承诺并订阅 [英] Promise and subscribe

问题描述

我有一个Angular2(ionic2)应用程序.我有一个请求城市的函数，但我收到一个错误，指出this.cityService.getAllCities()上不存在属性订阅.

I have an Angular2 (ionic2) application. I have a function that request cities but I get an error that Property subscribe does not exists on this.cityService.getAllCities().

cityPage.ts具有如下功能:

cityPage.ts has a function like this:

getCities(){
    this.cityService.getAllCities()
          .subscribe(cityData => { this.cityList = cityData; },
            err => console.log(err),
            () => console.log('Complete!')
    );
}

我的cityService.getAllCities()函数如下所示:

my cityService.getAllCities() function looks like this:

getAllCities(){

    return new Promise (resolve => {
        this.storage.ready().then(() => {

            this.storage.get('authData').then(authData => {
              let hdr = new Headers({'Content-Type': 'application/json', 'Authorization': 'Bearer ' +
                authData.access_token });
              let opt = new RequestOptions({ headers: hdr });
                return this.http.get(AppSettings.API_GET_CITIES).map(res => <CityModel[]> res.json(), opt);
            }).catch(() => {
              //resolve(false);
            });

        });

    });

  }

修改

根据评论，我已更改了如下功能:

Based on the comment I've changed my function like this:

getAllCities(){

    return Observable.create(resolve => {
        this.storage.ready().then(() => {

            this.storage.get('authData').then(authData => {
              let hdr = new Headers({'Content-Type': 'application/json', 'Authorization': 'Bearer ' +
                authData.access_token });

                console.log('access_token ' + authData.access_token);
              let opt = new RequestOptions({ headers: hdr });
                 return this.http.get(AppSettings.API_GET_CITIES,opt).map(res => <CityModel[]> res.json()).subscribe((result) => {
                  console.log(result);
                  resolve = result;
                });
            }).catch(() => {
              //resolve(false);
            });

        });

    });

  }

在我的console.log(result)中，我接收到数据，但是数据从未返回到我的getCities()函数中.同样，console.log('Complete!')也不会被调用.

In my console.log(result) I receive data, but the data is never returned to my getCities() function. Also the console.log('Complete!') is not called.

推荐答案

之所以引发错误，是因为.subscribe方法确实在Observable上可用，只要它发出数据就可以监听.在这里，从getAllCities方法返回的是promise，您可以在其上应用.then函数以获取从该Promise

The reason it is throwing an error, because .subscribe method does available on Observable to listen to, whenever it emits a data. Here from getAllCities method you're returning a promise you could apply .then function over it to get data returned from that Promise

getCities() {
  this.cityService.getAllCities()
    .then(
       cityData => { this.cityList = cityData; },
       err => console.log(err),
       () => console.log('Complete!')
  );
}

还可以通过在http.get() Observable上调用.toPromise()方法来从getAllCities方法返回promise.

And also return promise from getAllCities method by calling .toPromise() method over http.get() Observable.

getAllCities(){

    return new Promise (resolve => {
        this.storage.ready().then(() => {

            this.storage.get('authData').then(authData => {
              let hdr = new Headers({'Content-Type': 'application/json', 'Authorization': 'Bearer ' +
                authData.access_token });
              let opt = new RequestOptions({ headers: hdr });
              //returned promise from here.
                return this.http.get(AppSettings.API_GET_CITIES)
                   .map(res => <CityModel[]> res.json(), opt)
                   .toPromise();
            }).catch(() => {
              //resolve(false);
            });
        });
    });
}

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