Unit Tests, Mock HTTP & UI Tests in Kotlin Compose Desktop | Lesson 15

Learn how to test Kotlin Compose Desktop applications! We refactor a weather app for testability with injectable HttpClient, add pure functions, and write 16 tests covering unit tests, Ktor MockEngine API mocking, and Compose UI tests with createComposeRule. What You'll Learn: - Setting up test dependencies: compose.desktop.uiTestJUnit4, ktor-client-mock, coroutines-test - Refactoring for testability: injectable HttpClient parameter instead of global client - Pure functions for easy unit testing (weatherDescription WMO code mapping) - @Serializable data class deserialization tests with kotlinx-serialization - Ktor MockEngine for controlled HTTP responses without hitting real APIs - Compose UI testing with createComposeRule() and JUnit4 - onNodeWithTag / onNodeWithText for querying the semantic tree - Modifier.testTag("id") for stable UI test selectors - assertTextEquals, assertTextContains, assertExists for UI assertions - waitUntil for async operations in UI tests - Testing error states with mock server errors Timestamps: 0:00 - Introduction 0:15 - Preview: 16 Tests Across 3 Files 0:45 - Project Setup (Gradle with Test Dependencies) 2:48 - Weather Models & API (Refactored for Testability) 5:03 - App Content & Main (with testTag Modifiers) 9:27 - Unit Tests (WeatherModelsTest + WeatherApiTest) 14:18 - UI Tests (AppContentTest with Compose Rules) 17:51 - Run Tests (Compile + ./gradlew test + Results) 19:33 - Live Demo: Weather App in Action 20:13 - Recap & Key Takeaways 21:13 - Thanks for Watching Key Takeaways: 1. Test Setup — compose.desktop.uiTestJUnit4 + kotlin("test") for desktop UI and unit tests 2. Unit Tests — Test pure functions and serialization with @Test and assertEquals 3. Mock HTTP — Ktor MockEngine replaces real API calls with controlled responses 4. UI Tests — createComposeRule() + onNodeWithTag for composable assertions 5. Test Tags — Modifier.testTag("id") gives stable selectors for UI test queries Previous Lesson: Lesson 14: Tables & Data Grids — Data Explorer https://youtube.com/... Next Lesson: Lesson 16: Packaging & Distribution (coming soon) Tech Stack: Kotlin 2.1, Compose Multiplatform 1.7.3, Material3 Ktor 3.0.3, Ktor MockEngine, kotlinx-serialization 1.7.3 JUnit4, kotlinx-coroutines-test 1.9.0 Gradle 8.10, JVM 17+ Source Code: https://github.com/GoCelesteAI/kotlin_desktop_weather_testing Taught by CelesteAI If you found this helpful, please like and subscribe for more Kotlin Desktop tutorials! #kotlin #compose #desktop #material3 #testing #junit #mockengine #ktor #unittests #uitests #tutorial #programming