Article about this library: Every Composable deserves a ViewModel
The right scope for objects and View Models in Android Compose.
Resaca provides a simple way to keep a Jetpack ViewModel (or any other object) in memory during the lifecycle of a @Composable function and automatically
clean it up when not needed anymore. This means, it retains your object or ViewModel across recompositions, during configuration changes, and also when the
container Fragment or Compose Navigation destination goes into the backstack.
With Resaca you can create fine grained ViewModels for fine grained Composables and finally have reusable components across screens.
Compose allows the creation of fine-grained UI components that can be easily reused like Lego blocks 🧱. Well architected Android apps isolate functionality in small business logic components (like use cases, interactors, repositories, etc.) that are also reusable like Lego blocks 🧱.
Screens are built using Compose components together with business logic components, and the standard tool to connect these two types of components is a Jetpack ViewModel. Unfortunately, ViewModels can only be scoped to a whole screen (or larger scope), but not to smaller Compose components on the screen.
In practice, this means that we are gluing UI Lego blocks with business logic Lego blocks using a big glue class for the whole screen, the ViewModel 🗜.
Until now...
Just include the library (less than 5Kb):
Kotlin (KTS)
// In module's build.gradle.kts
dependencies {
// The latest version of the lib is available in the badget at the top from Maven Central, replace X.X.X with that version
implementation("io.github.sebaslogen:resaca:X.X.X")
}Groovy
dependencies {
// The latest version of the lib is available in the badget at the top from Maven Central, replace X.X.X with that version
implementation 'io.github.sebaslogen:resaca:X.X.X'
}Inside your @Composable function create and retrieve an object using rememberScoped to remember any type of object (except ViewModels). For ViewModels use
viewModelScoped. That's all 🪄✨
Examples:
Scope an object to a Composable
@Composable
fun DemoScopedObject() {
val myRepository: MyRepository = rememberScoped { MyRepository() }
DemoComposable(inputObject = myRepository)
}Scope a ViewModel to a Composable
@Composable
fun DemoScopedViewModel() {
val myScopedVM: MyViewModel = viewModelScoped()
DemoComposable(inputObject = myScopedVM)
}Scope a ViewModel with a dependency to a Composable
@Composable
fun DemoScopedViewModelWithDependency() {
val myScopedVM: MyViewModelWithDependencies = viewModelScoped { MyViewModelWithDependencies(myDependency) }
DemoComposable(inputObject = myScopedVM)
}Scope a ViewModel with a key to a Composable
@Composable
fun DemoViewModelWithKey() {
val scopedVMWithFirstKey: MyViewModel = viewModelScoped("myFirstKey") { MyViewModel("myFirstKey") }
val scopedVMWithSecondKey: MyViewModel = viewModelScoped("mySecondKey") { MyViewModel("mySecondKey") }
// We now have 2 ViewModels of the same type with different data inside the same Composable scope
DemoComposable(inputObject = scopedVMWithFirstKey)
DemoComposable(inputObject = scopedVMWithSecondKey)
}Scope a ViewModel with a dependency injected with Koin to a Composable
@Composable
fun DemoKoinInjectedViewModelWithDependency() {
val myInjectedScopedVM: MyViewModelWithDependencies = viewModelScoped() { getKoin().get { parametersOf(myConstructorDependency) } }
DemoComposable(inputObject = myInjectedScopedVM)
}Use a different ViewModel for each item in a LazyColumn and scope them to the Composable that contains the LazyColumn
@Composable
fun DemoManyViewModelsScopedOutsideTheLazyColumn(listItems: List<Int> = (1..1000).toList()) {
val keys = rememberKeysInScope(inputListOfKeys = listItems)
LazyColumn() {
items(items = listItems, key = { it }) { item ->
val myScopedVM: MyViewModel = viewModelScoped(key = item, keyInScopeResolver = keys)
DemoComposable(inputObject = myScopedVM)
}
}
}Once you use the rememberScoped or viewModelScoped functions, the same object will be restored as long as the Composable is part of the composition, even if
it temporarily leaves composition on configuration change (e.g. screen rotation, change to dark mode, etc.) or while being in the backstack.
For ViewModels, in addition to being forgotten when they're really not needed anymore, their coroutineScope will also be automatically canceled because
ViewModel's onCleared method will be automatically called.
💡 Optional key: a key can be provided to the call,
rememberScoped(key) { ... }orviewModelScoped(key) { ... }. This makes possible to forget an old object when there is new input data during a recomposition (e.g. a new input id for your ViewModel).
