Operators that transform items that are emitted by an Observable.
Buffer — periodically gather items from an Observable into bundles and emit these bundles rather than emitting the items one at a time
FlatMap — transform the items emitted by an Observable into Observables, then flatten the emissions from those into a single Observable
GroupBy — divide an Observable into a set of Observables that each emit a different group of items from the original Observable, organized by key
Map — transform the items emitted by an Observable by applying a function to each item
Scan — apply a function to each item emitted by an Observable, sequentially, and emit each successive value
Window — periodically subdivide items from an Observable into Observable windows and emit these windows rather than emitting the items one at a time
官网链接：reactivex
buffer&Window
同：将数据按序缓存，缓存大小由用户存入，每收集到count个数据发送一次（接收到onComplete()并未收集到count个数据时也会发送）；也可以按时间缓存，没隔一段时间发送一次。
异：buffer缓存后发射的是list数据集，而window缓存后会包装成一个Observable发射
注意：缓存中遇到onError()时，已缓存但未发射的数据将被抛弃,之后的数据也不会发送
Map&FlatMap
transform the items emitted by an Observable by applying a function to each item
transform the items emitted by an Observable into Observables, then flatten the emissions from those into a single Observable
异：发射的数据不同：function和Observable
GroupBy
将一组数据分组包装成Observable发射出去。每个数据会返回一个key,这些包含相同key的数据将被收集在一起
scan
累加器的效果
