LiveData transformations
Transformations 有两个:
- [
Transformations.map](https://developer.android.com/reference/androidx/lifecycle/Transformations.html#map(androidx.lifecycle.LiveData, androidx.arch.core.util.Function)) - [
Transformations.switchMap](https://developer.android.com/reference/androidx/lifecycle/Transformations.html#switchMap(androidx.lifecycle.LiveData, androidx.arch.core.util.Function>))
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