asLiveData -The Terminal Operator

Table of contents

Terminal Operator

Flow provides terminal operators that initiate the collection of elements from a flow and execute an action or return a result asynchronously.

Explaining asLiveData()

• asLiveData() extension function is part of the lifecycle-livedata library
• it’s used to convert a Flow into a LiveData object.
• can be particularly useful when one wants to integrate Kotlin Flow with the Android Architecture Components, especially when working with the ViewModel and observing data changes in UI components that rely on LiveData.

asLiveData() as Terminal Operator

As per the details mentioned above about Terminal operator, asLiveData() also initiate the collection of elements from a flow and return result asynchronously.

To use asLiveData() in Android Project

• Need to add these dependencies in build.gradle file in Android studio

implementation("androidx.lifecycle:lifecycle-viewmodel-ktx:$current_lifecycle_version")

implementation("androidx.lifecycle:lifecycle-livedata-ktx:$current_lifecycle_version")

• Update the ViewModel with below code snippet where asLiveData() acts as a bridge between Kotlin Flow and LiveData,

class FlowUseCaseViewModel(stockPriceDataSource: StockPriceDataSource) {
    val currentPriceLiveData : LiveData<UiState> =         stockPriceDataSource.latestStockList.map {stockList->
            UiState.Success(stockList) as UiState
        }.onStart(emit(UiState.Loading)).onCompletion{
            Timber.tag("Flow").d("Flow has completed")
        }.asLiveData()
    
}

Green Flag about asLiveData()

• LiveData is designed to be lifecycle-aware. By converting a Flow into a LiveData using asLiveData, data changes can be easily observed.
• Android’s data binding library works well with LiveData.
• If application has to support both older and newer Android versions, asLiveData provides a convenient way to work with Kotlin Flow in a backward-compatible manner.
• The asLiveData extension function allows to specify a dispatcher on which the Flow should be collected. This can be beneficial for handling asynchronous flows, ensuring that the Flow’s emission and collection are done on the appropriate dispatchers, such as the IO dispatcher.

import androidx.lifecycle.LiveData
import androidx.lifecycle.asLiveData
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.runBlocking

fun main() = runBlocking {
    val flow = flow {
        for (i in 1..5) {
            delay(1000) // Simulate asynchronous work
            emit(i)
        }
    }

    val liveData: LiveData<Int> = flow.asLiveData(Dispatchers.IO)

    liveData.observeForever { value ->
        println("Received: $value")
    }

    delay(6000) // Wait for the flow to complete
}

In this example :

• simple Flow that emits values from 1 to 5 with a delay to simulate asynchronous work.
• the asLiveData() extension function to convert the Flow into a LiveData object. The Dispatchers.IO parameter specifies the dispatcher on which the Flow is collected.
• We observe the LiveData object using observeForever() for demonstration purposes.

Some Important points to remember

1. If the LiveData instance is not properly removed or observed, it might lead to unintended and prolonged subscriptions, affecting the lifecycle of the associated components.
2. It may not work as seamlessly with MutableStateFlow instances.
3. State flows are designed to be mutable and updated, and using asLiveData with them might not capture the mutability aspects accurately.

Take-away

• LiveData and Flow are both are used for handling asynchronous data streams in Android app development.
• While LiveData simplifies UI updates and lifecycle management, Flow offers more flexibility and powerful operators for complex data processing.
• LiveData converted from a Flow using the asLiveData() function will behave slightly differently than expected. It will emit data only when there are active observers.

Happy Learning !!!