RxJS可观察到，从第一次发射开始既发射先前值，又发射当前值 [英] RxJS observable which emits both previous and current value starting from first emission

问题描述

我有一个 BehaviorSubject ，它会定期发出JavaScript对象.我想构造另一个可观察对象，该对象将发出基础可观察对象的先前值和当前值，以便比较两个对象并确定增量.

I have a BehaviorSubject which emits JavaScript objects periodically. I want to construct another observable which will emit both previous and current values of the underlying observable in order to compare two objects and determine the delta.

pairwise() 或 bufferCount(2, 1) 运算符正在寻找就像一个很好的拟合，但是它们仅在缓冲区填充后才开始发出，但是我要求此可观察对象从基础可观察对象的第一个事件开始发出.

The pairwise() or bufferCount(2, 1) operators are looking like a good fit, but they start emitting only after buffer is filled, but I require this observable to start emitting from the first event of the underlying observable.

subject.someBufferingOperator()
    .subscribe([previousValue, currentValue] => {
        /** Do something */
    })
;

第一次发射时，previousValue可能只是null.

是否可以使用一些内置运算符来获得所需的结果?

Is there some built-in operators that I can use to achieve the desired result?

推荐答案

实际上，这和配对

Actually, it was as easy as pairing pairwise() with startWith() operators:

subject
    .startWith(null) // emitting first empty value to fill-in the buffer
    .pairwise()
    .subscribe([previousValue, currentValue] => {
        if (null === previousValue) {
            console.log('Probably first emission...');
        }
    })
;

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