Question

完整的工作代码和框示例 here

我声明了一个简单的动作类型和一个分派它的异步thunk：

type ActionType = { type: "foo"; v: number };

const FooThunk: ThunkAction<Promise<ActionType>, any, any, ActionType> = (dispatch): Promise<ActionType>
=> {
  return new Promise<number>((resolve) => {
    setTimeout(() => {
      resolve(42);
    }, 100);
  }).then((v: number) => {
    return dispatch({ type: "foo", v });
  });
};

现在的问题是，当我调用dispatch(FooThunk)时得到的值是什么类型。 Typescript认为类型为ThunkAction<Promise<ActionType>, any, any, ActionType>，并在以下消息中抱怨（在沙盒的第38行）：

'ThunkAction<Promise<ActionType>, any, any, ActionType>' is missing the following properties from type 'Promise<ActionType>': then, catch, [Symbol.toStringTag]ts(2739)

但是，当我记录在运行时获得的值时（codesandbox的第48行），我清楚地看到它是Promise。在StackOverflow上搜索时，我发现矛盾的答案。 This answer说，派遣一个thunk会返回thunk本身。 this answer建议派遣一个thunk会返回一个Promise。

Typescript的类型系统似乎表示派发thunk的类型与thunk本身相同。但是在运行时，我得到一个Promise对象。我想念什么？

仅出于完整性目的，我附加了将在沙箱中找到的代码（上面提供的链接）：

import * as React from "react";
import { render } from "react-dom";
import { Provider } from "react-redux";
import { createStore } from "redux";
import { initialState, rootReducer } from "./rootReducer";
import "./styles.css";

import { ThunkDispatch as Dispatch, ThunkAction } from "redux-thunk";
import { connect, ConnectedProps } from "react-redux";

import { applyMiddleware } from "redux";

import thunk from "redux-thunk";

const store = createStore(
  rootReducer /* preloadedState, */,
  applyMiddleware(thunk)
);

//const store = createStore(rootReducer, initialState);

type ActionType = { type: "foo"; v: number };

const FooThunk: ThunkAction<Promise<ActionType>, any, any, ActionType> = (
  dispatch
): Promise<ActionType> => {
  return new Promise<number>((resolve) => {
    setTimeout(() => {
      resolve(42);
    }, 100);
  }).then((v: number) => {
    return dispatch({ type: "foo", v });
  });
};

const mapDispatchToProps = (dispatch: Dispatch<any, any, any>) => {
  return {
    dispatchFooThunk: (): Promise<ActionType> => dispatch(FooThunk)
  };
};
const connector = connect(null, mapDispatchToProps);

type PropsFromRedux = ConnectedProps<typeof connector>;

class FooComponent_ extends React.Component<PropsFromRedux> {
  componentDidMount() {
    const p = this.props.dispatchFooThunk();
    console.log(p); // if you examine log output you see clearly that this is a PROMISE !
  }
  render() {
    return <div>foo</div>;
  }
}

const FooComponent = connector(FooComponent_);

class App extends React.Component {
  render() {
    return (
      <Provider store={store}>
        <FooComponent />
      </Provider>
    );
  }
}

const rootElement = document.getElementById("root");
render(<App />, rootElement);

Answer 1

thunk的返回值是thunk动作本身的返回值。结果是：如果它是一个异步函数，它将返回一个Promise。

普通的Dispatch类型不知道如何处理重击。如果您使用的是redux工具包，this documentation part将介绍如何使用商店配置中推断的Dispatch类型正确键入useDispatch挂钩。如果您使用的是纯Redux，则只能使用ThunkDispatch类型。 This part of the react-redux documentation介绍了在这种情况下如何键入useDispatch挂钩。

PS：在您的特定情况下，您的ThunkDispatch<any, any, any>太普通了。最后一个any会非常匹配。尝试使用ThunkDispatch<any, any, AnyAction>，它将起作用。

打字稿显然为派遣的Thunk推导了错误的类型

1 个答案: