Recommended Drag Types - Web APIs 编辑

The HTML Drag and Drop API supports dragging various types of data, including plain text, URLs, HTML code, files, etc. The document describes best practices for common draggable data types.

Caution:
All methods and properties in this document with a moz prefix, such as mozSetDataAt(), will only work with Gecko-based browsers.

Dragging Text

For dragging text, use the text/plain type. The second data parameter should be the dragged string. For example:

event.dataTransfer.setData("text/plain", "This is text to drag");

Dragging text in textboxes and selections on web pages is done automatically by the browser, so you do not need to handle it yourself.

It is recommended to always add data of the text/plain type as a fallback for applications or drop targets that do not support other types, unless there is no logical text alternative. Always add this text/plain type last, as it is the least specific and shouldn’t be preferred.

Note: In older code, you may find text/unicode or the Text types. These are equivalent to text/plain, and will store and retrieve plain text data.

Dragging Links

Dragged hyperlinks should include data of two types: text/uri-list, and text/plain. Both types should use the link’s URL for their data. For example:

var dt = event.dataTransfer;
dt.setData("text/uri-list", "https://www.mozilla.org");
dt.setData("text/plain", "https://www.mozilla.org");

As usual, set the text/plain type last, as a fallback for the text/uri-list type.

Note: the URL type is uri-list with an I, not an L.

To drag multiple links, separate each link inside the text/uri-list data with a CRLF linebreak. Lines that begin with a number sign (#) are comments, and should not be considered URLs. You can use comments to indicate the purpose of a URL, the title associated with a URL, or other data.

The text/plain fallback for multiple links should include all URLs, but no comments.

For example, this sample text/uri-list data contains two links and a comment:

http://www.mozilla.org
#A second link
http://www.example.com

When retrieving a dropped link, ensure you handle when multiple links are dragged, including any comments. For convenience, the special type URL may be used to refer to the first valid link within data for the text/uri-list type.

Do not add data with the URL type — attempting to do so will set the value of the text/uri-list type instead.

var url = event.dataTransfer.getData("URL");

You may also see data with the Mozilla-specific type text/x-moz-url. If it appears, it should appear before the text/uri-list type. It holds the URLs of links followed by their titles, separated by a linebreak. For example:

http://www.mozilla.org
Mozilla
http://www.example.com
Example

Dragging HTML and XML

HTML content may use the text/html type. The data for this type should be serialized HTML source code. For example, it would be suitable to set its data to the value of the innerHTML property of an element.

XML content may use the text/xml type, but ensure that the data is well-formed XML.

You may also include a plain text representation of the HTML or XML data using the text/plain type. The data should be just the text without any of the source tags or attributes. For instance:

var dt = event.dataTransfer;
dt.setData("text/html", "Hello there, <strong>stranger</strong>");
dt.setData("text/plain", "Hello there, stranger");

Dragging Files

A local file is dragged using the application/x-moz-file type with a data value that is an nsIFile object. Non-privileged web pages canot retrieve or modify data of this type.

Because a file is not a string, you must use the mozSetDataAt() method to assign the data. Similarly, when retrieving the data, you must use the mozGetDataAt() method.

event.dataTransfer.mozSetDataAt("application/x-moz-file", file, 0);

If possible, include the file URL of the file using both the text/uri-list and text/plain types. These types should be added afterward so that the more specific application/x-moz-file type has higher priority.

Multiple files will be received during a drop as multiple items in the data transfer. See Dragging and Dropping Multiple Items for more details about this.

The following example shows how to create an area for receiving dropped files:

<listbox ondragenter="return checkDrag(event)"
         ondragover="return checkDrag(event)"
         ondrop="doDrop(event)"/>

<script>
function checkDrag(event) {
  return event.dataTransfer.types.contains("application/x-moz-file");
}

function doDrop(event) {
  var file = event.dataTransfer.mozGetDataAt("application/x-moz-file", 0);
  if (file instanceof Components.interfaces.nsIFile) {
    event.currentTarget.appendItem(file.leafName);
  }
}
</script>

In this example, the event returns false only if the data transfer contains the application/x-moz-file type. During the drop event, the data associated with the file type is retrieved, and the filename of the file is added to the listbox. Note that the instanceof operator is used here as the mozGetDataAt() method will return an nsISupports that needs to be checked and converted into an nsIFile. This is also a good extra check in case someone made a mistake and added a non-file for this type.

Updates to DataTransfer.types

The latest spec dictates that DataTransfer.types should return a frozen array of DOMStrings rather than a DOMStringList (this is supported in Firefox 52 and above).

