React 的生命周期有哪些
Explain the React component lifecycle and its methods
一句话：三个阶段 ——挂载、更新、卸载。
- 挂载：
constructor→getDerivedStateFromProps→render→componentDidMount（DOM 已经有了，发请求、订阅、操作 DOM 都在这） - 更新：
getDerivedStateFromProps→shouldComponentUpdate（返回 false 就跳过渲染）→render→getSnapshotBeforeUpdate→componentDidUpdate（这里改 state 必须加条件，否则死循环） - 卸载：
componentWillUnmount（清定时器、解绑监听、取消请求） - 出错：
getDerivedStateFromError+componentDidCatch（错误边界，见 #333）
三个被废弃的要知道：componentWillMount、componentWillReceiveProps、componentWillUpdate。原因是 Fiber 的 render 阶段可能被中断和重跑，这几个方法可能被调用多次，放在里面的副作用会重复执行。能说出这个原因很加分。
会追问：「请求为什么不放componentWillMount？」—— 除了上面的原因，它在 SSR 时也会执行，而且并不会更早拿到数据（请求是异步的，反正要等）。
In one line: three phases — mounting, updating, unmounting.
- Mounting:
constructor→getDerivedStateFromProps→render→componentDidMount(the DOM exists now, so fetching, subscribing and DOM work all belong here) - Updating:
getDerivedStateFromProps→shouldComponentUpdate(return false to skip the render) →render→getSnapshotBeforeUpdate→componentDidUpdate(setting state here needs a condition, or you get an infinite loop) - Unmounting:
componentWillUnmount(clear timers, detach listeners, cancel requests) - On error:
getDerivedStateFromError+componentDidCatch(error boundaries, see #333)
Know the three that were deprecated: componentWillMount, componentWillReceiveProps, componentWillUpdate. The reason is that Fiber can interrupt and re-run the render phase, so these could fire more than once and any side effect inside them would run twice. Giving that reason earns real credit.
Follow-up: “Why not fetch in componentWillMount?” — besides the reason above, it also runs during SSR, and it does not get the data any sooner: the request is async, so you wait either way.