How to implement drag and drop in cypress test?

I've written up a small example for how to implement drag and drop.

It works by adding a dragTo command like so:

/// <reference types="cypress"/>

it('works', () => {
  cy.visit('https://angular-oxkc7l-zirwfs.stackblitz.io/')
  cy.contains('To do', { timeout: 15000 }) // ensure page is loaded -__-

  const item = '.example-box:not(.cdk-drag-placeholder)'

  cy.get('#cdk-drop-list-1').children(item).should('have.length', 5)

  cy.get('.example-box:contains("Get to work")').dragTo('.example-box:contains("Get up")')
  cy.get('#cdk-drop-list-1').children(item).should('have.length', 6)

  // interpolates 10 extra mousemove events on the way
  cy.get('#cdk-drop-list-0').dragTo('#cdk-drop-list-1', { steps: 10 })
  cy.get('#cdk-drop-list-1').children(item).should('have.length', 7)

  // sets steps >= 10
  cy.get('#cdk-drop-list-0').dragTo('#cdk-drop-list-1', { smooth: true })
  cy.get('#cdk-drop-list-1').children(item).should('have.length', 8)

  cy.get('#cdk-drop-list-0').dragTo('#cdk-drop-list-1')
  cy.get('#cdk-drop-list-1').children(item).should('have.length', 9)
})

To add it, try putting this in your support/index.js or pasting it at the bottom of a spec file (warning: poor code quality):

const getCoords = ($el) => {
  const domRect = $el[0].getBoundingClientRect()
  const coords = { x: domRect.left + (domRect.width / 2 || 0), y: domRect.top + (domRect.height / 2 || 0) }

  return coords
}

const dragTo = (subject, to, opts) => {

  opts = Cypress._.defaults(opts, {
    // delay inbetween steps
    delay: 0,
    // interpolation between coords
    steps: 0,
    // >=10 steps
    smooth: false,
  })

  if (opts.smooth) {
    opts.steps = Math.max(opts.steps, 10)
  }

  const win = subject[0].ownerDocument.defaultView

  const elFromCoords = (coords) => win.document.elementFromPoint(coords.x, coords.y)
  const winMouseEvent = win.MouseEvent

  const send = (type, coords, el) => {

    el = el || elFromCoords(coords)

    el.dispatchEvent(
      new winMouseEvent(type, Object.assign({}, { clientX: coords.x, clientY: coords.y }, { bubbles: true, cancelable: true }))
    )
  }

  const toSel = to

  function drag (from, to, steps = 1) {

    const fromEl = elFromCoords(from)

    const _log = Cypress.log({
      $el: fromEl,
      name: 'drag to',
      message: toSel,
    })

    _log.snapshot('before', { next: 'after', at: 0 })

    _log.set({ coords: to })

    send('mouseover', from, fromEl)
    send('mousedown', from, fromEl)

    cy.then(() => {
      return Cypress.Promise.try(() => {

        if (steps > 0) {

          const dx = (to.x - from.x) / steps
          const dy = (to.y - from.y) / steps

          return Cypress.Promise.map(Array(steps).fill(), (v, i) => {
            i = steps - 1 - i

            let _to = {
              x: from.x + dx * (i),
              y: from.y + dy * (i),
            }

            send('mousemove', _to, fromEl)

            return Cypress.Promise.delay(opts.delay)

          }, { concurrency: 1 })
        }
      })
      .then(() => {

        send('mousemove', to, fromEl)
        send('mouseover', to)
        send('mousemove', to)
        send('mouseup', to)
        _log.snapshot('after', { at: 1 }).end()

      })

    })

  }

  const $el = subject
  const fromCoords = getCoords($el)
  const toCoords = getCoords(cy.$$(to))

  drag(fromCoords, toCoords, opts.steps)
}

Cypress.Commands.addAll(
  { prevSubject: 'element' },
  {
    dragTo,
  }
)

Dispatching MouseEvents seems to be the only way to test Angular Material drag and drop.

You should also be aware of the following issue, which tests in Protractor but also applies to this Cypress test

CDK DragDrop Regression between 7.0.0-beta.2 and 7.0.0-rc.2: Protractor tests stopped working #13642,

It seems that (for want of a better explanation) an additional nudge is needed on the mousemove.

The steps given as a workaround (Protractor syntax),

private async dragAndDrop ( $element, $destination ) {
  await browser.actions().mouseMove( $element ).perform();
  await browser.actions().mouseDown( $element ).perform();
  await browser.actions().mouseMove( {x: 10, y: 0 } ).perform();
  await browser.actions().mouseMove( $destination ).perform();
  return browser.actions().mouseUp().perform();
}

can be translated into a Cypress test, the simplest form I found is

it('works (simply)', () => {
  const draggable = Cypress.$('#cdk-drop-list-0 > :nth-child(1)')[0]  // Pick up this
  const droppable = Cypress.$('#cdk-drop-list-1 > :nth-child(4)')[0]  // Drop over this

  const coords = droppable.getBoundingClientRect()
  draggable.dispatchEvent(new MouseEvent('mousedown'));
  draggable.dispatchEvent(new MouseEvent('mousemove', {clientX: 10, clientY: 0}));
  draggable.dispatchEvent(new MouseEvent('mousemove', {
    clientX: coords.x+10,   
    clientY: coords.y+10  // A few extra pixels to get the ordering right
  }));
  draggable.dispatchEvent(new MouseEvent('mouseup'));

  cy.get('#cdk-drop-list-1').should('contain', 'Get to work');
  cy.get('#cdk-drop-list-1 > .cdk-drag').eq(3).should('contain', 'Get to work');