As a result, the contains method no longer works; the includes method should be used instead to check if a specific type of data is provided, using code like the following:

if ([...event.dataTransfer.types].includes('text/html')) {
  // Do something
}

You could use feature detection to determine which method is supported on types, then run code as appropriate.

Dragging Images

Direct image dragging is not common. In fact, Mozilla does not support direct image dragging on Mac or Linux. Instead, images are usually dragged only by their URLs. To do this, use the text/uri-list type as with other URLs. The data should be the URL of the image, or a data: URI if the image is not stored on a web site or disk.

As with links, the data for the text/plain type should also contain the URL. However, a data: URL is not usually useful in a text context, so you may wish to exclude the text/plain data in this situation.

In chrome or other privileged code, you may also use the image/jpeg, image/png or image/gif types, depending on the type of image. The data should be an object which implements the nsIInputStream interface. When this stream is read, it should provide the data bits for the image, as if the image was a file of that type.

You should also include the application/x-moz-file type if the image is located on disk. In fact, this a common way in which image files are dragged.

It is important to set the data in the right order, from most-specific to least-specific. The standard image type, such as image/jpeg, should come first, followed by the application/x-moz-file type. Next, you should set the text/uri-list data, and finally the text/plain data. For example:

var dt = event.dataTransfer;
dt.mozSetDataAt("image/png", stream, 0);
dt.mozSetDataAt("application/x-moz-file", file, 0);
dt.setData("text/uri-list", imageurl);
dt.setData("text/plain", imageurl);

Note the mozGetDataAt() method is used for non-textual data. As some contexts may only include some of these types, it is important to check which type is made available when receiving dropped images.

Dragging Nodes

Nodes and elements in a document may be dragged using the application/x-moz-node type. The data for the type should be a DOM node. This allows the drop target to receive the actual node where the drag was started from. Note that callers from a different domain will not be able to access the node even when it has been dropped.

You should always include a text/plain alternative for the node.

Dragging Custom Data

You can also use other types that you invent for custom purposes. Strive to always include a text/plain alternative, unless the dragged object is specific to a particular site or application. In this case, the custom type ensures that the data cannot be dropped elsewhere.

Dragging files to an operating system folder

You may want to add a file to an existing drag event session, and you may also want to write the file to disk when the drop operation happens over a folder in the operating system when your code receives notification of the target folder's location. This only works in extensions (or other privileged code) and the type application/moz-file-promise should be used. The following sample offers an overview of this advanced case:

// currentEvent is an existing drag operation event

currentEvent.dataTransfer.setData("text/x-moz-url", URL);
currentEvent.dataTransfer.setData("application/x-moz-file-promise-url", URL);
currentEvent.dataTransfer.setData("application/x-moz-file-promise-dest-filename", leafName);
currentEvent.dataTransfer.mozSetDataAt('application/x-moz-file-promise',
                  new dataProvider(success,error),
                  0, Components.interfaces.nsISupports);

function dataProvider(){}

dataProvider.prototype = {
  QueryInterface : function(iid) {
    if (iid.equals(Components.interfaces.nsIFlavorDataProvider)
                  || iid.equals(Components.interfaces.nsISupports))
      return this;
    throw Components.results.NS_NOINTERFACE;
  },
  getFlavorData : function(aTransferable, aFlavor, aData, aDataLen) {
    if (aFlavor == 'application/x-moz-file-promise') {

       var urlPrimitive = {};
       var dataSize = {};

       aTransferable.getTransferData('application/x-moz-file-promise-url', urlPrimitive, dataSize);
       var url = urlPrimitive.value.QueryInterface(Components.interfaces.nsISupportsString).data;
       console.log("URL file orignal is = " + url);

       var namePrimitive = {};
       aTransferable.getTransferData('application/x-moz-file-promise-dest-filename', namePrimitive, dataSize);
       var name = namePrimitive.value.QueryInterface(Components.interfaces.nsISupportsString).data;

       console.log("target filename is = " + name);

       var dirPrimitive = {};
       aTransferable.getTransferData('application/x-moz-file-promise-dir', dirPrimitive, dataSize);
       var dir = dirPrimitive.value.QueryInterface(Components.interfaces.nsILocalFile);

       console.log("target folder is = " + dir.path);

       var file = Cc['@mozilla.org/file/local;1'].createInstance(Components.interfaces.nsILocalFile);
       file.initWithPath(dir.path);
       file.appendRelativePath(name);

       console.log("output final path is =" + file.path);

       // now you can write or copy the file yourself…
    }
  }
}

See Also

• HTML Drag and Drop API (Overview)
• Drag Operations
• Dragging and Dropping Multiple Items
• HTML5 Living Standard: Drag and Drop