rxjs的第一个运算符被反复调用? [英] rxjs first operator is being repeatedly called?

问题描述

如果我使用的话，每3秒钟将有2条打印到控制台:

If I use, 2 gets printed to the console every 3 seconds:

Rx.Observable.interval(3000)
.mergeMap(() => Rx.Observable.of(3))
.concatMap(()=>Rx.Observable.of(2).first(x=>x == 2))
.subscribe((x)=>console.log(x))

如果我使用的话，只有2次打印到控制台:

If I use, 2 gets printed to the console only one time:

Rx.Observable.interval(3000)
.mergeMap(() => Rx.Observable.of(3))
.concatMap(()=>Rx.Observable.of(2))
.first(x=>x == 2)
.subscribe((x)=>console.log(x))

区别在于我链接了第一个运算符的位置，但是我不了解链接的流程.我认为在这两种情况下，由于我的可观察值为2，所以它应该只打印2次.

The difference is where I chained the first operator, but I don't understand the flow of the chain. I thought in both cases, it should only print 2 once since I have an observable of 2.

推荐答案

您对first()运算符感到困惑. first()的定义是:

You are confused with the first() operator. The definition of first() is:

返回满足谓词条件的可观察序列的第一个元素，如果不存在该元素，则返回默认值.如果未提供默认值，则将调用onError.

Returns the first element of an observable sequence that satisfies the condition in the predicate, or a default value if no such element exists. If no default value is given, then onError will be called.

注意单词可观察到的序列，又名流.两种情况的区别如下:

Note the word observable sequence, aka stream. The difference of the two scenarios are as follow:

您的first()应用于返回值2的可观察对象.Observable.interval()流不执行任何操作，该流每3秒不断发出一个事件.这就是为什么您的控制台永不停止记录的原因.

Your first() is applied to an observable that returns a value 2. Nothing is done to the Observable.interval() stream, which keeps on emitting an event every 3 seconds. That is why your console never stops logging.

first()应用于一个Observable.interval()，该Observable.interval()连续 每3秒发出一次事件.因此，一旦找到满足条件的x===2，您的first()运算符就基本将其斩波(或停止该序列).当条件满足时，流结束，然后返回该序列的FIRST元素.因此，您的控制台仅记录一次.

The first() is applied to an Observable.interval() that continuously emits an event, every 3 seconds. Hence, your first() operator basically chops the sequence (or stop it) as soon as it finds the fulfilling condition, which is x===2. When the condition is met, the stream ends, it then returns the FIRST element of that sequence. Hence your console only logs one time.

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