LiveData transformations and mediators

                  package com.example.myapp.ui

import androidx.lifecycle.*
import kotlinx.coroutines.launch

class UserViewModel(
    private val userRepository: UserRepository
) : ViewModel() {

    private val _userId = MutableLiveData<Int>()

    // Transform user ID to full user object
    val user: LiveData<User?> = _userId.switchMap { id ->
        liveData {
            val user = userRepository.getUser(id)
            emit(user)
        }
    }

    // Map user to display name
    val displayName: LiveData<String> = Transformations.map(user) { user ->
        user?.let { "${it.firstName} ${it.lastName}" } ?: "Unknown"
    }

    // Combine multiple LiveData sources
    private val _posts = MutableLiveData<List<Post>>()
    private val _comments = MutableLiveData<List<Comment>>()

    val feedItems: LiveData<FeedData> = MediatorLiveData<FeedData>().apply {
        var posts: List<Post>? = null
        var comments: List<Comment>? = null

        fun update() {
            if (posts != null && comments != null) {
                value = FeedData(posts!!, comments!!)
            }
        }

        addSource(_posts) { newPosts ->
            posts = newPosts
            update()
        }

        addSource(_comments) { newComments ->
            comments = newComments
            update()
        }
    }

    // Conditional transformation
    private val _searchQuery = MutableLiveData<String>()

    val searchResults: LiveData<List<Post>> = _searchQuery.switchMap { query ->
        if (query.isNullOrBlank()) {
            liveData { emit(emptyList<Post>()) }
        } else {
            liveData {
                val results = userRepository.searchPosts(query)
                emit(results)
            }
        }
    }

    // Custom transformation with distinct
    val postCount: LiveData<Int> = Transformations.distinctUntilChanged(
        Transformations.map(_posts) { it?.size ?: 0 }
    )

    fun setUserId(id: Int) {
        _userId.value = id
    }

    fun search(query: String) {
        _searchQuery.value = query
    }
}

data class FeedData(
    val posts: List<Post>,
    val comments: List<Comment>
)