如何在订阅Angular 4中返回值 [英] How to return value inside subscribe Angular 4

问题描述

我对角度可观测对象是陌生的.我有一个问题想要在订阅方法内返回一个值.我有 以下方法(getFirebaseData(idForm:string):observable <any[]>):

I'm new to observables in angular. I have a problem wanting to return a value inside a subscribe method. I have the following method (getFirebaseData(idForm:string):observable <any[]>):

getTotalQuestions(idForm:string){
let totalQuestions:number;
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => 
  {
    items.map(item => {
      totalQuestions=item.Total;
      console.log(totalQuestions);
    });
  }
);
console.log(totalQuestions);
return totalQuestions;
}

第一个console.log(totalQuestions)打印 4 ，第二个console.log(totalQuestions)打印未定义. 我知道订阅是异步操作，因此第二个console.log(totalQuestions)(为了编写代码")打印时未定义，但是在订阅方法完成后，我找不到返回变量的方法.现在，如果我更改对地图的订阅:

the firts console.log(totalQuestions) print 4 but the second console.log(totalQuestions) print undefined. I understand that subscribe is an asynchronous operation and that for that reason the second console.log(totalQuestions) ("In order to write the code") print undefined, but I can not find the way to return the variable after the subscribe method has been completed. Now, if I change the subscribe to map:

getTotalQuestions(idForm:string){
let totalQuestions:number;
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => 
  {
    items.map(item => {
      totalQuestions=item.Total;
      console.log(totalQuestions);
    });
  }
);
console.log(totalQuestions);
return totalQuestions;
}

第一个console.log(totalQuestions)不打印任何内容，而第二个console.log(totalQuestions)不打印任何内容.这是我不了解的，因为它会发生.

the firts console.log(totalQuestions) does not print anything and the second console.log(totalQuestions) print undefined. It's something that I do not understand because it happens.

希望您能帮助我阐明我不理解的概念.谢谢！

I hope you can help me clarify the concept that I do not understand. Thanks!

推荐答案

您不能像这样直接返回 totalQuestions ，您需要使用一个主题来实现.

You can't directly return totalQuestions like that, you need to use a subject to achieve that.

getTotalQuestions(idForm:string): Observable<string> {
let totalQuestions:number;
var subject = new Subject<string>();
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => {
    items.map(item => {

      totalQuestions=item.Total;
      console.log(totalQuestions);
      subject.next(totalQuestions);
    });
  }
);
  return subject.asObservable();
}

用法:getTotalQuestion(idForm).subscribe((r)=>console.log(r))

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