});

Notes

• The problem in the referenced issue is not limited to Protractor. If you remove the first mousemove in the Cypress test, it also fails.
• The cy.get(..).trigger() syntax does not seem to work with Angular, but native dispatchEvent() does.
• Dragging over a specific element in the target list (as opposed to just dropping on the list) gives precise positioning within the target list.
• dragstart, dragend may not be appropriate for Angular Material, as the code shows the event received is type CdkDragDrop rather than a DataTransfer object.
• If content is asynchronously fetched, you may have to switch from Cypress.$(...) to cy.get(...).then(el => {...}), to take advantage of cypress' auto retry in commands.
• I had to add a 10 second timeout to visit the Stackblitz url.

Async list fetching

If the list is fetched by an async Angular service (httpClient) during component construction, using this in the test

const draggable = Cypress.$('#cdk-drop-list-0 > :nth-child(1)')[0]

will not work, because the nth-child will not be present immediately, only after the fetch completes.

Instead, you can use cy.get() to provide retries up to a timeout (default 5 seconds).

cy.get('#cdk-drop-list-0 > :nth-child(1)').then(el => {
  const draggable = el[0]  // Pick up this
  cy.get('#cdk-drop-list-1 > :nth-child(4)').then(el => {
    const droppable = el[0]  // Drop over this

    const coords = droppable.getBoundingClientRect()
    draggable.dispatchEvent(new MouseEvent('mousemove'));
    draggable.dispatchEvent(new MouseEvent('mousedown'));
    draggable.dispatchEvent(new MouseEvent('mousemove', {clientX: 10, clientY: 0}));
    draggable.dispatchEvent(new MouseEvent('mousemove', {clientX: coords.x+10, clientY: coords.y+10}));
    draggable.dispatchEvent(new MouseEvent('mouseup'));

  })

  cy.get('#cdk-drop-list-1').should('contain', 'Get to work');
  cy.get('#cdk-drop-list-1 > .cdk-drag').eq(3).should('contain', 'Get to work');
})

or my preference is to use a 'canary' test to ensure loading is complete, something like

before(() => {
  cy.get('#cdk-drop-list-0 > :nth-child(1)') // Canary - wait 5s for data
})

it('should...', () => {
  const draggable = Cypress.$('#cdk-drop-list-0 > :nth-child(1)')[0]  // Pick up this
  const droppable = Cypress.$('#cdk-drop-list-1 > :nth-child(4)')[0]  // Drop over this
  ...
})

Typescript support

Warning - this is a quick hack to get over Typescript compiler problems, and could be improved.

• Cast MouseEvent to <any> as per MouseEvent in TypeScript error not matching signature

• Type return of getBoundingClientRect() to ClientRect, and use properties left and top instead of x and y.

const coords: ClientRect = droppable.getBoundingClientRect()
draggable.dispatchEvent(new (<any>MouseEvent)('mousemove'));
draggable.dispatchEvent(new (<any>MouseEvent)('mousedown'));
draggable.dispatchEvent(new (<any>MouseEvent)('mousemove', {clientX: 10.0, clientY: 0.0}));
draggable.dispatchEvent(new (<any>MouseEvent)('mousemove', {clientX: coords.left + 10.0, clientY: coords.top + 10.0}));
draggable.dispatchEvent(new (<any>MouseEvent)('mouseup'));

After a lot of battling, I managed to make the drag and drop work with this:

cy.get('.list .item')
      .contains(startpos)
      .trigger('dragstart', { dataTransfer: new DataTransfer });
cy.get('.list .item')
      .eq(endpos)
      .trigger('drop')
      .trigger('dragend');

Pretty easy to use.

In case anyone is also struggling with cdkDropListEntered not being triggered, you might want to check if there is any scrolling going on.

Since scrolling is handled by CDK (e.g. https://github.com/angular/components/blob/master/src/cdk/scrolling/viewport-ruler.ts#L131), I had to add the scroll position to any mouse events. The scroll position is computed like this (corresponding to above link):

const win = subject[0].ownerDocument.defaultView;
const window = win;
const document = window.document;
const documentElement = document.documentElement;
const documentRect = documentElement.getBoundingClientRect();

const top =
  -documentRect.top ||
  document.body.scrollTop ||
  window.scrollY ||
  documentElement.scrollTop ||
  0;

const left =
  -documentRect.left ||
  document.body.scrollLeft ||
  window.scrollX ||
  documentElement.scrollLeft ||
  0;

where subject is the result yielded e.g. by a cy.get command.