Drag and Drop API keynotes

I created an example about Javascript Drag-n-Drop APIs at https://hta218.github.io/dnd-keynotes

Below are some interesting key-notes I've learned about the APIs.

Basic concepts

• A typical drag operation begins when a user selects a draggable element, drags the element to a dropzone, and then releases the dragged element.

• Events:

Keynotes

• To make an element draggable, add draggable="true" attribute

  <div draggable="true">This element is draggable</div>

• dragstart

  • dragstart is the first event fired when a drag operation starts on a draggable element

  • use e.dataTransfer.setData() method to set any drag's data, this will stay during the drag operation

  // The `dragstart` event fires on the `draggable` element
  dragElem.addEventListener('dragstart', function (e) {
    // We can set data using `e.dataTransfer.setData` method
    e.dataTransfer.setData('text/plain', e.target.id)
    // Use e.dataTransfer.setDragImage() to change the drag image
    // e.dataTransfer.setDragImage(img | element, xOffset, yOffset)
  })

  • If you don't want the translucent image generated from the drag target during drag, use e.dataTransfer.setDragImage() to change it

• The dropEffect

  • The dropEffect property is used to control the feedback the user is given during a drag-and-drop operation

  dragElem.addEventListener('dragstart', function (e) {
    e.dataTransfer.setData('text/plain', e.target.id)
    // The `move` value works on window, not on macOS - It might be the problem of browser along with OS
    e.dataTransfer.dropEffect = 'move' // or "copy"
  })

  • dropEffect property could be:

    1. move: dragged data will be moved to the dropzone.
    2. copy: dragged data will be copied to the dropzone.
    3. ..

• The dropzone

  • To make an element becomes a dropzone, it must have both dragover and drop event handler.

  • Remember to call e.preventDefault() in dragover handler or the browser won't let you drop anything inside

  dropzone.addEventListener('dragover', function handleDragOver(e) {
    // The `dropzone` element must have both `dragover` and `drop` event
    // Remember to preventDefault the behavior or the browser or it will not let you drop anything inside
    e.preventDefault()
    e.dataTransfer.dropEffect = 'move'
  })
  dropzone.addEventListener('drop', function handleDrop(e) {
    // NOTE: there must be a handler for dragover to use drop event
    e.preventDefault()
    // Use `e.dataTransfer.getData` method to retrieve drag's data and process them
    let data = e.dataTransfer.getData('text/plain')
    // NOTE: Keep mind that we can only use the `dataTransfer.getData()` in the `drop-handler`
    // `getData()` will return empty string inside handle dragover or dragenter
  })

  • Keep mind that we can only use the dataTransfer.getData() in the drop-handler (it will return empty string inside dragover or dragenter handler)

• dragend

  • The dragend event fires after a drag operation finished regardless of whether the drag completed or was canceled

  // The `dragend` event fired on the `draggable` element (not the dropzone element)
  dragElem.addEventListener('dragend', function handleDragEnd(e) {
    // We can check if the drag was successful or not by checking the ʻe.dataTransfer.dropEffect` value
    let dropEffect = e.dataTransfer.dropEffect
    // If that fails, the value of `e.dataTransfer.dropEffect` will be "none"
  })

  • If the drag operation failed, the value of e.dataTransfer.dropEffect will be "none"

Refs

• DigitalOcean
• MDN Documentations
• https://web.dev/drag-and-drop/
• https://html.spec.whatwg.org/multipage/dnd.html#dnd