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Managing ViewModel State with StateFlow: Stopping Race Situations | by Mahmoud Afarideh | March 2023

As an Android developer, managing your app’s UI state is a vital side of constructing a sturdy and dependable app. It ensures that your utility’s person interface works as anticipated, offering a seamless person expertise. You may select to make use of the ViewModel to handle the state and expose it to the UI layer utilizing LiveData or StateFlow. StateFlow is a more recent addition to the Kotlin coroutines library that provides a number of benefits over LiveData, comparable to higher help for coroutines and flow-based programming.

One factor that I like extra about StateFlow is its nice integration with Jetpack Compose, a contemporary UI toolkit for constructing Android apps.

Nevertheless, when utilizing StateFlow to handle the ViewModel’s state, you must watch out about the way you replace the state, particularly when there are a number of threads concerned. Updating state in a thread-safe method can result in concurrency situations, the place the ultimate state is inconsistent and unpredictable.

On this article, we’ll talk about race situation challenge when updating ViewModel’s state utilizing StateFlow and supply an instance of the way to remedy it. This will occur not solely in ViewModels but additionally in different conditions the place your StateFlow is up to date concurrently!

As an instance you will have a ViewModel that maintains the state of the display screen in your utility. The state is represented by an information layer containing person interface-oriented information, comparable to the present textual content in an EditText or the visibility of a ProgressBar. This is an instance of what an information layer may appear to be:

information class ScreenState(
val textual content: String = "",
val isLoading: Boolean = false

You expose this state to the person interface layer by utilizing the StateFlow object:

class MyViewModel : ViewModel() {
personal val _stateFlow = MutableStateFlow(ScreenState())

val stateFlow: StateFlow
get() = _stateFlow.asStateFlow()

// ...

Now suppose you will have a perform in your ViewModel that updates state primarily based on person motion or API name:

enjoyable onTextChanged(textual content: String) {
val currentState = _stateFlow.worth
val newState = currentState.copy(textual content = textual content)
_stateFlow.worth = newState

This perform retrieves the present state, creates a brand new state with the up to date textual content, and units the StateFlow worth to the brand new state. Nevertheless, this strategy is just not thread-safe. If a number of threads name this perform concurrently, they will learn and write state on the similar time, resulting in inconsistent and unpredictable outcomes.

For instance, for instance you will have two threads calling this perform concurrently. The primary thread retrieves the present state, and earlier than it may possibly set a brand new state, the second thread additionally retrieves the present state and units its personal new state. When the primary thread units a brand new state, it overwrites the modifications of the second thread, leading to inconsistent state.

To work across the race situation challenge when updating the ViewModel’s state utilizing StateFlow, you need to use the replace perform offered by StateFlow. This perform means that you can replace state in a thread-safe method, guaranteeing that just one thread can replace state at a time.

Right here is an instance of how one can modify it onTextChanged perform use replace:

enjoyable onTextChanged(textual content: String) {
_stateFlow.replace { currentState ->
currentState.copy(textual content = textual content)

On this instance, we use replace perform to change the present state. The replace the perform takes a lambda that takes the present state as a parameter and returns the brand new state. Contained in the lambda, we create a brand new state with the up to date textual content and return it. The replace perform ensures that this lambda is executed atomically, stopping race situations.

Let’s take a more in-depth have a look at StateFlow’s replace performance:

public inline enjoyable  MutableStateFlow.replace(perform: (T) -> T) {
whereas (true) {
val prevValue = worth
val nextValue = perform(prevValue)
if (compareAndSet(prevValue, nextValue)) {

The replace the primary perform takes the present worth of StateFlow, apply the replace perform to generate the following worth, then attempt to set the brand new worth. To set the worth, it compares the present worth with the earlier worth that was used to generate the brand new worth. If the present worth and the earlier worth should not equal, the perform returns false, indicating that the replace failed. The whereas the loop will proceed till compareAndSet returns true, which means the state has been up to date in a thread-safe means.

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Updated: May 8, 2023 — 2:46 am

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