by viewModel

Learn ViewModel injection with by viewModel().

What is by viewModel?

by viewModel() is the Koin delegate used to get a ViewModel inside an Activity or Fragment. It works with AndroidX ViewModel.

Example: class MyActivity : AppCompatActivity() { val vm: MyViewModel by viewModel() }. This delegate ties the ViewModel to the Activity lifecycle. When the screen rotates, the ViewModel survives; the new Activity gets the same ViewModel. viewModel is also used in module declarations: viewModel { MyViewModel(get()) }. This lets the ViewModel receive dependencies through Koin.

The viewModel module declaration

To add a ViewModel to a Koin module use viewModel { ... }. This is preferred over single or factory for ViewModels.

Example: module { viewModel { MyViewModel(get()) } }. Here MyViewModel receives other dependencies via get(). The viewModel block ties the ViewModel to the lifecycle. Koin manages the ViewModel lifecycle in line with AndroidX. This prevents memory leaks.

Exercise Define a class with by viewModel

Define class MyViewModel. Define class MyActivity with val vm: MyViewModel by viewModel() inside. In main, print println("ViewModel ready").

Show example solution

class MyViewModel

class MyActivity {
    val vm: MyViewModel by viewModel()
}

fun main() {
    println("ViewModel ready")
}

Test yourself

Complete the by viewModel call

class MyActivity : AppCompatActivity() {
    val vm: MyViewModel ____()
}

Show answer

class MyActivity : AppCompatActivity() {
    val vm: MyViewModel by viewModel()
}

When is by viewModel() preferred?
- To make an HTTP request
- For a database connection
- To serialize JSON
- When getting a ViewModel in an Activity
Show answer

When getting a ViewModel in an Activity — by viewModel() is used to get a ViewModel.
Match by viewModel parts

by viewModel

ViewModel delegate
MyActivity

Owning class
MyViewModel

ViewModel
viewModel

Module declaration

Next lesson: Summary - DI

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