Function whenTransitionEnds

packages/runtime-dom/src/components/Transition.ts:348–387

(
  el: Element & { _endId?: number },
  expectedType: TransitionProps['type'] | undefined,
  explicitTimeout: number | null,
  resolve: () => void,
)

let endId = 0

function whenTransitionEnds(
  el: Element & { _endId?: number },
  expectedType: TransitionProps['type'] | undefined,
  explicitTimeout: number | null,
  resolve: () => void,
) {
  const id = (el._endId = ++endId)
  const resolveIfNotStale = () => {
    if (id === el._endId) {
      resolve()
    }
  }

  if (explicitTimeout != null) {
    return setTimeout(resolveIfNotStale, explicitTimeout)
  }

  const { type, timeout, propCount } = getTransitionInfo(el, expectedType)
  if (!type) {
    return resolve()
  }

  const endEvent = type + 'end'
  let ended = 0
  const end = () => {
    el.removeEventListener(endEvent, onEnd)
    resolveIfNotStale()
  }
  const onEnd = (e: Event) => {
    if (e.target === el && ++ended >= propCount) {
      end()
    }
  }
  setTimeout(() => {
    if (ended < propCount) {
      end()
    }
  }, timeout + 1)
  el.addEventListener(endEvent, onEnd)
}

interface CSSTransitionInfo {
  type: AnimationTypes | null