如何從Promise返回Observable

Question

我需要從promise中返回Observable，但是當我嘗試學習angular時，我對語法感到困惑。

我有一個以下函數getCountries（該函數由另一個實用程序訂閱），在這里我需要調用以檢索國家列表.'lang'尚未定義，我補充說這只是為了完成我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;
    }
  }

由於api需要language（lang）並且此信息存儲在存儲中，可以按以下方式從存儲中檢索：

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

現在，我受困的地方是在我從上述承諾中檢索語言信息后如何調用我的api。 我需要確保該函數（getCountries）仍然返回它最初返回的可觀察值。 謝謝你的幫助。

有人建議將它們包裝在Observable.fromPromise中，我也做了同樣的事情，但是這破壞了代碼。我肯定錯過了我所知道的東西。在這里是：它錯過了返回的東西，在代碼后發布了我的堆棧跟蹤信息：

 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

可以通過： Observable.fromPromise()將Promise（輕松）轉換為Observable。

Observable.fromPromise(this.storage.get(AppSettings.STORAGE_KEYS.LANGUAGE_APP))
.subscribe((language) => {
    //here you can use language
});

Answer 2

只需使用Observable.fromPromise。 所以在你的情況下

 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; }); } }