如何在运行下一行代码Angular2之前等待AngularFire2订阅完成

问题描述

我的Angular2 Type Script代码有问题。我尝试通过订阅FirebaseListObserver来访问Firebase数据库中的问题：

  this.af。 list（'/ questions'）。subscribe（val => {
 this.questions = val 
 console.log（this.questions）// works 
}）
 console .log（this.questions）//显示未定义的
  

我不知道如何等待订阅在运行一个依赖于 this.questions 的函数之前获得一个值。

我尝试使用async /等待，但也没有等待订阅获得一个值。

我也尝试在一个承诺里面订阅，而且还返回一个空的数组。您可以从订阅回调中运行该函数，或者您可以在将 questions 转换为setter / getter：

  private _questions = [] ; 
 
设置问题（问题）{
 this._questions = questions; 
 
 //对这里的问题做出反应
} 
 
得到问题（）{
 return this._questions; 
 
 
 
 
 
 $ b如果你熟悉早期版本的话，的角度。因此，在 set 函数中，您可以检查 questions 是否有一个值并作出反应，调用所需的任何函数。  
I have a problem with my Angular2 Type Script code. I'm trying to access Questions from a Firebase Database, which I am doing by subscribing to a FirebaseListObserver:
this.af.list('/questions').subscribe( val => {
     this.questions = val
     console.log(this.questions) // works
})
console.log(this.questions) // displays undefined
I dont know how to wait for the subscription to get a value, though, before running a function which depends on this.questions.
I tried to use async/await, but also didn't wait for the subscription to get a value.
I also tried to subscribe inside a promise, but also returned an empty array.
 解决方案 
You can either run the function from within the subscribe callback or you can turn questions into a setter/getter:
private _questions = [];

set questions(questions) {
  this._questions = questions;

  // React to questions changes here
}

get questions() {
 return this._questions;
}
Which will act kinda like a watcher if you're familiar with the early version of angular. So in the set function you can check if questions has a value and react to it, calling whatever function you need.
                        
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