1. 拖放(Drag 和 Drop) 是HTML5标准的组成部分
2. 拖动开始
   * ondragstart: 调用一个函数，drag（event）,它规定了被拖动的数据
3. 设置拖动数据:
   * setData(): 设置被拖动的数据类型和值
4. 放入位置:
   * ondragover: 事件规定在何处防止被拖动的数据
5. 放置:
   * ondrop: 当放置被拖动数据是，会发生drop事件
 
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>HTML5 拖放 API</title>
    <style>
        .box {
            width: 400px;
            height: 400px;
            border: 1px solid red;
            border-radius: 24px;
            padding: 32px;
            float: left;
        }
        #box2 {
            margin-left: 20px;
        }
        #pic{
            width: 200px;
            height: 150px;
            margin-top: 10px;
        }
    </style>
</head>
<body>
    <div id="box1" class="box"></div>
    <div id="box2" class="box"></div>
    <img id="pic" src="../source/img/12.jpg" alt="">
    <script>
        let box1 = document.getElementById('box1');
        let box2 = document.getElementById('box2');
        let pic = document.getElementById('pic');
        let body = document.body;

        box1.ondragover = defaultStop;
        box2.ondragover = defaultStop;

        pic.ondragstart = function(e) {
            // 设置拖放数据
            console.log('设置数据:', e.dataTransfer)
            e.dataTransfer.setData('img', 'pic');
        }
        // 放置数据
        box1.ondrop = dropData;

        box2.ondrop = dropData;

        function defaultStop (e) {
            e.preventDefault();
        }

        function dropData(e) {
            e.preventDefault();
            let img = document.getElementById(e.dataTransfer.getData('img'));
            e.target.appendChild(img);
        }
    </script>
</body>
</html>