HTML Drag and Drop API - Web APIs 编辑

HTML Drag and Drop interfaces enable applications to use drag-and-drop features in browsers. The user may select draggable elements with a mouse, drag those elements to a droppable element, and drop them by releasing the mouse button. A translucent representation of the draggable elements follows the pointer during the drag operation.

For web sites, extensions, and XUL applications, you can customize which elements can become draggable, the type of feedback the draggable elements produce, and the droppable elements.

This overview of HTML Drag and Drop includes a description of the interfaces, basic steps to add drag-and-drop support to an application, and an interoperability summary of the interfaces.

Drag Events

HTML drag-and-drop uses the DOM event model and drag events inherited from mouse events. A typical drag operation begins when a user selects a draggable element, drags the element to a droppable element, and then releases the dragged element.

During drag operations, several event types are fired, and some events might fire many times, such as the drag and dragover events.

Each drag event type has an associated global event handler:

Note: Neither dragstart nor dragend events are fired when dragging a file into the browser from the OS.

Interfaces

The HTML Drag and Drop interfaces are DragEvent, DataTransfer, DataTransferItem and DataTransferItemList.

The DragEvent interface has a constructor and one dataTransfer property, which is a DataTransfer object.

DataTransfer objects include the drag event's state, such as the type of drag being done (like copy or move), the drag's data (one or more items), and the MIME type of each drag item. DataTransfer objects also have methods to add or remove items to the drag's data.

The DragEvent and DataTransfer interfaces should be the only ones needed to add HTML Drag and Drop capabilities to an application. (Firefox supports some Gecko-specific extensions to the DataTransfer object, but those extensions will only work on Firefox.)

Each DataTransfer object contains an items property, which is a list of DataTransferItem objects. A DataTransferItem object represents a single drag item, each with a kind property (either string or file) and a type property for the data item's MIME type. The DataTransferItem object also has methods to get the drag item's data.

The DataTransferItemList object is a list of DataTransferItem objects. The list object has methods to add a drag item to the list, remove a drag item from the list, and clear the list of all drag items.

A key difference between the DataTransfer and DataTransferItem interfaces is that the former uses the synchronous getData() method to access a drag item's data, but the latter instead uses the asynchronous getAsString() method.

Note: DragEvent and DataTransfer are broadly supported on desktop browsers. However, the DataTransferItem and DataTransferItemList interfaces have limited browser support. See Interoperability for more information about drag-and-drop interoperability.

Gecko-specific interfaces

Mozilla and Firefox support some features not in the standard drag-and-drop model. These are convenience functions to help with dragging multiple items or non-string data (such as files). For more information, see Dragging and Dropping Multiple Items. Additionally, see the DataTransfer reference page for all of the Gecko-specific properties and Gecko-specific methods.

The basics

This section is a summary of the basic steps to add drag-and-drop functionality to an application.

Identify what is draggable

Making an element draggable requires adding the draggable attribute and the ondragstart global event handler, as shown in the following code sample:

<script>
  function dragstart_handler(ev) {
    // Add the target element's id to the data transfer object
    ev.dataTransfer.setData("text/plain", ev.target.id);
  }

  window.addEventListener('DOMContentLoaded', () => {
    // Get the element by id
    const element = document.getElementById("p1");
    // Add the ondragstart event listener
    element.addEventListener("dragstart", dragstart_handler);
  });
</script>

<p id="p1" draggable="true">This element is draggable.</p>

For more information, see:

• Draggable attribute reference
• Drag operations guide

Define the drag's data

The application is free to include any number of data items in a drag operation. Each data item is a string of a particular type — typically a MIME type such as text/html.

Each drag event has a dataTransfer property that holds the event's data. This property (which is a DataTransfer object) also has methods to manage drag data. The setData() method is used to add an item to the drag data, as shown in the following example.

function dragstart_handler(ev) {
  // Add different types of drag data
  ev.dataTransfer.setData("text/plain", ev.target.innerText);
  ev.dataTransfer.setData("text/html", ev.target.outerHTML);
  ev.dataTransfer.setData("text/uri-list", ev.target.ownerDocument.location.href);
}

• For a list of common data types used in drag-and-drop (such as text, HTML, links, and files), see Recommended Drag Types.
• For more information about drag data, see Drag Data.

