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akmhmgc
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Jan 3, 2023
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| useEffect(() => { | ||
| fetch("http://localhost:3000/trips") | ||
| .then((response) => response.json()) | ||
| .then((json) => setTrips(json)); |
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Reactのレンダー（DOMの変更を起こす）後に何らかのアクション（＝副作用、Effectと呼ぶ）をおこないたい場合、useEffectを使用する。
コンポーネント内で定義している理由は、stateなどの変数にアクセスすることができるからである。
akmhmgc
commented
Jan 3, 2023
| fetch(url) | ||
| .then((response) => response.json()) | ||
| .then((json) => setTrips(json)); | ||
| }, [url]); |
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第二引数として監視する変数を指定すると副作用をスキップすることができる
https://ja.reactjs.org/docs/hooks-effect.html#tip-optimizing-performance-by-skipping-effects
akmhmgc
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Jan 3, 2023
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| useEffect(() => { | ||
| fetchTrips(); | ||
| }, [fetchTrips]); |
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useEffectの第二引数に関数を指定する場合、関数が定義されると発火する
Reactがrenderを行うたびにコンポーネント内の関数は再定義されるので、useCallbackを使用してrenderのたびにあたらしい関数を作らないようにしないと無限ループとなる。
useCallbackを使用すると関数を毎回作らなくなるが、第二引数を指定するとそれが変化した場合は関数を再定義するようになる
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