tryOnBeforeMount 
安全的 onBeforeMount。如果它在组件生命周期内,则调用 onBeforeMount();如果不在,则直接调用该函数。
用法 
ts
import { tryOnBeforeMount } from '@vueuse/core'
tryOnBeforeMount(() => {
})类型声明 
ts
/**
 * Call onBeforeMount() if it's inside a component lifecycle, if not, just call the function
 *
 * @param fn
 * @param sync if set to false, it will run in the nextTick() of Vue
 * @param target
 */
export declare function tryOnBeforeMount(
  fn: Fn,
  sync?: boolean,
  target?: ComponentInternalInstance | null,
): void