拖动 Drag

1.在对Drag传递数据的时候，只能在dragstart中存放数据，在drop事件中获取数据，同时必须得有dragover事件并且必须在dragover事件中添加

event.preventDefault();

box2.addEventListener('dragover',function(){

event.preventDefault();

});

2.对拖动的效果设置的时候

　　将dataTransfer.effectAllowed = "move"放置到dragstart中表示允许拖动哪种dropEffect;将ev.dataTransfer.dropEffect = "move"放置到dragover中(但是书上写的是放置到dragenter中，但是使用之后没有效果，不知是不是写的不对，才没效果)，这只是将拖动的元素设计了一个拖动的效果，最后在drop中将拖动的元素添加到被放置目标中。

<!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">
    <style>
        div {
  margin: 0em;
  padding: 2em;
}

#source {
  color: blue;
  border: 1px solid black;
}

#target {
  border: 1px solid black;
}

    </style>


    <title>Document</title>
</head>
<body>
    <div>
        <p id="source" ondragstart="dragstart_handler(event);" draggable="true">
          Select this element, drag it to the Drop Zone and then release the selection to move the element.
        </p>
      </div>
      <div id="target" ondrop="drop_handler(event);" ondragover="dragover_handler(event);">Drop Zone</div>
      <script>
        function dragstart_handler(ev) {
            console.log("dragStart: dropEffect = " + ev.dataTransfer.dropEffect + " ; effectAllowed = " + ev.dataTransfer.effectAllowed);
            ev.dataTransfer.setData("text", ev.target.id);
            ev.dataTransfer.effectAllowed = "move";
        }

        function drop_handler(ev) {
            console.log("drop: dropEffect = " + ev.dataTransfer.dropEffect + " ; effectAllowed = " + ev.dataTransfer.effectAllowed);
            ev.preventDefault();
            var data = ev.dataTransfer.getData("text");
            ev.target.appendChild(document.getElementById(data));
        }

        function dragover_handler(ev) {
            console.log("dragOver: dropEffect = " + ev.dataTransfer.dropEffect + " ; effectAllowed = " + ev.dataTransfer.effectAllowed);
            ev.preventDefault();
            ev.dataTransfer.dropEffect = "move"
        }
      </script>
</body>

</html>

来源：https://developer.mozilla.org/zh-CN/docs/Web/API/DataTransfer/dropEffect