Level 18

Testing (ViewModel & Compose UI)

Coroutine Tests & Compose Test Rule

Tests lock in behavior. Unit-test `ViewModel` logic with a test dispatcher, and assert your UI with the Compose testing rule using semantics (no Espresso view-matching).

📚 Explanation

`runTest` runs coroutines on a virtual clock, so `delay` is skipped and assertions see the final state deterministically. For UI, `createComposeRule()` hosts a Composable in isolation; you query the semantics tree with finders like `onNodeWithText`, drive interactions with `performClick()`, and assert with `assertIsDisplayed()` — fast, reliable tests with no emulator-specific view matching.

Note: Code execution is not available for Android/Kotlin in the browser. Use Android Studio to run and test the app.

kotlin

import androidx.compose.ui.test.*
import androidx.compose.ui.test.junit4.createComposeRule
import kotlinx.coroutines.test.runTest
import org.junit.Assert.assertEquals
import org.junit.Rule
import org.junit.Test

// 1. Unit test a ViewModel with a virtual-time dispatcher
class CounterViewModelTest {
    @Test
    fun increment_updatesState() = runTest {
        val vm = CounterViewModel()

        vm.increment()

        // runTest skips real delays and drains pending coroutines
        assertEquals(1, vm.state.value.count)
    }
}

// 2. UI test a Composable against its semantics tree
class GreetingTest {
    @get:Rule val composeRule = createComposeRule()

    @Test
    fun button_click_incrementsLabel() {
        composeRule.setContent { CounterScreen() }

        composeRule.onNodeWithText("Clicked 0 times").assertIsDisplayed()
        composeRule.onNodeWithText("Clicked 0 times").performClick()
        composeRule.onNodeWithText("Clicked 1 times").assertIsDisplayed()
    }
}

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Testing (ViewModel & Compose UI)

Coroutine Tests & Compose Test Rule

Tests lock in behavior. Unit-test `ViewModel` logic with a test dispatcher, and assert your UI with the Compose testing rule using semantics (no Espresso view-matching).

📚 Explanation

Note: Code execution is not available for Android/Kotlin in the browser. Use Android Studio to run and test the app.

import androidx.compose.ui.test.*

import androidx.compose.ui.test.junit4.createComposeRule

import kotlinx.coroutines.test.runTest

import org.junit.Assert.assertEquals

import org.junit.Rule

import org.junit.Test

// 1. Unit test a ViewModel with a virtual-time dispatcher

class CounterViewModelTest {

@Test

fun increment_updatesState() = runTest {

val vm = CounterViewModel()

vm.increment()

// runTest skips real delays and drains pending coroutines

assertEquals(1, vm.state.value.count)

}

// 2. UI test a Composable against its semantics tree

class GreetingTest {

@get:Rule val composeRule = createComposeRule()

@Test

fun button_click_incrementsLabel() {

composeRule.setContent { CounterScreen() }

composeRule.onNodeWithText("Clicked 0 times").assertIsDisplayed()

composeRule.onNodeWithText("Clicked 0 times").performClick()

composeRule.onNodeWithText("Clicked 1 times").assertIsDisplayed()

}