RxJS序列相当于promise.then()？

Question

我过去经常发展很多,现在我转向RxJS.RxJS的文档没有提供关于如何从promise链到观察者序列的非常明确的例子.

例如,我通常用多个步骤编写promise链,比如

// a function that returns a promise
getPromise()
.then(function(result) {
   // do something
})
.then(function(result) {
   // do something
})
.then(function(result) {
   // do something
})
.catch(function(err) {
    // handle error
});

我应该如何以RxJS风格重写这个承诺链？

Answer 1

对于数据流(相当于then):

Rx.Observable.fromPromise(...)
  .flatMap(function(result) {
   // do something
  })
  .flatMap(function(result) {
   // do something
  })
  .subscribe(function onNext(result) {
    // end of chain
  }, function onError(error) {
    // process the error
  });

承诺可以转换为可观察的Rx.Observable.fromPromise.

一些承诺运营商有直接翻译.例如RSVP.all,或者jQuery.when可以替换为Rx.Observable.forkJoin.

请记住,您有一堆允许异步转换数据的运算符,并执行您不能或很难用promises执行的任务.Rxjs使用异步数据序列(序列即超过1个异步值)显示其所有功能.

对于错误管理,主题有点复杂.

• 还有捕获,最后是运营商
• retryWhen 如果出错,也可以帮助重复序列
• 您还可以使用该onError功能处理订户本身的错误.

要获得精确的语义,请深入了解可在Web上找到的文档和示例,或在此处提出具体问题.

对于使用Rxjs进行更深入的错误管理,这肯定是一个很好的起点:https://xgrommx.github.io/rx-book/content/getting_started_with_rxjs/creating_and_querying_observable_sequences/error_handling.html

Answer 2

一个更现代的选择:

import {from as fromPromise} from 'rxjs';
import {catchError, flatMap} from 'rxjs/operators';

fromPromise(...).pipe(
   flatMap(result => {
       // do something
   }),
   flatMap(result => {
       // do something
   }),
   flatMap(result => {
       // do something
   }),
   catchError(error => {
       // handle error
   })
)

Answer 3

Update May 2019, using RxJs 6

Agree with the provided answers above, wished to add a concrete example with some toy data & simple promises (with setTimeout) using RxJs v6 to add clarity.

Just update the passed id (currently hard-coded as 1) to something that does not exist to execute the error handling logic too. Importantly, also note the use of of with catchError message.

import { from as fromPromise, of } from "rxjs";
import { catchError, flatMap, tap } from "rxjs/operators";

const posts = [
  { title: "I love JavaScript", author: "Wes Bos", id: 1 },
  { title: "CSS!", author: "Chris Coyier", id: 2 },
  { title: "Dev tools tricks", author: "Addy Osmani", id: 3 }
];

const authors = [
  { name: "Wes Bos", twitter: "@wesbos", bio: "Canadian Developer" },
  {
    name: "Chris Coyier",
    twitter: "@chriscoyier",
    bio: "CSS Tricks and CodePen"
  },
  { name: "Addy Osmani", twitter: "@addyosmani", bio: "Googler" }
];

function getPostById(id) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const post = posts.find(post => post.id === id);
      if (post) {
        console.log("ok, post found!");
        resolve(post);
      } else {
        reject(Error("Post not found!"));
      }
    }, 200);
  });
}

function hydrateAuthor(post) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const authorDetails = authors.find(person => person.name === post.author);
      if (authorDetails) {
        post.author = authorDetails;
        console.log("ok, post hydrated with author info");
        resolve(post);
      } else {
        reject(Error("Author not Found!"));
      }
    }, 200);
  });
}

function dehydratePostTitle(post) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      delete post.title;
      console.log("ok, applied transformation to remove title");
      resolve(post);
    }, 200);
  });
}

// ok, here is how it looks regarding this question..
let source$ = fromPromise(getPostById(1)).pipe(
  flatMap(post => {
    return hydrateAuthor(post);
  }),
  flatMap(post => {
    return dehydratePostTitle(post);
  }),
  catchError(error => of(`Caught error: ${error}`))
);

source$.subscribe(console.log);

Output Data:

ok, post found!
ok, post hydrated with author info
ok, applied transformation to remove title
{ author:
   { name: 'Wes Bos',
     twitter: '@wesbos',
     bio: 'Canadian Developer' },
  id: 1 }

The key part, is equivalent to the following using plain promise control flow:

getPostById(1)
  .then(post => {
    return hydrateAuthor(post);
  })
  .then(post => {
    return dehydratePostTitle(post);
  })
  .then(author => {
    console.log(author);
  })
  .catch(err => {
    console.error(err);
  });