Define the drag image

By default, the browser supplies an image that appears beside the pointer during a drag operation. However, an application may define a custom image with the setDragImage() method, as shown in the following example.

function dragstart_handler(ev) {
  // Create an image and then use it for the drag image.
  // NOTE: change "example.gif" to a real image URL or the image
  // will not be created and the default drag image will be used.
  let img = new Image();
  img.src = 'example.gif';
  ev.dataTransfer.setDragImage(img, 10, 10);
}

Learn more about drag feedback images in:

• Setting the Drag Feedback Image

Define the drag effect

The dropEffect property is used to control the feedback the user is given during a drag-and-drop operation. It typically affects which cursor the browser displays while dragging. For example, when the user hovers over a drop target, the browser's cursor may indicate the type of operation that will occur.

Three effects may be defined:

1. copy indicates that the dragged data will be copied from its present location to the drop location.
2. move indicates that the dragged data will be moved from its present location to the drop location.
3. link indicates that some form of relationship or connection will be created between the source and drop locations.

During the drag operation, drag effects may be modified to indicate that certain effects are allowed at certain locations.

The following example shows how to use this property.

function dragstart_handler(ev) {
  ev.dataTransfer.dropEffect = "copy";
}

For more details, see:

• Drag Effects

Define a drop zone

By default, the browser prevents anything from happening when dropping something onto most HTML elements. To change that behavior so that an element becomes a drop zone or is droppable, the element must have both ondragover and ondrop event handler attributes.

The following example shows how to use those attributes, and includes basic event handlers for each attribute.

<script>
function dragover_handler(ev) {
 ev.preventDefault();
 ev.dataTransfer.dropEffect = "move";
}
function drop_handler(ev) {
 ev.preventDefault();
 // Get the id of the target and add the moved element to the target's DOM
 const data = ev.dataTransfer.getData("text/plain");
 ev.target.appendChild(document.getElementById(data));
}
</script>

<p id="target" ondrop="drop_handler(event)" ondragover="dragover_handler(event)">Drop Zone</p>

Note that each handler calls preventDefault() to prevent additional event processing for this event (such as touch events or pointer events).

For more information, see:

• Specifying Drop Targets

Handle the drop effect

The handler for the drop event is free to process the drag data in an application-specific way.

Typically, an application uses the getData() method to retrieve drag items and then process them accordingly. Additionally, application semantics may differ depending on the value of the dropEffect and/or the state of modifier keys.

The following example shows a drop handler getting the source element's id from the drag data, and then using the id to move the source element to the drop element:

<script>
function dragstart_handler(ev) {
 // Add the target element's id to the data transfer object
 ev.dataTransfer.setData("application/my-app", ev.target.id);
 ev.dataTransfer.effectAllowed = "move";
}
function dragover_handler(ev) {
 ev.preventDefault();
 ev.dataTransfer.dropEffect = "move"
}
function drop_handler(ev) {
 ev.preventDefault();
 // Get the id of the target and add the moved element to the target's DOM
 const data = ev.dataTransfer.getData("application/my-app");
 ev.target.appendChild(document.getElementById(data));
}
</script>

<p id="p1" draggable="true" ondragstart="dragstart_handler(event)">This element is draggable.</p>
<div id="target" ondrop="drop_handler(event)" ondragover="dragover_handler(event)">Drop Zone</div>

For more information, see:

• Performing a Drop

Drag end

At the end of a drag operation, the dragend event fires at the source element — the element that was the target of the drag start.

This event fires regardless of whether the drag completed or was canceled. The dragend event handler can check the value of the dropEffect property to determine if the drag operation succeeded or not.

For more information about handling the end of a drag operation, see:

• Finishing a Drag

Interoperability

As can be seen in the DataTransferItem interface's Browser Compatibility table, drag-and-drop interoperability is relatively broad among desktop browsers (except the DataTransferItem and DataTransferItemList interfaces have less support). This data also indicates drag-and-drop support among mobile browsers is very low.

Examples and demos

• Copying and moving elements with the DataTransfer interface
• Copying and moving elements with the DataTransferListItem interface
• Dragging and dropping files (Firefox only): http://jsfiddle.net/9C2EF/
• Dragging and dropping files (All browsers): https://jsbin.com/hiqasek/

• A parking project using the Drag and Drop API: https://park.glitch.me/ (You can edit here)

Specifications

See also

• Drag Operations
• Dragging and Dropping Multiple Items
• Recommended Drag Types
• HTML5 Living Standard: Drag and Drop
• Drag and Drop interoperability data from CanIUse