Function onLeave

packages/runtime-dom/src/components/Transition.ts:249–284

(
      el: Element & { _isLeaving?: boolean; _enterCancelled?: boolean },
      done,
    )

    onEnter: makeEnterHook(false),
    onAppear: makeEnterHook(true),
    onLeave(
      el: Element & { _isLeaving?: boolean; _enterCancelled?: boolean },
      done,
    ) {
      el._isLeaving = true
      const resolve = () => finishLeave(el, done)
      addTransitionClass(el, leaveFromClass)
      if (__COMPAT__ && legacyClassEnabled && legacyLeaveFromClass) {
        addTransitionClass(el, legacyLeaveFromClass)
      }
      // add *-leave-active class before reflow so in the case of a cancelled enter transition
      // the css will not get the final state (#10677)
      if (!el._enterCancelled) {
        // force reflow so *-leave-from classes immediately take effect (#2593)
        forceReflow(el)
        addTransitionClass(el, leaveActiveClass)
      } else {
        addTransitionClass(el, leaveActiveClass)
        forceReflow(el)
      }
      nextFrame(() => {
        if (!el._isLeaving) {
          // cancelled
          return
        }
        removeTransitionClass(el, leaveFromClass)
        if (__COMPAT__ && legacyClassEnabled && legacyLeaveFromClass) {
          removeTransitionClass(el, legacyLeaveFromClass)
        }
        addTransitionClass(el, leaveToClass)
        if (!hasExplicitCallback(onLeave)) {
          whenTransitionEnds(el, type, leaveDuration, resolve)
        }
      })
      callHook(onLeave, [el, resolve])
    },
    onEnterCancelled(el) {
      finishEnter(el, false, undefined, true)
      callHook(onEnterCancelled, [el])