RxJS - take n last elements from an observable

问题:

I want to take 3 last elements from an observable. Let's say that my timeline looks like this:

--a---b-c---d---e---f-g-h-i------j->

where: a, b, c, d, e, f, g, h, i, j are emitted values

Whenever a new value is emitted I want to get it immediately, so it can look like this:

[a]
[a, b]
[a, b, c]
[b, c, d]
[c, d, e]
[d, e, f]
[e, f, g]
[f, g, h]
... and so on

I think that this is super useful. Imagine building a chat where you want to display 10 last messages. Whenever a new message comes you want to update your view.

My attempt: demo

回答1:

You can use scan for this:

from(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u'])
  .pipe(
    scan((acc, val) => {
      acc.push(val);
      return acc.slice(-3);
    }, []),
  )
  .subscribe(console.log);

This will print:

[ 'a' ]
[ 'a', 'b' ]
[ 'a', 'b', 'c' ]
[ 'b', 'c', 'd' ]
[ 'c', 'd', 'e' ]
...
[ 's', 't', 'u' ]

The bufferCount won't do what you want. It'll emit only when each buffer is exactly === 3 which means you won't get any emission until you post at least 3 messages.

Jan 2019: Updated for RxJS 6

回答2:

You can look at Observable#bufferCount function. One difference is that it wants at least 3 times to emit (first parameter, in this example).

const source = Rx.Observable.interval(1000);
const example = source.bufferCount(3,1)
const subscribe = example.subscribe(val => console.log(val));

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