Android ViewModellのテスト。 savedStateHandle で値の保存と復元が行われることを確かめる。

MainViewModel.kt

package com.example.myapplication

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.SavedStateHandle
import androidx.lifecycle.ViewModel
import androidx.lifecycle.asLiveData
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.launch

class MainViewModel(private val savedStateHandle: SavedStateHandle) : ViewModel() {
    var value1: Int
        get() = savedStateHandle.get<Int>("value1") ?: 0
        set(v) = savedStateHandle.set("value1", v)

    private val _value2: MutableLiveData<Int> = savedStateHandle.getLiveData("value2", -1)
    val value2 = _value2 as LiveData<Int>

    fun setValue2(v: Int) {
        viewModelScope.launch {
            _value2.value = v
        }
    }

    val value3 = savedStateHandle.getStateFlow("value3", -987).asLiveData()

    fun setValue3(v: Int) {
        viewModelScope.launch {
            savedStateHandle["value3"] = v
        }
    }
}

MainViewModelTest.kt

package com.example.myapplication

import androidx.arch.core.executor.testing.InstantTaskExecutorRule
import androidx.lifecycle.LiveData
import androidx.lifecycle.SavedStateHandle
import androidx.lifecycle.testing.TestLifecycleOwner
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.test.resetMain
import kotlinx.coroutines.test.runTest
import kotlinx.coroutines.test.setMain
import kotlinx.coroutines.withContext
import org.junit.After
import org.junit.Assert.*
import org.junit.Before
import org.junit.Rule
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4
import java.util.concurrent.CountDownLatch
import java.util.concurrent.TimeUnit

@ExperimentalCoroutinesApi
@RunWith(JUnit4::class)
class MainViewModelTest {
    @get:Rule
    val instantTaskExecutorRule = InstantTaskExecutorRule()

    @Before
    fun setUp() {
        Dispatchers.setMain(Dispatchers.Unconfined)
    }

    @After
    fun tearDown() {
        Dispatchers.resetMain()
    }

    /**
     * SavedStateHandleのテスト
     */
    @Test
    fun test1() = runTest {
        val handle = SavedStateHandle()
        val viewModel = MainViewModel(handle)

        // -------------------------------------
        // 初期値
        // -------------------------------------
        waitForLiveDataChange(-1, viewModel.value2)
        waitForLiveDataChange(-987, viewModel.value3)

        assertEquals(viewModel.value1, 0)
        assertEquals(-1, viewModel.value2.value)
        assertEquals(-987, viewModel.value3.value)

        // -------------------------------------
        // 値のセットが行われる
        // -------------------------------------
        viewModel.value1 = 2
        viewModel.setValue2(-999)
        viewModel.setValue3(1024)

        waitForLiveDataChange(-999, viewModel.value2)
        waitForLiveDataChange(1024, viewModel.value3)

        assertEquals(2, viewModel.value1)
        assertEquals(-999, viewModel.value2.value)
        assertEquals(1024, viewModel.value3.value)

        // -------------------------------------
        // SavedStateHandleにより値が復元される
        // -------------------------------------
        val viewModel2 = MainViewModel(handle)

        assertEquals(2, viewModel2.value1)
        waitForLiveDataChange(-999, viewModel.value2)
        waitForLiveDataChange(1024, viewModel.value3)
    }

    /**
     * LiveDataの値が変更されるまで待つ
     */
    private suspend fun <T> waitForLiveDataChange(
        expected: T,
        liveData: LiveData<T>,
        timeout: Long = 3000
    ) {
        val latch = CountDownLatch(1)
        val observer = object: Observer<T> {
            override fun onChanged(value: T) {
                if (value == expected) {
                    latch.countDown()
                    liveData.removeObserver(this)
                }
            }
        }
        liveData.observeForever(observer)

        withContext(Dispatchers.IO) {
            latch.await(timeout, TimeUnit.MILLISECONDS)
        }
    }
}