blocks|key|4541708|text|不是百分之百确定您需要什么，但是如果您只想观察第一个值，那么使用first()或take(1)|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4541709|observable.first().subscribe(func);|code-block|syntax|javascript|4541710|注意:当满足条件时，.take(1)和.first()都会自动取消订阅|4541711|从RxJS+5.5%2B更新|4541712|来自Coderer的评论。|4541713|import+{+first+}+from+'rxjs/operators'
++++
observable
++.pipe(first())
++.subscribe(func);|4541714|Here's+why|4541715|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/28007777/create-one-time-subscription/28030416#comment91800031_28030416|1|https://github.com/ReactiveX/rxjs/blob/22c6137116268af91fafa14cc89d5db4f234bec3/doc/pipeable-operators.md#why^0|W|7|14|7|0|0|A|8|J|8|0|0|2|7|0|0|0|0|A|1|0^^$0|@$1|2|3|4|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1A|8|@$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|1F|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|1G|8|@]|D|@$9|1H|A|1I|1|1J]]|E|$]]|$1|Q|3|R|5|H|7|1K|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1L|8|@]|D|@$9|1M|A|1N|1|1O]]|E|$]]|$1|U|3|-4|5|6|7|1P|8|@]|D|@]|E|$]]]|V|$W|$5|X|Y|Z|E|$10|11]]|12|$5|X|Y|Z|E|$10|13]]]]

Not 100% certain about what you need, but if you only want to observe the first value, then use either <code>first()</code> or <code>take(1)</code>:
<pre><code>observable.first().subscribe(func);
</code></pre>
note: <code>.take(1)</code> and <code>.first()</code> both unsubscribe automatically when their condition is met
<h1>Update from RxJS 5.5+</h1>
From comment by <a href="https://stackoverflow.com/questions/28007777/create-one-time-subscription/28030416#comment91800031_28030416">Coderer</a>.
<pre class="lang-js prettyprint-override"><code>import { first } from 'rxjs/operators'
 
observable
 .pipe(first())
 .subscribe(func);
</code></pre>
<a href="https://github.com/ReactiveX/rxjs/blob/22c6137116268af91fafa14cc89d5db4f234bec3/doc/pipeable-operators.md#why" rel="noreferrer">Here's why</a>

blocks|key|4541743|text|RxJS有一些我见过的最好的文档。点击下面的链接，你会看到一个非常有用的表格，它将用例映射到运算符。例如，在“我想取第一个值”用例下有三个运算符：first、firstOrDefault和sample。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4541744|请注意，如果一个可观察的序列在没有通知的情况下完成，则first操作符通知订阅者一个错误，而firstOrDefault操作符向订阅者提供默认值。|4541745|operator+use+case+lookup|4541746|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/Reactive-Extensions/RxJS/blob/master/doc/gettingstarted/which-instance.md^0|21|5|27|E|2M|6|0|R|5|1A|E|0|0|O|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|13|8|@]|D|@$9|14|A|15|1|16]]|E|$]]|$1|J|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|K|$L|$5|M|N|O|E|$P|Q]]]]

RxJS has some of the best documentation I've ever come across. Following the bellow link will take you to an exceedingly helpful table mapping use cases to operators. For instance, under the "I want to take the first value" use case are three operators: <code>first</code>, <code>firstOrDefault</code>, and <code>sample</code>.

Note, if an observable sequence completes with no notifications, then the <code>first</code> operator notifies subscribers with an error while the <code>firstOrDefault</code> operator provides a default value to subscribers.

<a href="https://github.com/Reactive-Extensions/RxJS/blob/master/doc/gettingstarted/which-instance.md">operator use case lookup</a>

blocks|key|34591|text|为了补充@Brandon's+answer，使用first()等对于基于其Observable更新BehaviorSubject也是必不可少的。例如(未测试)：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|34592|var+subject+=+new+BehaviorSubject({1:'apple',2:'banana'});
var+observable+=+subject.asObservable();

observable
++.pipe(
++++first(),+//+<--+Ensures+no+stack+overflow
++++flatMap(function(obj)+{
++++++obj[3]+=+'pear';
++++++return+of(obj);
++++})
++)
++.subscribe(function(obj)+{
++++subject.next(obj);
++});|code-block|syntax|javascript|34593|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/28030416/493807^0|O|7|11|A|1D|F|4|H|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

To supplement <a href="https://stackoverflow.com/a/28030416/493807">@Brandon's answer</a>, using <code>first()</code> or the like is also essential for updating a <code>BehaviorSubject</code> based on its <code>Observable</code>. For example (untested):

