处理reference type 的 useEffect dependency（通过useRef hook）

当通过字面量语法设置的参数被作为useEffect dependency时，由于数组或对象本身是reference type。这样在component 每次re-render时，即使一模一样的输入也会因为存储地址的不同而被React 认为dependency 发生变化，进而触发 infinite loop。

解决此问题

方案一：useRef hook

源码：

import { useEffect, useState, useRef } from "react"

export const useCollection = (collection, _query, _orderBy) => {
  const [documents, setDocuments] = useState(null)
  const [error, setError] = useState(null)

  // if we don't use a ref --> infinite loop in useEffect
  // _query is an array and is "different" on every function call
  const query = useRef(_query).current
  const orderBy = useRef(_orderBy).current

  useEffect(() => {
    let ref = projectFirestore.collection(collection)

    if (query) {
      ref = ref.where(...query)
    }
    if (orderBy) {
      ref = ref.orderBy(...orderBy)
    }

    const unsubscribe = ref.onSnapshot(snapshot => {
      let results = []
      snapshot.docs.forEach(doc => {
        console.log(doc)
        results.push({...doc.data(), id: doc.id})
      });
      
      // update state
      setDocuments(results)
      setError(null)
    }, error => {
      console.log(error)
      setError('could not fetch the data')
    })

    // unsubscribe on unmount
    return () => unsubscribe()

  }, [collection, query, orderBy])

  return { documents, error }
}

方案二：

用JSON.stringify() 函数包裹变量，似乎也是近似可行的方法之一。（待考证）