本节包含很多小函数，每个函数负责在实现拖放功能时处理一些烦人的重复工作。
This section contains a lot of small functions, each of which takes care of a bit of the annoying boilerplate that usually comes with implementing drag-and-drop functions.
这就是实现地图拖放功能的方法——在拖动地图时，服务器发送用户请求的地区的图像，地图立即装载它。
This is how the drag-and-drop capability of the map comes to life — when the map is dragged, the server sends the image for the area you are requesting, and the map loads it immediately.
把“ Sortable Column Title ”拖放到希望实现排序功能的列的标题单元格中。
Drag and drop the "Sortable column title" to the title cell of a column that you want to have sorting capabilities.
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