mergeMap 运算符的用例是什么? [英] What are use cases for mergeMap operator?
问题描述
我根本不明白 mergeMap 的目的.我听说有两种解释:
I don't understand the purpose of mergeMap at all. I have heard are two explanations:
- 这就像 .NET LINQ 中的
SelectAll()- 不是. - 它是 RxJS
merge和map的组合 - 不(或者我无法复制).
- It's like
SelectAll()in .NET LINQ - nope. - It's a combination of RxJS
mergeandmap- nope (or I can't replicate this).
考虑以下代码:
var obs1 = new Rx.Observable.interval(1000);
var obs2 = new Rx.Observable.interval(1000);
//Just a merge and a map, works fine
obs1.merge(obs2).map(x=> x+'a').subscribe(
next => console.log(next)
)
//Who know what - seems to do the same thing as a plain map on 1 observable
obs1.mergeMap(val => Rx.Observable.of(val + `B`))
.subscribe(
next => console.log(next)
)
标有谁知道什么"的最后一段只不过是 obs1 上的地图 - 有什么意义?
The last piece labelled "Who knows what" does nothing more than a map on obs1 - what's the point?
mergeMap 实际上是做什么的?什么是有效用例的示例?(最好有一些代码)
What does mergeMap actually do? What is an example of a valid use case? (Preferably with some code)
根本没有帮助我的文章(上面的mergeMap代码来自其中之一):1, 2
Articles that didn't help me at all (mergeMap code from above is from one of these): 1, 2
推荐答案
tl;dr; mergeMap 比 map 强大得多.理解mergeMap是获得Rx全部功能的必要条件.
tl;dr; mergeMap is way more powerful than map. Understanding mergeMap is the necessary condition to access full power of Rx.
mergeMap和map都作用于单个流(相对于zip、combineLatest)
both
mergeMapandmapacts on a single stream (vs.zip,combineLatest)
mergeMap 和 map 都可以转换流的元素(相对于 filter、delay)
both mergeMap and map can transform elements of a stream (vs. filter, delay)
不能改变源流的大小(假设:
map本身不会throw)；对于源中的每个元素，只发出一个mapped元素；map不能忽略元素(例如filter)；
cannot change size of the source stream (assumption:
mapitself does notthrow); for each element from source exactly onemappedelement is emitted;mapcannot ignore elements (like for examplefilter);
在默认调度器的情况下，转换是同步发生的；要 100% 清楚:源流可能会异步传递其元素，但每个下一个元素都会立即mapped 并进一步重新发出；map 不能像 delay
in case of the default scheduler the transformation happens synchronously; to be 100% clear: the source stream may deliver its elements asynchronously, but each next element is immediately mapped and re-emitted further; map cannot shift elements in time like for example delay
对返回值没有限制
id: x =>x
可以改变源流的大小；对于每个元素，可能有任意数量(0、1 或许多)创建/发出的新元素
can change size of the source stream; for each element there might be arbitrary number (0, 1 or many) of new elements created/emitted
它提供了对异步性的完全控制——无论何时创建/发出新元素，以及应同时处理多少来自源流的元素；例如，假设源流发出 10 个元素，但 maxConcurrency 设置为 2，则将立即处理两个第一个元素，其余 8 个元素将被缓冲；一旦一个处理过的 completed 源流中的下一个元素将被处理等等 - 这有点棘手，但看看下面的例子
it offers full control over asynchronicity - both when new elements are created/emitted and how many elements from the source stream should be processed concurrently; for example assume source stream emitted 10 elements but maxConcurrency is set to 2 then two first elements will be processed immediately and the rest 8 buffered; once one of the processed completed the next element from source stream will be processed and so on - it is bit tricky, but take a look at the example below
所有其他操作符都可以通过 mergeMap 和 Observable 构造函数来实现
all other operators can be implemented with just mergeMap and Observable constructor
可用于递归异步操作
返回值必须是 Observable 类型(或者 Rx 必须知道如何从中创建可观察的 - 例如承诺、数组)
return values has to be of Observable type (or Rx has to know how to create observable out of it - e.g. promise, array)
id: x =>Rx.Observable.of(x)
let array = [1,2,3]
fn map mergeMap
x => x*x [1,4,9] error /*expects array as return value*/
x => [x,x*x] [[1,1],[2,4],[3,9]] [1,1,2,4,3,9]
类比没有显示全貌，它基本上对应于 .mergeMap 和 maxConcurrency 设置为 1.在这种情况下，元素将按上述顺序排序，但在一般情况下不必如此.我们唯一的保证是新元素的发射将按照它们在底层流中的位置进行排序.例如:[3,1,2,4,9,1] 和 [2,3,1,1,9,4] 是有效的，但是 [1,1,4,2,3,9] 不是(因为 4 在底层流中的 2 之后发出).
The analogy does not show full picture and it basically corresponds to .mergeMap with maxConcurrency set to 1. In such a case elements will be ordered as above, but in general case it does not have to be so. The only guarantee we have is that emission of new elements will be order by their position in the underlying stream. For example: [3,1,2,4,9,1] and [2,3,1,1,9,4] are valid, but [1,1,4,2,3,9] is not (since 4 was emitted after 2 in the underlying stream).
// implement .map with .mergeMap
Rx.Observable.prototype.mapWithMergeMap = function(mapFn) {
return this.mergeMap(x => Rx.Observable.of(mapFn(x)));
}
Rx.Observable.range(1, 3)
.mapWithMergeMap(x => x * x)
.subscribe(x => console.log('mapWithMergeMap', x))
// implement .filter with .mergeMap
Rx.Observable.prototype.filterWithMergeMap = function(filterFn) {
return this.mergeMap(x =>
filterFn(x) ?
Rx.Observable.of(x) :
Rx.Observable.empty()); // return no element
}
Rx.Observable.range(1, 3)
.filterWithMergeMap(x => x === 3)
.subscribe(x => console.log('filterWithMergeMap', x))
// implement .delay with .mergeMap
Rx.Observable.prototype.delayWithMergeMap = function(delayMs) {
return this.mergeMap(x =>
Rx.Observable.create(obs => {
// setTimeout is naive - one should use scheduler instead
const token = setTimeout(() => {
obs.next(x);
obs.complete();
}, delayMs)
return () => clearTimeout(token);
}))
}
Rx.Observable.range(1, 3)
.delayWithMergeMap(500)
.take(2)
.subscribe(x => console.log('delayWithMergeMap', x))
// recursive count
const count = (from, to, interval) => {
if (from > to) return Rx.Observable.empty();
return Rx.Observable.timer(interval)
.mergeMap(() =>
count(from + 1, to, interval)
.startWith(from))
}
count(1, 3, 1000).subscribe(x => console.log('count', x))
// just an example of bit different implementation with no returns
const countMoreRxWay = (from, to, interval) =>
Rx.Observable.if(
() => from > to,
Rx.Observable.empty(),
Rx.Observable.timer(interval)
.mergeMap(() => countMoreRxWay(from + 1, to, interval)
.startWith(from)))
const maxConcurrencyExample = () =>
Rx.Observable.range(1,7)
.do(x => console.log('emitted', x))
.mergeMap(x => Rx.Observable.timer(1000).mapTo(x), 2)
.do(x => console.log('processed', x))
.subscribe()
setTimeout(maxConcurrencyExample, 3100)<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.1.1/Rx.min.js"></script>这篇关于mergeMap 运算符的用例是什么?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
