AngularFire2 Firestore Take(1)对文档值的更改

问题描述

使用AngularFire2，我尝试从单个Firestore文档中获取数据. 有效的方法是:

Using AngularFire2 I try to get data from a single Firestore document. What works is:

this.addressRef = afs.doc<Address>('addresses/key')
this.addressRef.valueChanges().subscribe(val => {......}

我只想运行一次订阅代码.通常，我使用take(1)实现此目的(与Firebase DB一起使用). 但是此代码:

I want to run the subscription code just once. Normally I use take(1) to achieve this (works with the Firebase DB). But this code:

this.addressRef.valueChanges().take(1).subscribe(val => {......}

给出错误:TypeError:this.addressRef.valueChanges(...).take不是函数.

gives error: TypeError: this.addressRef.valueChanges(...).take is not a function.

VS Code列出了执行动作.这是AngularFire2中的bug，是我做错了还是获取数据后停止订阅的另一种(更好)方式?我也尝试了topromise，但这也失败了.

VS Code is listing the take action. Is this a bug in AngularFire2, am I doing this wrong or is there an other (better) way to stop the subscription after the data is fetched? I also tried topromise but this also failed.

推荐答案

更新

由于此答案已引起广泛关注，因此我提供了更新的答案.通常，您应该在使用可观察变量进行编程时尝试做出反应.有一些很棒的教程，但是这是很好的

Because this answer has received a lot of attention, I have provided an updated answer. You should, in general, try to think reactively when programming with observables. There are some great tutorials, but this one is good.

新解决方案

user$ = this.afDs.doc<User>('users/NaGjauNy3vYOzp759xsc')
  .valueChanges().pipe(
    take(1) // Here you can limit to only emit once, using the take operator
  )

在模板中，您订阅了可观察对象.

And in your template, you subscribe to your observable.

{{(user$ | async).name}}

上下文

这种解决问题的方法有很多好处:

This way of solving the problem has multiple benefits:

• 如果您不希望转换数据，只需抛出一个地图运算符
• 您可以使用rxjs过滤器运算符过滤事件
• 基于用户$流创建其他流
• 易于调试，因为其他开发人员可以更透明的方式查看应用程序中的数据流

例如(创建另一个流，该流仅在用户为admin时发出，并创建一个新标题):

For example (creating another stream, that only emits if the user is admin, and creates a new title):

adminTitle$ = this.user$.pipe(
  filter(user => user.title === 'admin'),
  map(user => `${user.name} (admin)`)
)

旧答案

您可以通过执行以下操作来实现:

You can achieve this by doing the following:

this.userDoc = afDs.doc<User>('users/NaGjauNy3vYOzp759xsc');
this.userDoc.valueChanges()
  .pipe(take(1))
  .subscribe(v => {
    this.user = v;
  });

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