blocks|key|4417724|text|Subject或Observable没有当前值。当发出一个值时，它会被传递给订阅者，然后Observable就会使用它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4417725|如果你想要一个当前值，使用BehaviorSubject，它就是为这个目的而设计的。BehaviorSubject保留最后发出的值，并立即将其发送给新订阅者。|4417726|它还有一个getValue()方法来获取当前值。|4417727|entityMap^0|0|7|8|A|18|A|0|D|F|16|F|0|5|A|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|K|$]]

A <code>Subject</code> or <code>Observable</code> doesn't have a current value. When a value is emitted, it is passed to subscribers and the <code>Observable</code> is done with it.

If you want to have a current value, use <code>BehaviorSubject</code> which is designed for exactly that purpose. <code>BehaviorSubject</code> keeps the last emitted value and emits it immediately to new subscribers.

It also has a method <code>getValue()</code> to get the current value.

blocks|key|4417897|text|我也遇到过类似的情况，即延迟订阅的用户在主题价值到达后才订阅该主题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4417898|我发现与BehaviorSubject类似的ReplaySubject在这种情况下很有魅力。这里有一个更好的解释的链接：http://reactivex.io/rxjs/manual/overview.html#replaysubject|offset|length|4417899|entityMap|0|LINK|mutability|MUTABLE|url|http://reactivex.io/rxjs/class/es6/ReplaySubject.js~ReplaySubject.html|1|http://reactivex.io/rxjs/manual/overview.html#replaysubject^0|0|M|D|0|1O|1N|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@$D|R|E|S|1|T]|$D|U|E|V|1|W]]|A|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]|N|$5|I|J|K|A|$L|O]]]]

I had similar situation where late subscribers subscribe to the Subject after its value arrived. 

I found <a href="http://reactivex.io/rxjs/class/es6/ReplaySubject.js~ReplaySubject.html" rel="noreferrer">ReplaySubject</a> which is similar to BehaviorSubject works like a charm in this case.
And here is a link to better explanation: <a href="http://reactivex.io/rxjs/manual/overview.html#replaysubject" rel="noreferrer">http://reactivex.io/rxjs/manual/overview.html#replaysubject</a>

blocks|key|4417948|text|我在子组件中遇到了同样的问题，最初它必须具有主题的当前值，然后订阅主题以侦听更改。我只是维护服务中的当前值，以便组件可以访问它，例如：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4417949|import+{Storage}+from+'./storage';
import+{Injectable}+from+'angular2/core';
import+{Subject}++++from+'rxjs/Subject';

@Injectable()
export+class+SessionStorage+extends+Storage+{

++isLoggedIn:+boolean;

++private+_isLoggedInSource+=+new+Subject<boolean>();
++isLoggedIn+=+this._isLoggedInSource.asObservable();
++constructor()+{
++++super('session');
++++this.currIsLoggedIn+=+false;
++}
++setIsLoggedIn(value:+boolean)+{
++++this.setItem('_isLoggedIn',+value,+()+=>+{
++++++this._isLoggedInSource.next(value);
++++});
++++this.isLoggedIn+=+value;
++}
}|code-block|syntax|javascript|4417950|需要当前值的组件可以直接从服务访问它，即：|4417951|sessionStorage.isLoggedIn|4417952|不确定这是否是正确的做法:)|4417953|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I encountered the same problem in child components where initially it would have to have the current value of the Subject, then subscribe to the Subject to listen to changes. I just maintain the current value in the Service so it is available for components to access, e.g. :

<pre><code>import {Storage} from './storage';
import {Injectable} from 'angular2/core';
import {Subject} from 'rxjs/Subject';

@Injectable()
export class SessionStorage extends Storage {

 isLoggedIn: boolean;

 private _isLoggedInSource = new Subject&lt;boolean&gt;();
 isLoggedIn = this._isLoggedInSource.asObservable();
 constructor() {
 super('session');
 this.currIsLoggedIn = false;
 }
 setIsLoggedIn(value: boolean) {
 this.setItem('_isLoggedIn', value, () =&gt; {
 this._isLoggedInSource.next(value);
 });
 this.isLoggedIn = value;
 }
}
</code></pre>

A component that needs the current value could just then access it from the service, i.e,:

<pre><code>sessionStorage.isLoggedIn
</code></pre>

Not sure if this is the right practice :)

blocks|key|4417841|text|您可以将最后一个发射值与可观察值分开存储。然后在需要的时候阅读它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4417842|let+lastValue:+number;

const+subscription+=+new+Service().start();
subscription
++++.subscribe((data)+=>+{
++++++++lastValue+=+data;
++++}
);|code-block|syntax|javascript|4417843|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You could store the last emitted value separately from the Observable. Then read it when needed.

