This content originally appeared on DEV Community and was authored by Aleksei Laptev
Starting
What is Preview? It's a functionality that gives your an opportunity for view a composable UI during development without the need to build and run on an emulator. (If you use the Android Studio obviously)
Problem
In fact you must have a some data that you wanna see in preview. But sometimes it's impossible.
Imagine, you have a screen like this:
@Composable
fun SomeScreen(
viewModel: SomeViewModel = hiltViewModel(),
){
//some UI
}
And view model of that screen like this:
class SomeViewModel @Inject constructor(
private val someUseCase1: someUseCase1,
private val someUseCase2: someUseCase2
//etc
) : ViewModel() {
val someDataFLow1 = someUseCase1.getSomeData()
.stateIn(
scope = viewModelScope,
started = SharingStarted.WhileSubscribed(5_000),
initialValue = null
)
//etc
}
And that's all. How can you make a preview? - Directly, no how.
Solution
Use MockK library. It's an open-source and excellent library for testing. Okay, it's for testing... But why can't we use it for debugging and preview? - Of course, we can!
First of all we have to add dependencies
//Compose
implementation(platform("androidx.compose:compose-bom:<bom-version>"))
//other compose dependencies
//Preview
debugImplementation("androidx.compose.ui:ui-tooling")
debugImplementation("androidx.compose.ui:ui-tooling-preview")
//MockK
debugImplementation("io.mockk:mockk:<mockk-version>")
Here's our project structure
└── src/
├── main/
│ └── kotlin/
│ └── yourpackage/
│ ├── SomeScreen.kt
│ └── SomeViewModel.kt
│
└── debug/
└── kotlin/
└── yourpackage/
└── preview/
├── SomeScreenPreview.kt
└── MockEntities.kt
We have to add a mock ViewModel by MockK library and setup it's behavior:
internal object MockEntities {
private val mockData = listOf(
SomeDataItem(id = 1),
SomeDataItem(id = 2),
SomeDataItem(id = 3),
SomeDataItem(id = 4),
)
private val mockDataFlow1 = MutableStateFlow(mockData)
val someViewModel = mockk<SomeViewModel>().apply {
every { someDataFLow1 } returns mockDataFlow1
}
}
And a final function is a Preview itself:
@Composable
@Preview
private fun SomeScreenPreview() {
YourTheme {
SomeScreen(
viewModel = MockEntities.someViewModel
)
}
}
So, that was a little trick. I hope it helps you in your work :)
This content originally appeared on DEV Community and was authored by Aleksei Laptev
Aleksei Laptev | Sciencx (2025-11-01T20:16:55+00:00) Using MockK library in Jetpack Compose Preview. Retrieved from https://www.scien.cx/2025/11/01/using-mockk-library-in-jetpack-compose-preview/
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