⚠️ Note that ViewModels remembered withviewModelScopedshould not be created using any of the ComposeviewModel()orViewModelProvidersfactories, otherwise they will be retained in the scope of the screen regardless ofviewModelScoped. Also, if a ViewModel is remembered withrememberScoped, instead ofviewModelScoped, then its clean-up method won't be called, so it's always better to useviewModelScopedfor ViewModels.
Here are some sample use cases reported by the users of this library:
- ❤️ Isolated and stateful UI components like a favorite button that are widely used across the screens. This
FavoriteViewModelcan be very small, focused and only require an id to work without affecting the rest of the screen's UI and state. - 🗪 Dialog pop-ups can have their own business-logic with state that is better to isolate in a separate ViewModel but the lifespan of these dialogs might be short, so it's important to clean-up the ViewModel associated to a Dialog after it has been closed.
- 📃 A LazyColumn with a ViewModel per list item. Each item can have its own complex logic in an isolated ViewModel that will be lazily loaded when the item is visible for the first time. The ViewModel will cleared and destroyed when the item is not part of the list in the source data or the whole LazyColumn is removed.
- 📄📄 Multiple instances of the same type of ViewModel in a screen with a view-pager. This screen will have multiple sub-pages that use the same ViewModel
class with different ids. For example, a screen of holiday destinations with multiple pages and each page with its own
HolidayDestinationViewModel.
Demo app documentation can be found here.
| Before | After backstack navigation & configuration change |
|---|---|
 |
 |
Resaca works in Kotlin Multiplaform and also in Compose Multiplatform for Android and iOS targets:
Since version 4.0, Resaca supports Compose Multiplatform for Android and iOS targets.
To see an example of usage and configuration check the Sample Compose Multiplatform project in the samplecmp module.
Resaca is a Kotlin Multiplatform library and can be used in any Kotlin Multiplatform project that targets Android or iOS. Nevertheless, Resaca does not make sense in a SwiftUI project because it's a Compose only library, instead, if you want to scope ViewModels to SwiftUI views, you can look at the solutions provided in the first comments of this ticket #91.
This library does not influence how your app provides or creates objects so it's dependency injection strategy and framework agnostic.
Nevertheless, this library supports two of the main dependency injection frameworks:
Hilt details
HILT (Dagger) support is available in a small extension of this library: resaca-hilt.
Documentation and installation instructions are available here.
Koin details
Koin is out of the box supported by simply changing the way you request a dependency.
Instead of using the getViewModel or koinViewModel functions from Koin, you have to use the standard way of getting a dependency from Koin getKoin().get().
Usage example: val viewModel: MyViewModel = viewModelScoped(myId) { getKoin().get { parametersOf(myId) } }
Note: if you plan to use a ViewModel with a SavedStateHandle, then you need to use the
koinViewModelScopedfunction from the small extension library resaca-koin.
This is handy for the typical case where you have a lazy list of items and you want to have a separate ViewModel for each item in the list, using the viewModelScoped function.
How to use `rememberKeysInScope` to control the lifecycle of a scoped object in a Lazy* list
When using the Lazy* family of Composables it is recommended that -just above the call to the Lazy* Composable- you use rememberKeysInScope with a list of
keys corresponding to the items used in the Lazy* Composable to obtain a KeyInScopeResolver (it's already highly recommended in Compose that items in a Lazy* list have unique keys).
Then, in the Lazy* Composable, once you are creating an item and you need an object or ViewModel for that item,
all you have to do is include in the call to rememberScoped/viewModelScoped the key for the current item and the KeyInScopeResolver you previously got from rememberKeysInScope.
With this setup, when an item of the Lazy* list becomes visible for the first time, its associated rememberScoped/viewModelScoped object will be created and even if the item is scrolled away, the scoped object will still be alive. Only once the associated key is not present anymore in the list provided to rememberKeysInScope and the item is either not part of the Lazy* list anymore or scrolled away, then the associated object will be cleared and destroyed.
🏷️ Example of a separate ViewModel for each item in a LazyColumn and scope them to the Composable that contains the LazyColumn
@Composable
fun DemoManyViewModelsScopedOutsideTheLazyColumn(listItems: List<Int> = (1..1000).toList()) {
val keys = rememberKeysInScope(inputListOfKeys = listItems)
LazyColumn() {
items(items = listItems, key = { it }) { item ->
val myScopedVM: MyViewModel = viewModelScoped(key = item, keyInScopeResolver = keys)
DemoComposable(inputObject = myScopedVM)
}
}
}When a Composable is used more than once in the same screen with the same input, then the ViewModel (or business logic object) should be provided only once
with viewModelScoped at a higher level in the composition tree using Compose's State Hoisting.