<pre><code>var subject = new BehaviorSubject({1:'apple',2:'banana'});
var observable = subject.asObservable();

observable
 .pipe(
 first(), // &lt;-- Ensures no stack overflow
 flatMap(function(obj) {
 obj[3] = 'pear';
 return of(obj);
 })
 )
 .subscribe(function(obj) {
 subject.next(obj);
 });
</code></pre>

blocks|key|4541805|text|干净方便的版本|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4541806|在M+Fuat+NUROĞLU关于将可观察对象转换为承诺的惊人答案的基础上进行扩展，这里是它非常方便的版本。|4541807|const+value+=+await+observable.toPromise();

console.log(value)|code-block|syntax|javascript|4541808|这样做的美妙之处在于，我们可以像使用普通变量一样使用该值，而无需引入另一个嵌套块！|4541809|当您需要从多个观察值中获取多个值时，这特别方便。整洁干净。|4541810|const+content+=+await+contentObservable.toPromise();
const+isAuthenticated+=+await+isAuthenticatedObservable.toPromise();

if(isAuthenticated){
+++service.foo(content)
}|4541811|当然，如果您要使用此方法，则必须将包含函数设置为async。如果不希望包含函数是异步的，也可以只对promise执行.then操作|offset|length|style|CODE|4541812|我不确定这种方法是否有权衡，请随时在评论中让我知道，这样我们就知道了。|4541813|附注:如果你喜欢这个答案，别忘了给M+Fuat+NUROĞLU的答案加票:)|4541814|entityMap^0|0|0|0|0|0|0|O|5|1M|5|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|13|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|15|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|16|8|@$Q|17|R|18|S|T]|$Q|19|R|1A|S|T]]|9|@]|A|$]]|$1|U|3|V|5|6|7|1B|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|Z|$]]

<h1>Clean and Convenient Version</h1>

Expanding on M Fuat NUROĞLU's amazing answer on converting the observable to a promise, here's the very convenient version of it.

<pre class="lang-js prettyprint-override"><code>const value = await observable.toPromise();

console.log(value)
</code></pre>

The beauty of this is that we can use that value like a normal variable without introducing another nested block!

This is especially handy when you need to get multiple values from multiple observables. Neat and clean.

<pre class="lang-js prettyprint-override"><code>const content = await contentObservable.toPromise();
const isAuthenticated = await isAuthenticatedObservable.toPromise();

if(isAuthenticated){
 service.foo(content)
}
</code></pre>

Of course, you will have to make your containing function <code>async</code> if you are to go with this route. You can also just <code>.then</code> the promise if you don't want the containing function to be async

I'm not sure if there are tradeoffs with this approach, feel free to let me know in the comments so we are aware.

P.S. If you liked this answer, don't forget to upvote M Fuat NUROĞLU's Answer as well :)

blocks|key|34666|text|我也有类似的问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|34667|下面是后来从不同的状态转换器调用的。因为我不想这么做。|34668|function+foo()+{
++++//+this+was+called+many+times+which+was+not+needed
++++observable.subscribe(func);
++++changeObservableState("new+value");
}|code-block|syntax|javascript|34669|我决定在订阅后试用unsubscribe()，如下所示。|offset|length|style|CODE|34670|function+foo()+{
++++//+this+was+called+ONE+TIME
++++observable.subscribe(func).unsubscribe();
++++changeObservableState("new+value");
}|34671|subscribe(func).unsubscribe();就像subscribeOnce(func)一样。|34672|我希望这对你也有帮助。|34673|entityMap^0|0|0|0|9|D|0|0|0|U|W|J|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|Z|8|@$K|10|L|11|M|N]]|9|@]|A|$]]|$1|O|3|P|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|Q|3|R|5|6|7|13|8|@$K|14|L|15|M|N]|$K|16|L|17|M|N]]|9|@]|A|$]]|$1|S|3|T|5|6|7|18|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|19|8|@]|9|@]|A|$]]]|V|$]]

I had similar question.

Below was called even later on from different state changers. As I did not want to.

<pre class="lang-js prettyprint-override"><code>function foo() {
 // this was called many times which was not needed
 observable.subscribe(func);
 changeObservableState("new value");
}
</code></pre>

I decided to try <code>unsubscribe()</code> after subscribe as below.

<pre class="lang-js prettyprint-override"><code>function foo() {
 // this was called ONE TIME
 observable.subscribe(func).unsubscribe();
 changeObservableState("new value");
}
</code></pre>

<code>subscribe(func).unsubscribe();</code> is like <code>subscribeOnce(func)</code>.

I hope that helped you too.

blocks|key|34697|text|observable.pipe(+take+(1)).subscribe()使用take+1订阅一次然后退出|type|unstyled|depth|inlineStyleRanges|entityRanges|data|34698|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

observable.pipe(take(1)).subscribe() use take 1 it subscribe for one time then exit

I need to create a subscription to an <code>Observable</code> that is immediately disposed of when it is first called.

Is there something like:

<pre><code>observable.subscribeOnce(func);
</code></pre>

My use case, I am creating a subscription in an express route handler and the subscription is being called multiple times per request.

Create one-time subscription

我需要创建一个对Observable的订阅，它在第一次被调用时会立即被处理掉。有没有类似这样的东西：observable.subscribeOnce(func);我的用例是，我在一个快速路由处理程序中创建了一个订阅，该订阅在每个请求中被多次调用。

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