Hello, quick question about the SAM converter and ...

# android

[pdegand]

Hello, quick question about the SAM converter and live data :
Why do I have to write liveData.observe(this, Observer { }) when I can write mediatorLiveData.addSource(liveData) { } ?
In both case, the 2d param of the observe or addSource is a android.arch.lifecycle.Observer and this interface has only 1 method ...
I'm confused

[louiscad]

I'm puzzled by this too, and ended up writing an extension in a library to get the nicer syntax: https://github.com/LouisCAD/Splitties/blob/master/arch-lifecycle/README.md#observe-and-observenotnull-extension-functions-on-lifecycleowner

[russhwolf]

Just spitballing, but my guess is it's due to the difference in type parameters. In MediatorLiveData.addSource, both the source parameter and the lambda share a generic parameter, while in LiveData.observe the generic is only on the lambda. I'm guessing this means that the compiler can infer the generic type on the MediatorLiveData method but not the LiveData one, so you must use a syntax that specifies it.

[arekolek]

See https://stackoverflow.com/questions/46542037/cant-replace-sam-constructor-with-lambda-when-first-argument-is-a-class-with-on

[arekolek]

The way I see it, kotlin compiler doesn't want to generate 2^n variants for a method that has n parameters that are eligible for SAM conversion, so instead it generates only two variants: the one where all are lambdas and the one where there are no lambdas

[arekolek]

See also https://youtrack.jetbrains.net/issue/KT-14984