {
cur = parseInt(css(obj, key));
}
target = attr[key];
speed = ( target - cur ) / 8;
speed = speed > 0 ? Math.ceil(speed) : Math.floor(speed);
if (cur != target) {
bFlag = false;
if (key == 'opacity') {
obj.style.opacity = ( cur + speed ) / 100;
obj.style.filter = "alpha(opacity:" + ( cur + speed ) + ")";
} else {
obj.style[key] = cur + speed + "px";
}
}
}
if (bFlag) {
clearInterval(obj.timer);
fn && fn.call(obj);
}
}, 30 );
}有了这匀速运动框架，我们就来做幻灯片：
上下幻灯片的html样式文件：
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>slide - by ghostwu</title> <link rel="stylesheet" href="css/slide3.css" rel="external nofollow" > <script src="js/animate.js"></script> <script src="js/slide.js"></script> </head> <body> <p id="slide"> <p id="slide-img"> <p id="img-container"> <img src="./img/1.jpg" alt=""> <img src="./img/2.jpg" alt=""> <img src="./img/3.jpg" alt=""> <img src="./img/4.jpg" alt=""> <img src="./img/5.jpg" alt=""> </p> </p> <p id="slide-nums"> <ul> <li class="active"></li> <li></li> <li></li> <li></li> <li></li> </ul> </p> </p> </body> </html>
slide3.css文件：
* {
margin: 0;
padding: 0;
}
li {
list-style-type: none;
}
#slide {
width: 800px;
height: 450px;
position: relative;
margin:20px auto;
}
#slide-img {
position: relative;
width: 800px;
height: 450px;
overflow: hidden;
}
#img-container {
position: absolute;
left: 0px;
top: 0px;
height: 2250px;
/*font-size:0px;*/
}
#img-container img {
display: block;
float: left;
}
#slide-nums {
position: absolute;
right:10px;
bottom:10px;
}
#slide-nums li {
float: left;
margin:0px 10px;
background: white;
width: 20px;
height: 20px;
text-align: center;
line-height: 20px;
border-radius:10px;
text-indent:-999px;
opacity:0.6;
filter:alpha(opacity:60);
cursor:pointer;
}
#slide-nums li.active {
background: red;
}animate.js文件:
function css(obj, attr, value) {
if (arguments.length == 3) {
obj.style[attr] = value;
} else {
if (obj.currentStyle) {
return obj.currentStyle[attr];
} else {
return getComputedStyle(obj, false)[attr];
}
}
}
function animate(obj, attr, fn) {
clearInterval(obj.timer);
var cur = 0;
var target = 0;
var speed = 0;
obj.timer = setInterval(function () {
var bFlag = true;
for (var key in attr) {
if (key == 'opacity ') {
cur = css(obj, 'opacity') * 100;
} else {
cur = parseInt(css(obj, key));
}
target = attr[key];
speed = ( target - cur ) / 8;
speed = speed > 0 ? Math.ceil(speed) : Math.floor(speed);
if (cur != target) {
bFlag = false;
if (key == 'opacity') {
obj.style.opacity = ( cur + speed ) / 100;
obj.style.filter = "alpha(opacity:" + ( cur + speed ) + ")";
} else {
obj.style[key] = cur + speed + "px";
}
}
}
if (bFlag) {
clearInterval(obj.timer);
fn && fn.call(obj);
}
}, 30 );
}slide.js文件：
window.onload = function () {
function Slide() {
this.oImgContainer = document.getElementById("img-container");
this.aLi = document.getElementsByTagName("li");
this.index = 0;
}
Slide.prototype.bind = function () {
var that = this;
for (var i = 0; i < this.aLi.length; i++) {
this.aLi[i].index = i;
this.aLi[i].onmouseover = function () {
that.moveTop( this.index );
}
}
}
Slide.prototype.moveTop = function (i) {
this.index = i;
for( var j = 0; j < this.aLi.length; j++ ){
this.aLi[j].className = '';
}
this.aLi[this.index].className = 'active';
animate( this.oImgContainer, {
"top" : -this.index * 450,
"left" : 0
});
}
var oSlide = new Slide();
oSlide.bind();
}左右幻灯片只需要改下样式即可
样式文件：
* {
margin: 0;
padding: 0;
}
li {
list-style-type: none;
}
#slide {
width: 800px;
height: 450px;
position: relative;
margin:20px auto;
}
#slide-img {
position: relative;
width: 800px;
height: 450px;
overflow: hidden;
}
#img-container {
position: absolute;
left: 0px;
top: 0px;
width: 4000px;
}
#img-container img {
display: block;
float: left;
}
#slide-nums {
position: absolute;
right:10px;
bottom:10px;
}
#slide-nums li {
float: left;
margin:0px 10px;
background: white;
width: 20px;
height: 20px;
text-align: center;
line-height: 20px;
border-radius:10px;
text-indent:-999px;
opacity:0.6;
filter:alpha(opacity:60);
cursor:pointer;
}
#slide-nums li.active {
background: red;
}js调用文件：
window.onload = function () {
function Slide() {
this.oImgContainer = document.getElementById("img-container");
this.aLi = document.getElementsByTagName("li");
this.index = 0;
}
Slide.prototype.bind = function () {
var that = this;
for (var i = 0; i < this.aLi.length; i++) {
this.aLi[i].index = i;
this.aLi[i].onmouseover = function () {
that.moveLeft( this.index );
}
}
}
Slide.prototype.moveLeft = function (i) {
this.index = i;
for( var j = 0; j < this.aLi.length; j++ ){
this.aLi[j].className = '';
}
this.aLi[this.index].className = 'active';
animate( this.oImgContainer, {
"left" : -this.index * 800
});
}
var oSlide = new Slide();
oSlide.bind();
}
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时间：2022-05-15 12:00
