为什么 react 异步接收 redux 状态的更新 [英] Why react receives updates for redux state asynchronously
问题描述
我的 React/Redux 应用程序有一个小问题.
当我调度一个动作来改变我的 redux 状态时,React 组件异步接收改变的状态.但我想同步处理这些变化.
代码在这里:
//组件类 App 扩展了 React.Component {handleButtonChange = e =>{this.props.change("Hello World");console.log(this.props.title);//打印旧值setTimeout(() => {console.log(this.props.title);//打印新值}, 100);};使成为() {返回 (<div className="应用程序"><button onClick={this.handleButtonChange}>更改标题按钮>
);}}const mapStateToProps = state =>({标题:state.title});const mapDispatchToProps = dispatch =>({更改:标题 =>调度(更改(标题))});导出默认连接(mapStateToProps,mapDispatchToProps)(应用程序);
有什么想法吗?
完整示例在这里:
<小时>因此,如果你想第一次使用新的计算道具,它被更新了,你应该在componentDidUpdate()
不过,我也看到你想在分派动作后直接使用新的props
:
this.props.change("Hello World");//到此为止,创建一个新组件,合并 props,然后执行下一行console.log(this.props.title);
我认为这在某种程度上是不可能的.
返工演示:https://codesandbox.io/s/wol55qqx0k
I have a little problem in my React/Redux application.
When I dispatch an action to change my redux state, React component receives the changed state asynchronously. But I want to handle the changes synchronously.
The code is here:
// Component
class App extends React.Component {
handleButtonChange = e => {
this.props.change("Hello World");
console.log(this.props.title); // print old value
setTimeout(() => {
console.log(this.props.title); // print new value
}, 100);
};
render() {
return (
<div className="App">
<button onClick={this.handleButtonChange}>
Change the title
</button>
</div>
);
}
}
const mapStateToProps = state => ({
title: state.title
});
const mapDispatchToProps = dispatch => ({
change: title => dispatch(change(title))
});
export default connect(
mapStateToProps,
mapDispatchToProps
)(App);
Any ideas?
The full example is here: https://codesandbox.io/s/lxjkvvk3pl
The connect()
method, it does not modify the component class passed to it; instead, it returns a new, connected component class for you to use.
The mapStateToProps()
function is called any time the connector component needs to compute new props, as a result of a store state change.
Reference: react-redux api
When you dispatch an action:
this.props.change("Hello World");
console.log(this.props.title);
Which will make the store state change, as a result, the mapStateToProps()
function will be invoked and the connect()
function will get the current component and return for you a new component with the updated props
.
The console.log(this.props.title)
is executed with the value of the property title of the old props object of the old component.
I put some lifecycle hooks in your codes to see what actually happened:
import React from "react";
import { connect } from "react-redux";
import { change } from "./actions";
class App extends React.Component {
constructor(props) {
super(props);
console.log("contructor(): " + this.props.title);
}
handleButtonChange = e => {
this.props.change("Hello World");
console.log("handleButtonChange(): " + this.props.title);
};
componentDidMount() {
console.log("componentDidMount(): " + this.props.title);
}
componentDidUpdate(prevProps) {
// Typical usage (don't forget to compare props):
if (this.props.title !== prevProps.title) {
console.log("componentDidUpdate(): " + this.props.title);
}
}
componentWillUnmount() {
console.log("componentWillUnmount(): " + this.props.title);
}
render() {
console.log("render(): " + this.props.title);
return (
<div className="App">
<button onClick={this.handleButtonChange}>Change the title</button>
</div>
);
}
}
const mapStateToProps = state => ({
title: state.title
});
const mapDispatchToProps = dispatch => ({
change: title => dispatch(change(title))
});
export default connect(
mapStateToProps,
mapDispatchToProps
)(App);
And the results after clicking four times on the button are:
Therefore, in case you want to use the new computed props for the first time, it's updated, you should do it in the componentDidUpdate()
However, I also see that you want to use the new props
directly after dispatching an action:
this.props.change("Hello World");
// stop here, creating a new component, merging props, then executing the next line
console.log(this.props.title);
I think that's somehow impossible.
The rework demo: https://codesandbox.io/s/wol55qqx0k
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