无法读取 Redux thunk 操作上未定义的属性“then”

Question

我是 React、Redux 和 Thunk 的新手，并且一直在关注有关该主题的教程，最近一次是在 Pluralsight 上的 Building Applications with React and Redux in ES6。我一直在将本教程从 ES6 转换为 Typescript 3，并将 React v3 转换为 React v4。

我遇到了很多我能够按照这种模式解决的问题（尤其是与路由相关的任何问题），但我遇到了一个我无法解决的问题。从各种来源 inc Cannot read property '.then' of undefined when testing async action creators with redux and react 听起来我不能在返回 void 的函数上使用 .then() 函数，但它是一个（应该）返回承诺的异步函数，但是智能感知另有说明。代码如下。

saveCourse = (event: any) => {
    event.preventDefault();
    this.props.actions.saveCourse(this.state.course)
        .then(this.setState({ fireRedirect: true }));
}

上面的代码在我的组件上，props.actions通过mapStateToProps连接到redux store，这个函数在按钮点击时被调用。这是上面这个函数上的.then() 出错了。这个函数调用下面的动作

export const saveCourse = (course: Course) => {
    return (dispatch: any, getState: any) => {
        dispatch(beginAjaxCall());
        courseApi.saveCourse(course).then((savedCourse: Course) => {
            course.id ? dispatch(updateCourseSuccess(savedCourse)) :
                dispatch(createCourseSuccess(savedCourse));
        }).catch(error => {
            throw (error);
        });
    }
}

上面的动作是asyc调用，这里的.then()没有报错但是VS Code说整个saveCourse函数返回void。

从本教程来看，确实没有任何差异应该有所作为（箭头函数而不是常规等......）所以我想知道我缺少的版本之间是否存在模糊的变化，但不确定在哪里看，可能缺少一些基本的东西。谁能明白为什么我不能在saveCourse() 函数上执行.then()？

如果您需要更多信息，请告诉我。

编辑：courseApi 只是一个模拟 api，代码如下。

import delay from './delay';

const courses = [
{
    id: "react-flux-building-applications",
    title: "Building Applications in React and Flux",
    watchHref: "http://www.pluralsight.com/courses/react-flux-building-applications",
    authorId: "cory-house",
    length: "5:08",
    category: "JavaScript"
},
{
    id: "clean-code",
    title: "Clean Code: Writing Code for Humans",
    watchHref: "http://www.pluralsight.com/courses/writing-clean-code-humans",
    authorId: "cory-house",
    length: "3:10",
    category: "Software Practices"
},
{
    id: "architecture",
    title: "Architecting Applications for the Real World",
    watchHref: "http://www.pluralsight.com/courses/architecting-applications-dotnet",
    authorId: "cory-house",
    length: "2:52",
    category: "Software Architecture"
},
{
    id: "career-reboot-for-developer-mind",
    title: "Becoming an Outlier: Reprogramming the Developer Mind",
    watchHref: "http://www.pluralsight.com/courses/career-reboot-for-developer-mind",
    authorId: "cory-house",
    length: "2:30",
    category: "Career"
},
{
    id: "web-components-shadow-dom",
    title: "Web Component Fundamentals",
    watchHref: "http://www.pluralsight.com/courses/web-components-shadow-dom",
    authorId: "cory-house",
    length: "5:10",
    category: "HTML5"
}
];

function replaceAll(str: any, find: any, replace: any) {
return str.replace(new RegExp(find, 'g'), replace);
}

//This would be performed on the server in a real app. Just stubbing in.
const generateId = (course: any) => {
return replaceAll(course.title, ' ', '-');
};

class CourseApi {
static getAllCourses() {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(Object.assign([], courses));
        }, delay);
    });
}

static saveCourse(course: any) {
    course = Object.assign({}, course); // to avoid manipulating object         passed in.
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            // Simulate server-side validation
            const minCourseTitleLength = 1;
            if (course.title.length < minCourseTitleLength) {
                reject(`Title must be at least ${minCourseTitleLength} characters.`);
            }

            if (course.id) {
                const existingCourseIndex = courses.findIndex(a => a.id == course.id);
                courses.splice(existingCourseIndex, 1, course);
            } else {
                //Just simulating creation here.
                //The server would generate ids and watchHref's for new courses in a real app.
                //Cloning so copy returned is passed by value rather than by reference.
                course.id = generateId(course);
                course.watchHref = `http://www.pluralsight.com/courses/${course.id}`;
                courses.push(course);
            }

            resolve(course);
        }, delay);
    });
}

static deleteCourse(courseId: any) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            const indexOfCourseToDelete = courses.findIndex(course =>
                course.id == courseId
            );
            courses.splice(indexOfCourseToDelete, 1);
            resolve();
        }, delay);
    });
}
}

export default CourseApi;

Answer 1

您遇到的问题是您没有在 Action Creator 中返回承诺。

在这里查看 redux-thunk 的示例：

https://github.com/reduxjs/redux-thunk

function makeASandwichWithSecretSauce(forPerson) {

  // Invert control!
  // Return a function that accepts `dispatch` so we can dispatch later.
  // Thunk middleware knows how to turn thunk async actions into actions.

  return function (dispatch) {
    return fetchSecretSauce().then(
      sauce => dispatch(makeASandwich(forPerson, sauce)),
      error => dispatch(apologize('The Sandwich Shop', forPerson, error))
    );
  };
}

他们正在返回由fetchSecretSauce 生成的承诺。

您需要在示例中类似地返回承诺：

export const saveCourse = (course: Course) => {
    return (dispatch: any, getState: any) => {
        dispatch(beginAjaxCall());
        return courseApi.saveCourse(course).then((savedCourse: Course) => {
            course.id ? dispatch(updateCourseSuccess(savedCourse)) :
                dispatch(createCourseSuccess(savedCourse));
        }).catch(error => {
            throw (error);
        });
    }
}

Answer 2

除了在 Action Creator 中返回承诺之外，您可能还想确保在您的 mapDispatchToProps 中，映射的相应函数应声明为 async。可能是await里面的dispatch语句。

我遇到了类似的问题，错误消息完全相同，但除了返回承诺之外，这是我一开始没有注意到的其他问题。