HTML |拖放

拖放是一个非常互动和用户友好的概念，它可以更轻松地通过抓取对象将其移动到不同的位置。这允许用户在元素上单击并按住鼠标按钮，将其拖动到另一个位置，然后释放鼠标按钮将元素放在那里。在 HTML 5 中，拖放更容易编码，并且其中的任何元素都是可拖动的。

拖放事件：有许多拖放事件，其中一些列在下面：

Events	Description
ondragstart	Calls a function, drag(event), that specifies what data to be dragged
ondragenter	To determine whether or not the drop target is to accept the drop. If the drop is to be accepted, then this event has to be canceled
ondragleave	Occurs when the mouse leaves an element before a valid drop target while the drag is ocurring
ondragover	Specifies where the dragged data can be dropped
ondrop	Specifies where the drop was occurred at the end of the drag operation
ondragend	Occurs when the user has finished dragging an element

数据传输对象：数据传输属性在整个拖放过程发生时使用。它用于保存从源拖动的数据并将其放到所需位置。这些是与之相关的属性：

Attributes	Description
dataTransfer.setData(format, data)	Sets the data to be dragged
dataTransfer.clearData(format)	Calling this function with no argument clears all the data. Calling it with a format argument removes only that specific data.
dataTransfer.getData(format)	Returns the data of the specified format. If there is no such data, returns the empty string.
dataTransfer.types	This property returns an array of all the formats that were set in the dragstart event.
dataTransfer.files	It is used to return all the files that are to be dropped.
dataTransfer.setDragImage(element, x, y)	It is used to display an existing image as the drag image instead of the default image alongside the cursor. The coordinates specify the drop location
dataTransfer.addElement(element)	Adds the specified element to be drawn as a drag feedback image
dataTransfer.effectAllowed(value)	Tells the browser that only the listed type(s) of operations are allowed for the user. The property can be set to the following values: none, copy, copyLink, copyMove, link, linkMove, move, all, and uninitialized.
dataTransfer.dropEffect(value)	Controls the feedback that the user is given during the dragenter and dragover events. When the user hovers over a target element, the browser’s cursor will indicate what type of operation is going to take place (e.g. a copy, a move, etc.). The effect can take on one of the following values: none, copy, link, move.

拖放过程：

• 第 1 步：使对象可拖动
  • 首先将 draggable 属性设置为 true 使该元素可拖动
  • 然后，指定拖动元素时应发生的情况。 ondragstart 属性调用一个函数，drag(event)，它指定要拖动的数据。 dataTransfer.setData() 方法设置拖拽数据的数据类型和值
  • 事件侦听器 ondragstart 将设置允许的效果(复制、移动、链接或某些组合)。

  function dragStart(ev) {
      ev.dataTransfer.setData("text", ev.target.id);
  }
  

• 第 2 步：删除对象
  • ondragover 事件指定可以放置拖动数据的位置。默认情况下，不能将数据/元素放入其他元素中。要允许放置，它必须阻止元素的默认处理。这是通过调用 event.preventDefault() 方法完成的
  • 最后是 drop 事件，它允许执行实际的 drop

  function dragDrop(ev) {
      ev.preventDefault();
      var data = ev.dataTransfer.getData("text");
      ev.target.appendChild(document.getElementById(data));
  }
  

示例 1：

    
        Drag and Drop box
          
        
            function allowDrop(ev) {
                ev.preventDefault();
            }
              
            function dragStart(ev) {
                ev.dataTransfer.setData("text", ev.target.id);
            }
              
            function dragDrop(ev) {
                ev.preventDefault();
                var data = ev.dataTransfer.getData("text");
                ev.target.appendChild(document.getElementById(data));
            }
        
          
        
            #box {
                margin: auto;
                width: 50%;
                height:200px;
                border: 3px solid green;
                padding: 10px;
            }
            #box1, #box2, #box3 {
                float: left;
                margin: 5px;
                padding: 10px;
            }
            #box1 {
                width: 50px;
                height: 50px;
                background-color: #F5B5C5;
            }
            #box2 {
                width: 100px;
                height: 100px;
                background-color: #B5D5F5;
            }
            #box3 {
                width: 150px;
                height: 150px;
                background-color: #BEA7CC;
            }
            p {
                font-size:20px;
                font-weight:bold;
                text-align:center;
            }
            .gfg {
                font-size:40px;
                color:#009900;
                font-weight:bold;
                text-align:center;
            }
        
    
      
    
        GeeksforGeeks
        Drag and drop of boxes
        
            
            
            
        
    
                                

输出：

解释：

• 通过将要拖动的元素的draggable 属性设置为true 来开始拖动。
• 使用 dataTransfer.getData() 方法获取拖动的数据。此方法将返回在 setData() 方法中设置为相同类型的任何数据。
• 调用event.preventDefault()方法以通过阻止元素的默认处理来允许删除其他元素中的元素。
• 该元素存储在我们附加到 drop 元素中的变量 data 中。

示例 2：此示例说明图像拖放。

    
        
            function allowDrop(ev) {
                ev.preventDefault();
            }
              
            function dragStart(ev) {
                ev.dataTransfer.setData("text", ev.target.id);
            }
              
            function dragDrop(ev) {
                ev.preventDefault();
                var data = ev.dataTransfer.getData("text");
                ev.target.appendChild(document.getElementById(data));
            }
        
        
            .div1 {
                width: 240px;
                height: 75px;
                padding: 10px;
                border: 1px solid black;
                background-color: #F5F5F5;
            }
            p {
                font-size:20px;
                font-weight:bold;
            }
            .gfg {
                font-size:40px;
                color:#009900;
                font-weight:bold;
            }
        
    
    
        GeeksforGeeks
         Image Drag and Drop in boxes
        
        
        
        
    
                                    

输出：