kotlin中的LiveData地图转换

Question

Transformations.map in LiveData transformations take two arguments :

1. @NonNull LiveData source
2. @NonNull final Function func

I tried to make the function like this:

val localLiveData = #some live data of type LiveData<User>
Transformations.map(localLiveData, s->{return s.name = "Hi"})

but this shows error cannot unresolved "s"

finally i got it working by this :

Transformations.map(localLiveData) {
              s.name = "Hi"
                return@map s
            }

How this thing is working map has only one argument? (noob in kotlin)

Answer 1

Most of the problems here are with Kotlin's lambda syntax, which is slightly different from that of some other languages.

In Kotlin, a lambda must have braces. But the -> is optional in some cases (if the lambda takes no parameters; or if it takes one and you're referring to it with the dummy name it ).

This is one reason why your first version fails; it would need the s -> moved inside the braces. (Another is that in Kotlin, an assignment is not an expression, and doesn't return a value, so you can't use it in a return .)

Your second works because in Kotlin, if the last parameter is a lambda, it can be moved outside the parenthesis. (This allows for higher-order functions that look like language syntax. In fact, if the lambda is the only parameter, you can omit the parentheses entirely!)

I don't know LiveData, but I wonder if the return@map is doing the right thing: it will return not just from the lambda, but from the map() method itself. (Such non-local returns aren't needed very often, and can be confusing.)

Also, a lambda doesn't need an explicit return ; it returns the value of its last expression.

So I suspect that a more concise version would be:

Transformations.map(localLiveData) { it.name = "Hi"; it }