Pros, cons and alternatives to remember:
Remember will keep our object alive as long as the Composable is not disposed of. Unfortunately, there are a few cases where our Composable will be disposed of and then added again, breaking the lifecycle parity with the remember function. 😢
Pros and Cons
Pros
- Simple API
Cons
- remember value will NOT survive a configuration change
- remember value will NOT survive when going into the backstack
- remember value will NOT survive a process death
RememberSaveable will follow the lifecycle of the Composable, even in the few cases where the Composable is temporarily disposed of. But the object we want to remember needs to implement Parcelable or the Saver interface in an additional class. 😢 Implementing these interfaces might not trivial.
Pros and Cons
Pros
- rememberSaveable value will survive a configuration change
- rememberSaveable value will survive when going into the backstack
- rememberSaveable value will survive a process death
Cons
- Complex integration work is required to correctly implement Parcelable or Saver
RememberScoped function keeps
objects in memory during the lifecycle of the Composable, even in a few cases where the Composable is disposed of, and then added again. Therefore, it will
retain objects longer than the remember function but shorter than rememberSaveable because there is no serialization involved.
Pros and Cons
Pros
- Simple API
- rememberScoped/viewModelScoped value will survive a configuration change
- rememberScoped/viewModelScoped value will survive when going into the backstack
Cons
- rememberScoped/viewModelScoped value will NOT survive a process death
How does the lifecycle of the Resaca scoped objects work and some lifecycle illustrated examples:
RememberScoped function keeps objects in memory during the lifecycle of the Composable, even in a few cases where the Composable is disposed of, and then added again.
This project uses internally a ViewModel as a container to store all scoped ViewModels and scoped objects.
What happens when a Composable is disposed?
When a Composable is disposed of, we don't know for sure if it will return again later. So at the moment of disposal, we mark in our container the associated object to be disposed of after the next frame when the Activity is resumed. During the span of time of this next frame, a few things can happen:
- The Composable is not part of the composition anymore after the next frame and the associated object is disposed of. 🚮
- The LifecycleOwner of the disposed Composable (i.e. the navigation destination where the Composable lived) is paused (e.g. screen went to background) before
the next frame happened. Then the disposal of the scoped object is canceled, but the object is still marked for disposal at a later stage.
- This can happen when the application goes through a configuration change and the container Activity is recreated.
- Also when the Composable is part of a Fragment that has been pushed to the backstack.
- And also when the Composable is part of a Compose Navigation destination that has been pushed to the backstack.
- When the LifecycleOwner of the disposed Composable is resumed (e.g. Fragment comes back to foreground), then the disposal of the associated object is
scheduled again to happen after the next frame when the Activity is resumed. At this point two things can happen:
- The Composable becomes part of the composition again and the
rememberScoped/viewModelScopedfunction restores the associated object while also cancelling any pending disposal in the next frame when the Activity is resumed. 🎉 - The Composable is not part of the composition anymore after the next frame and then the associated object is disposed of. 🚮
- The Composable becomes part of the composition again and the
Note:
- To know that the same Composable is being added to the composition again after being disposed of, we generate a random ID and store it with
rememberSaveable, which survives recomposition, recreation and even process death. - To detect when the requester Composable is not needed anymore (has left composition and the screen for good), the ScopedViewModelContainer also observes the resume/pause Lifecycle events of the owner of this ScopedViewModelContainer (i.e. Activity, Fragment, or Compose Navigation destination)
This diagram shows the lifecycle of three Composables (A, B, and C) with their respective objects scoped with the rememberScoped function. All these
Composables are part of a Composable destination which is part of a Fragment which is part of an Activity which is part of the App. The horizontal arrows
represent different lifecycle events, events like Composable being disposed of, Composable screen going into the backstack, Fragment going into the backstack
and returning from backstack, or Activity recreated after a configuration change.
The existing alternatives to replicate the lifecycle of the objects in the diagram without using rememberScoped are:
- Object A lifecycle could only be achieved using the Compose
viewModel()orViewModelProvidersfactories. - Object B lifecycle could only be achieved using the Compose
remember()function. - Object C lifecycle could not be achieved neither by using ViewModel provider functions nor Compose
rememberfunctions.
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