<pre><code>let lastValue: number;

const subscription = new Service().start();
subscription
 .subscribe((data) =&gt; {
 lastValue = data;
 }
);
</code></pre>

blocks|key|4418032|text|const+observable+=+of('response')

function+hasValue(value:+any)+{
++return+value+!==+null+&&+value+!==+undefined;
}

function+getValue<T>(observable:+Observable<T>):+Promise<T>+{
++return+observable
++++.pipe(
++++++filter(hasValue),
++++++first()
++++)
++++.toPromise();
}

const+result+=+await+getValue(observable)
//+Do+the+logic+with+the+result
//+.................
//+.................
//+.................|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|4418033|你可以在这里查看关于如何实现它的完整文章。https://www.imkrish.com/blog/development/simple-way-get-value-from-observable|unstyled|offset|length|4418034|entityMap|0|LINK|mutability|MUTABLE|url|https://www.imkrish.com/blog/development/simple-way-get-value-from-observable^0|0|L|25|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|R|8|@]|9|@$G|S|H|T|1|U]]|A|$]]|$1|I|3|-4|5|F|7|V|8|@]|9|@]|A|$]]]|J|$K|$5|L|M|N|A|$O|P]]]]

<pre><code>const observable = of('response')

function hasValue(value: any) {
 return value !== null &amp;&amp; value !== undefined;
}

function getValue&lt;T&gt;(observable: Observable&lt;T&gt;): Promise&lt;T&gt; {
 return observable
 .pipe(
 filter(hasValue),
 first()
 )
 .toPromise();
}

const result = await getValue(observable)
// Do the logic with the result
// .................
// .................
// .................
</code></pre>
You can check the full article on how to implement it from here.
<a href="https://www.imkrish.com/blog/development/simple-way-get-value-from-observable" rel="nofollow noreferrer">https://www.imkrish.com/blog/development/simple-way-get-value-from-observable</a>

blocks|key|4417933|text|最好的方法是使用Behaviur+Subject，下面是一个示例：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4417934|var+sub+=+new+rxjs.BehaviorSubject([0,+1])
sub.next([2,+3])
setTimeout(()+=>+{sub.next([4,+5])},+1500)
sub.subscribe(a+=>+console.log(a))+//2,+3+(current+value)+->+wait+2+sec+->+4,+5|code-block|syntax|javascript|4417935|entityMap^0|8|G|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

The best way to do this is using <code>Behaviur Subject</code>, here is an example:

<pre><code>var sub = new rxjs.BehaviorSubject([0, 1])
sub.next([2, 3])
setTimeout(() =&gt; {sub.next([4, 5])}, 1500)
sub.subscribe(a =&gt; console.log(a)) //2, 3 (current value) -&gt; wait 2 sec -&gt; 4, 5
</code></pre>

blocks|key|4418004|text|可以创建订阅，并在获取第一个发出的项后将其销毁。管道是一个函数，它使用一个可观察对象作为其输入，并返回另一个可观察对象作为输出，而不修改第一个可观察对象。Angular+8.1.0。软件包："rxjs":+"6.5.3"、"rxjs-observable":+"0.0.7"|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4418005|++ngOnInit()+{

++++...

++++//+If+loading+with+previously+saved+value
++++if+(this.controlValue)+{

++++++//+Take+says+once+you+have+1,+then+close+the+subscription
++++++this.selectList.pipe(take(1)).subscribe(x+=>+{
++++++++let+opt+=+x.find(y+=>+y.value+===+this.controlValue);
++++++++this.updateValue(opt);
++++++});

++++}
++}|code-block|syntax|javascript|4418006|entityMap^0|2N|F|33|Q|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

A subscription can be created and after taking the first emitted item destroyed. Pipe is a function that uses an Observable as its input and returns another Observable as output, while not modifying the first observable. Angular 8.1.0. Packages: <code>&quot;rxjs&quot;: &quot;6.5.3&quot;</code>, <code>&quot;rxjs-observable&quot;: &quot;0.0.7&quot;</code>
<pre><code> ngOnInit() {

 ...

 // If loading with previously saved value
 if (this.controlValue) {

 // Take says once you have 1, then close the subscription
 this.selectList.pipe(take(1)).subscribe(x =&gt; {
 let opt = x.find(y =&gt; y.value === this.controlValue);
 this.updateValue(opt);
 });

 }
 }
</code></pre>

I have an Angular 2 service:

<pre><code>import {Storage} from './storage';
import {Injectable} from 'angular2/core';
import {Subject} from 'rxjs/Subject';

@Injectable()
export class SessionStorage extends Storage {
 private _isLoggedInSource = new Subject&lt;boolean&gt;();
 isLoggedIn = this._isLoggedInSource.asObservable();
 constructor() {
 super('session');
 }
 setIsLoggedIn(value: boolean) {
 this.setItem('_isLoggedIn', value, () =&gt; {
 this._isLoggedInSource.next(value);
 });
 }
}
</code></pre>

Everything works great. But I have another component which doesn't need to subscribe, it just needs to get the current value of isLoggedIn at a certain point in time. How can I do this?

How to get current value of RxJS Subject or Observable?

我有一个Angular 2服务：import {Storage} from './storage';import {Injectable} from 'angular2/core';import {Subject}    from 'rxjs/Subject';@Injectable()export class Ses...

问如何获取RxJS主题或观察值的当前值？
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