offsetLeft与offsetTop获取当前元素距离其父级参照物（position定位时的参照物）偏移值
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
.box {
width: 100px;
height: 100px;
position: absolute;
left: 0;
top: 0;
background-color: yellow;
cursor: move;
}
</style>
</head>
<body>
<div class="box" id="box"></div>
<!-- 只要鼠标在盒子滑动就触发move -->
<script>
let box = document.getElementById('box');
box.addEventListener("mousedown", down)
//
function down(ev) {
this.startX = ev.clientX;
this.startY = ev.clientY;
this.startL = this.offsetLeft;
this.startT = this.offsetTop;
this.addEventListener('mousemove', move)
this.addEventListener('mouseup', up)
}
function move(ev) {
let curL = ev.clientX - this.startX + this.startL;
let curT = ev.clientY - this.startY + this.startT;
this.style.cssText = `left:${curL}px;top:${curT}px`
}
function up(ev) {
this.removeEventListener('mousemove', move)
this.removeEventListener('mouseup', up)
}
// 鼠标移动太快，鼠标会脱离盒子
// 鼠标移动过快，鼠标会脱离拖拽的盒子，在盒子外面移动无法触发盒子的mousemove，
// 盒子不会跟着计算最新的位置
// 在盒子外面松开鼠标，也不会触发盒子的mouseup，导致mousemove无法移除，当鼠标
// 重新进入盒子不论是否按住，盒子都会跟着走
</script>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
.box {
width: 100px;
height: 100px;
position: absolute;
left: 0;
top: 0;
background-color: yellow;
cursor: move;
}
</style>
</head>
<body>
<div class="box" id="box"></div>
<!-- 只要鼠标在盒子滑动就触发move -->
<script>
// 解决
// * [IE和火狐浏览器中]
// * setCapture / releaseCapture：可以实现把元数和鼠标绑定在一起（或者移除绑定）
// 的效果，来防止鼠标焦点丢失
// [谷歌中的解决方案]
// 孙猴子（鼠标）蹦的在欢快，也逃离不了如来佛祖（document）的五指山，
// 所以我们在项目中，move和up方法绑定给document，而不是盒子
let box = document.getElementById('box');
box.addEventListener("mousedown", down)
//
function down(ev) {
this.startX = ev.clientX;
this.startY = ev.clientY;
this.startL = this.offsetLeft;
this.startT = this.offsetTop;
// this.addEventListener('mousemove', move)
// this.addEventListener('mouseup', up)
this._MOVE = move.bind(this);
this._UP = up.bind(this);
document.addEventListener('mousemove', this._MOVE)
document.addEventListener('mouseup', this._UP)
// 这样处理后move/up的this是document(但是我们需要保证this是盒子)，move.bind()返回的是代理函数
// 以后移除的也是代理函数才行 每bind一次都返回一个新的代理函数
// this.setCapture
}
function move(ev) {
let curL = ev.clientX - this.startX + this.startL;
let curT = ev.clientY - this.startY + this.startT;
// this.style.cssText = `left:${curL}px;top:${curT}px`
this.style.left = curL + 'px'
this.style.top = curT + 'px'
}
function up(ev) {
document.removeEventListener('mousemove', this._MOVE)
document.removeEventListener('mouseup', this._UP)
// this.releaseCapture
}
</script>
</body>
</html>
