[英]React context api returning undefined when setting the state
我最近開始學習反應,我正在使用上下文 api 來存儲我的全局狀態。
在MyProvider.js文件中,我定義了我的提供者,它只存儲了 2 個 json obj 數組
import {MyContext} from "./MyContext";
import React, {useState} from "react";
export const MyProvider = (props) => {
const intialState = {
featuredListings: [],
setFeaturedListings: (val) => setState({...state, featuredListings: val}),
featuredVendors: [],
setFeaturedVendors: (val) => setState({...state, featuredVendors: val})
}
const [state, setState] = useState(intialState)
return (
<MyContext.Provider
value={state}
>
{props.children}
</MyContext.Provider>
);
}
我通過這樣做將我的App.js中的所有組件包裝在 Provider 中,一邊不使用 ReachRouter 來處理路由,
<MyProvider>
<div className="content">
<Header/>
<Router>
<Home path="/"/>
</Router>
</div>
<Footer />
</MyProvider>
在我的Home.js文件中,我在 useEffect 鈎子中進行了一個網絡調用,它成功地返回了我期望的 json,並使用該 json 響應更新上下文的狀態,以便它可以全局可見。
我的代碼是
export const Home = () => {
let state = useContext(MyContext)
async function apiCalls() {
const featuredVendors = await getFeaturedVendors()
console.log("featuredVendors - ", featuredVendors) // the correct response is returned
state.setFeaturedVendors(featuredVendors)
const featuredListings = await getFeaturedListing()
console.log("featuredListings - ", featuredListings) // the correct response is returned
state.setFeaturedListings(featuredListings)
}
useEffect(() => {
apiCalls()
}, []);
return (
<div>
{console.log(state.featuredVendors)} // empty array
{console.log(state.featuredListings)} // contains correct values
</div>
)
}
]
為了消除任何歧義,我的上下文是在一個名為MyContext.js的單獨文件中創建的,我像這樣創建了上下文
export const MyContext = React.createContext()
為什么state.featuredVendors在我設置時沒有更新?
我注意到的另一個奇怪的事情是,如果我重新排列調用的順序,即先調用getFeaturedListing,然后調用getFeaturedVendors,那么我的狀態只更新featuresVendors和 featuresListings 將是一個空數組。
當您調用useState
,initialValue 僅設置一次。 當您的 MyProvider 首次安裝時,狀態將使用您的setFeaturedListings
和setFeaturedVendors
方法進行初始化,但只要state
值發生變化,這些方法就不會更新。 因此,當您傳播值時state
的值將始終是其初始值。
setState
也可以使用始終接收當前值作為參數的函數調用,因此您可以重寫這些方法來傳播該值,如下所示:
const intialState = {
featuredListings: [],
setFeaturedListings: (val) => setState(state => ({...state, featuredListings: val})),
featuredVendors: [],
setFeaturedVendors: (val) => setState(state => ({...state, featuredVendors: val}))
}
或者,您可以將這些功能完全移出您的狀態。
export const MyProvider = (props) => {
const intialState = {
featuredListings: [],
featuredVendors: [],
}
const [state, setState] = useState(intialState)
return (
<MyContext.Provider
value={{
...state,
setFeaturedListings: (val) => setState({...state, featuredListings: val}),
setFeaturedVendors: (val) => setState({...state, featuredVendors: val})
}}
>
{props.children}
</MyContext.Provider>
);
}
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.