背景
熟悉 React 的小伙伴对这个错误信息一定不陌生:
Warning: Can't call setState (or forceUpdate) on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.
之前也看到过, 但是一直没处理。 早上趁喝茶的功夫决定看一下。
复现的路径: 在某个请求结果没回来之前就切换页面, 必现。
原因
这种问题出现一般是, 请求发出得到结果之后, 进行了 setState 的操作。 形如:
class Demo extends Component {
constructor(props) {
super(props);
this.state = {
news: [],
};
}
componentDidMount() {
axios
.get('https://hn.algolia.com/api/v1/search?query=react')
.then(result =>
this.setState({
news: result.data,
}),
);
}
render() {
return (
<ul>
{this.state.news.map(topic => (
<li key={topic.objectID}>{topic.title}</li>
))}
</ul>
);
}
}
在结果回来之前, 我们切换路由, 组件销毁, 但是请求是异步的, 结果回来之后, 组件已经销毁了, 这个之后执行 setState 可能会有意想不到的后果, 正如error 信息提示的那样.
知道原因之后, 就比较好解决了:
- 在组件销毁之前, cancel 掉发出的请求。
找资料的过程中也发现了一些 脑洞大开的解决办法:
几种开脑洞的解决办法
标志位法(不推荐
)
这个方法的思路也很简单, 给个标志位, 比如叫 _isMounted
:
class Demo extends Component {
_isMounted = false;
constructor(props) {
super(props);
this.state = {
news: [],
};
}
componentDidMount() {
this._isMounted = true;
axios
.get('https://hn.algolia.com/api/v1/search?query=react')
.then(result => {
if (this._isMounted) {
this.setState({
news: result.data.hits,
});
}
});
}
componentWillUnmount() {
this._isMounted = false;
}
render() {
// ...
}
}
Unmount 之后, 标志位为false , 不执行setState. 自然也就不会报错。
顺着这个思路,干脆写个组件统一处理, 形如
import React from 'react';
function inject_prevent_setState_after_unount(target) {
let componentWillUnmount = target.prototype.componentWillUnmount;
target.prototype.componentWillUnmount = function() {
if (componentWillUnmount) componentWillUnmount.call(this, ...arguments);
this.unmount = true;
};
let setState = target.prototype.setState;
target.prototype.setState = function() {
if (this.unmount) return;
setState.call(this, ...arguments);
};
}
@inject_prevent_setState_after_unount
class BaseComponent extends React.Component {}
export default BaseComponent;
然后在业务代码里直接继承这个组件, 这个和上面的标志位法其实是一个道理,虽然也能 hack 掉错误信息, 但是请求的副作用依旧会发生
,这种做法也是不推荐
的。
官网上也有对这个情景的描述:
The primary use case for isMounted() is to avoid calling setState() after a component has unmounted, because calling setState() after a component has unmounted will emit a warning. The “setState warning” exists to help you catch bugs, because calling setState() on an unmounted component is an indication that your app/component has somehow failed to clean up properly. Specifically, calling setState() in an unmounted component means that your app is still holding a reference to the component after the component has been unmounted - which often indicates a memory leak!
To avoid the error message, people often add lines like this:
if (this.isMounted()) { // This is bad.
this.setState({...});
}
官网推荐的做法是: 在componentWillUnmount 的时候取消掉所有的请求。
形如:
class MyComponent extends React.Component {
componentDidMount() {
mydatastore.subscribe(this);
}
render() {
...
}
componentWillUnmount() {
mydatastore.unsubscribe(this);
}
}
实现一个可以 cancel 的 Promise
我在这也给出一个简单的实现:
// cancelablePromise.js
export const cancelablePromise = promise => {
let hasCanceled = false;
const wrappedPromise = new Promise((resolve, reject) => {
promise.then(
value => (hasCanceled ? reject({ isCanceled: true, value }) : resolve(value)),
error => reject({ isCanceled: hasCanceled, error }),
);
});
return {
promise: wrappedPromise,
cancel: () => (hasCanceled = true),
};
};
在你的组件中:
import React, { Component } from "react";
import cancelablePromise from "./cancelable-promise";
class MyComponent extends Component {
state = {
data: [],
error: null,
};
pendingPromises = [];
componentWillUnmount = () =>
this.pendingPromises.map(p => p.cancel());
appendPendingPromise = promise =>
this.pendingPromises = [...this.pendingPromises, promise];
removePendingPromise = promise =>
this.pendingPromises = this.pendingPromises.filter(p => p !== promise);
handleOnClick = () => {
const wrappedPromise = cancelablePromise(fetchData());
this.appendPendingPromise(wrappedPromise);
return wrappedPromise.promise
.then(() => this.setState({ data }))
.then(() => this.removePendingPromise(wrappedPromise))
.catch(errorInfo => {
if (!errorInfo.isCanceled) {
this.setState({ error: errorInfo.error });
this.removePendingPromise(wrappedPromise);
}
});
}
render() {
const { data, error } = this.state;
if (error) {
return (
<div className="error">
There was an error fetching data: {error}
</div>
);
}
return (
<div className="data">
<button onClick={this.handleClick}>reload data!</button>
<ul className="data-list">
{data.map((item, i) => <li key={i}>{item}</li>)}
</ul>
</div>
);
}
}
这样就保证了setState 的时候, 上下文是完整的, 进而从根本上解决报错的问题。
结语
上面介绍了解决warning几种方式,最好的办法当然是用cancelablePromise来处理
, 但是这种方式有一定的侵入性,带来了额外的开发成本。 如果你实在不能忍受那个报错, 可以使用这种方式, 当然也可以选择无视它
。
具体如何选择还需要各位看官老爷自行斟酌。
以上, 希望对大家有所启发, 谢谢。
参考资料:
https://reactjs.org/blog/2015...
https://github.com/facebook/r...
**粗体** _斜体_ [链接](http://example.com) `代码` - 列表 > 引用
。你还可以使用@
来通知其他用户。