步骤
使用 JavaScript 实现拖拽的步骤：
- 让元素捕获事件（mousedown, mousemove & mouseup）
- 单击并不释放，触发 mousedown，标记开始拖拽，并获取元素和鼠标的位置
- 拖动鼠标，触发 mousemove，不断的获取鼠标的位置，并通过计算重新确定元素的位置
- 释放师表，触发 mouseup，结束拖拽，确定元素位置并更新
代码1：
demo1.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
#block {
width: 200px;
height: 200px;
background-color: yellow;
position: absolute;
}
</style>
</head>
<body>
<div id="block"></div>
<script>
/**
* 拖动元素
* @param elementId 元素id
*/
function drag(elementId) {
var element = document.getElementById(elementId);
var position = {
offsetX: 0, //点击处偏移元素的X
offsetY: 0, //偏移Y值
state: 0 //是否正处于拖拽状态，1表示正在拖拽，0表示释放
};
//获得兼容的event对象
function getEvent(event) {
return event || window.event;
}
//元素被鼠标拖住
element.addEventListener(‘mousedown‘, function (event) {
//获得偏移的位置以及更改状态
var e = getEvent(event);
position.offsetX = e.offsetX;
position.offsetY = e.offsetY;
position.state = 1;
}, false);
//元素移动过程中
document.addEventListener(‘mousemove‘, function (event) {
var e = getEvent(event);
if (position.state) {
position.endX = e.clientX;
position.endY = e.clientY;
//设置绝对位置在文档中，鼠标当前位置-开始拖拽时的偏移位置
element.style.position = ‘absolute‘;
element.style.top = position.endY - position.offsetY + ‘px‘;
element.style.left = position.endX - position.offsetX + ‘px‘;
}
}, false);
//释放拖拽状态
element.addEventListener(‘mouseup‘, function (event) {
position.state = 0;
}, false);
}
drag(‘block‘);
</script>
</body>
</html>
demo2.html
<!DOCTYPE html>
<html lang="en">
<body>
<div id="box" class="box" style="width: 100px;height: 100px; position: absolute;background: red;cursor: move;"></div>
<script>
(function () {
var dragging = false;
var boxX, boxY, mouseX, mouseY, offsetX, offsetY;
var box = document.getElementById(‘box‘);
box.onmousedown = down;
document.onmousemove = move;
document.onmouseup = up;
function down(e) {
dragging = true;
boxX = box.offsetLeft;
boxY = box.offsetTop;
mouseX = parseInt(getMouseXY(e).x);
mouseY = parseInt(getMouseXY(e).y);
offsetX = mouseX - boxX;
offsetY = mouseY - boxY;
}
function move(e) {
if (dragging) {
var x = getMouseXY(e).x - offsetX;
var y = getMouseXY(e).y - offsetY;
var width = document.documentElement.clientWidth - box.offsetWidth;
var height = document.documentElement.clientHeight - box.offsetHeight;
x = Math.min(Math.max(0, x), width);
y = Math.min(Math.max(0, y), height);
box.style.left = x + ‘px‘;
box.style.top = y + ‘px‘;
}
}
function up(e) {
dragging = false;
}
function getMouseXY(e) {
var x = 0, y = 0;
e = e || window.event;
if (e.pageX) {
x = e.pageX;
y = e.pageY;
} else {
x = e.clientX + document.body.scrollLeft - document.body.clientLeft;
y = e.clientY + document.body.scrollTop - document.body.clientTop;
}
return {
x: x,
y: y
};
}
})()
</script>
</body>
</html>
原文地址：https://www.cnblogs.com/taobr/p/9940421.html