Function onBeforeAppear

packages/runtime-dom/src/components/Transition.ts:239–246

(el)

      addTransitionClass(el, enterActiveClass)
    },
    onBeforeAppear(el) {
      callHook(onBeforeAppear, [el])
      addTransitionClass(el, appearFromClass)
      if (__COMPAT__ && legacyClassEnabled && legacyAppearFromClass) {
        addTransitionClass(el, legacyAppearFromClass)
      }
      addTransitionClass(el, appearActiveClass)
    },
    onEnter: makeEnterHook(false),
    onAppear: makeEnterHook(true),
    onLeave(