Question

我是React，Redux和Thunk的新手，并且一直在关注该主题的教程，最近是Building Applications with React and Redux in ES6的Pluralsight。我一直在将本教程从ES6转换为Typescript 3，并将React v3转换为React v4。

按照这种模式，我遇到了很多可以解决的问题（最明显的是与路由相关的问题），但是遇到了无法解决的问题。从各种来源的公司Cannot read property '.then' of undefined when testing async action creators with redux and react 看来，我无法在返回void的函数上使用.then()函数，但它是一个异步函数（应该）返回promise，但是intellisense说除此以外。下面的代码。

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存储，并在单击按钮时调用此函数。上面的函数中的.then()出错了。该函数调用以下操作

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);
        });
    }
}

以上操作是asyc调用，这里的.then()并没有出错，但是VS Code指出整个saveCourse函数都返回void。

在本教程中，实际上没有任何差异应该有所作为（箭头功能代替常规功能等），所以我想知道我缺少的版本之间是否有晦涩的变化，但不确定在哪里外观，可能会缺少一些基本的东西。谁能看到为什么我无法在.then()函数上执行saveCourse()吗？

让我知道是否需要更多信息。

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中兑现承诺。

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fetchSecretSauce产生的承诺。

您需要在示例中类似地返回诺言：

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中返回诺言外，您可能还需要确保在mapDispatchToProps中，应将映射的相应函数声明为async。它可能await位于内部的调度语句。

我遇到了一个完全相同的错误消息，但除了返回承诺外，这是我一开始没有注意到的其他问题。

无法读取Redux thunk操作上未定义的属性'then'

2 个答案: