Here is a simple thumb-rule that I use help me decide as when to use flatMap()
over map()
in Rx's Observable
.
Once you come to a decision that you're going to employ a map
transformation, you'd write your transformation code to return some Object right?
If what you're returning as end result of your transformation is:
a non-observable object then you'd use just map()
. And map()
wraps that object in an Observable and emits it.
an Observable
object, then you'd use flatMap()
. And flatMap()
unwraps the Observable, picks the returned object, wraps it with its own Observable and emits it.
Say for example we've a method titleCase(String inputParam) that returns Titled Cased String object of the input param. The return type of this method can be String
or Observable<String>
.
If the return type of titleCase(..)
were to be mere String
, then you'd use map(s -> titleCase(s))
If the return type of titleCase(..)
were to be Observable<String>
, then you'd use flatMap(s -> titleCase(s))
Hope that clarifies.