javascript - 如何从 Promise 返回 Observable

Question

我需要从 Promise 返回 Observable，但是当我尝试学习 Angular 时，我被语法所困扰。所以事情是这样的:

我有一个以下函数 getCountries(这是由另一个实用程序订阅的)，我需要调用它来检索国家/地区列表。“lang”尚未定义。我添加它只是为了完成我的 api 的 url。

getCountries(): Observable<Country[]> {
    if (this.countries) {
       return Observable.of(this.countries);
    } else if (this.countriesObservable) {
      return this.countriesObservable;
    } else {
//lang is not defined yet it has to be retrieved from storage

      this.countriesObservable = this.http.get(AppSettings.API_URL + '?locale=' + lang).map(json => {
        delete this.countriesObservable; // when the cached countries is available we don't need  reference anymore
        this.countries = json as Country[];
        this.countries = this.countries.sort(function (a, b) { return (a.Name > b.Name) ? 1 : ((b.Name > a.Name) ? -1 : 0); });
        return this.countries;
      }).share();
      return this.countriesObservable;
    }
  }

由于 api 需要语言(lang)，并且此信息存储在存储中，可以按如下方式从存储中检索:

  this.storage.get(AppSettings.STORAGE_KEYS.LANGUAGE_APP).then(language=>{
        //here I can use language
      });

现在我遇到的问题是如何在从上述 promise 中检索语言信息后调用我的 api。我需要确保函数(getCountries)仍然返回它最初返回的可观察值。感谢您的帮助。

有些人建议将它们包装在 Observable.fromPromise 中，我也这样做了，但这破坏了代码。我一定错过了一些我知道的东西。这里是:是否错过了返回某些内容，在之后发布我的堆栈跟踪代码:

 getCountries(): Observable<Country[]> {
    if (this.countries) {
       return Observable.of(this.countries);
    } else if (this.countriesObservable) {
      return this.countriesObservable;
    } else {
      Observable.fromPromise(this.storage.get(AppSettings.STORAGE_KEYS.LANGUAGE_APP)).subscribe(language=>{
        //here I can use language
        this.countriesObservable = this.http.get(AppSettings.API_URL + '?locale=' + language).map(json => {
          delete this.countriesObservable; 
          this.countries = json as Country[];
          this.countries = this.countries.sort(function (a, b) { return (a.Name > b.Name) ? 1 : ((b.Name > a.Name) ? -1 : 0); });
          return this.countries;
        }).share();
        return this.countriesObservable;
      });
    }
  }



Error: Uncaught (in promise): TypeError: Cannot read property 'subscribe' of undefined
TypeError: Cannot read property 'subscribe' of undefined
    at http://localhost:8100/build/main.js:71:33
    at new t (http://localhost:8100/build/polyfills.js:3:20886)
    at UtilsProvider.webpackJsonp.10.UtilsProvider.getCountryById (http://localhost:8100/build/main.js:70:16)
    at MyApp.webpackJsonp.851.MyApp.updateUserInfo (http://localhost:8100/build/main.js:10067:47)
    at http://localhost:8100/build/main.js:9774:23
    at SafeSubscriber.schedulerFn [as _next] (http://localhost:8100/build/vendor.js:4004:36)
    at SafeSubscriber.__tryOrUnsub (http://localhost:8100/build/vendor.js:16987:16)
    at SafeSubscriber.next (http://localhost:8100/build/vendor.js:16934:22)
    at Subscriber._next (http://localhost:8100/build/vendor.js:16874:26)
    at Subscriber.next (http://localhost:8100/build/vendor.js:16838:18)
    at c (http://localhost:8100/build/polyfills.js:3:19132)
    at c (http://localhost:8100/build/polyfills.js:3:18841)
    at http://localhost:8100/build/polyfills.js:3:19613
    at t.invokeTask (http://localhost:8100/build/polyfills.js:3:15040)
    at Object.onInvokeTask (http://localhost:8100/build/vendor.js:4238:33)
    at t.invokeTask (http://localhost:8100/build/polyfills.js:3:14961)
    at r.runTask (http://localhost:8100/build/polyfills.js:3:10214)
    at o (http://localhost:8100/build/polyfills.js:3:7274)
    at e.invokeTask [as invoke] (http://localhost:8100/build/polyfills.js:3:16203)
    at p (http://localhost:8100/build/polyfills.js:2:26993)

Answer 1

只需使用 Observable.fromPromise。所以在你的情况下，它会是

getCountries(): Observable<Country[]> {
    if (this.countries) {
       return Observable.of(this.countries);
    } else if (this.countriesObservable) {
      return this.countriesObservable;
    } else {
      return Observable.fromPromise(this.storage.get(AppSettings.STORAGE_KEYS.LANGUAGE_APP)).mergeMap(language=>{
        //here I can use language
        this.countriesObservable = this.http.get(AppSettings.API_URL + '?locale=' + language).map(json => {
          delete this.countriesObservable; // when the cached countries is available we don't need the  reference anymore
          this.countries = json as Country[];
          this.countries = this.countries.sort(function (a, b) { return (a.Name > b.Name) ? 1 : ((b.Name > a.Name) ? -1 : 0); });
          return this.countries;
        }).share();
        return this.countriesObservable;
      });
    }
  }