1. Introduction

Kotlin’s Flow API has revolutionized asynchronous programming by providing a powerful and concise way to handle streams of data. One of the key components of this API is MutableStateFlow, a mutable state holder that emits values to its collectors. However, confusion often arises when developers encounter the value property and the emit() function in the context of MutableStateFlow.

In this tutorial, we’ll delve into the differences between value and emit().

2. Understanding MutableStateFlow

Before diving into the differences, let’s quickly review what MutableStateFlow is. MutableStateFlow is a special kind of Flow that represents a mutable value. It holds a current value and allows collectors to receive updates whenever the value changes. Specifically, we can create new instances with a starting value:

val mutableStateFlow = MutableStateFlow("Initial Value")

Furthermore, we can subscribe to the value with the collect() function:

fun main() = runBlocking {
    val mutableStateFlow = MutableStateFlow("Initial Value")
    val job = launch {
        mutableStateFlow.collect { value ->
            println("Received value: $value")
        }
    }
    mutableStateFlow.value = "Updated Value 1"
    mutableStateFlow.value = "Updated Value 2"
    job.cancel()
}

In the provided example, we create a MutableStateFlow with an initial value of “Initial Value.” Then, we start a coroutine using the launch() function. We subscribe to values emitted by the MutableStateFlow with the collect() function. Eventually, The values of the MutableStateFlow update sequentially. The subscriber created with the collect() block receives each update and prints each value from within the coroutine.

3. The value Property

The value property is a read-write property of MutableStateFlow that represents its current value. It allows us to set or retrieve the current state without subscribing to the flow:

@Test
fun `initial value should be Baeldung Team`() {
    val mutableStateFlow = MutableStateFlow("Baeldung Team")
    val currentValue = mutableStateFlow.value
    assertEquals("Baeldung Team", currentValue)
}

In this code, we create a MutableStateFlow with an initial value of “Baeldung Team.” By accessing the value property, we obtain a point-in-time snapshot of the mutable state, and in this case, it reflects the initial value. Additionally, the value property is thread-safe, allowing for concurrent thread access.

4. The emit() Function

The emit() function is used to update the value of a MutableStateFlow. It’s a suspending function, meaning it must be called within a coroutine or coroutine context:

@Test
fun `emit function should update the value`() = runBlocking {
    val mutableStateFlow = MutableStateFlow("Baledung Team")
    mutableStateFlow.emit("Baledung Team Kotlin")
    assertEquals("Baledung Team Kotlin", mutableStateFlow.value)
}

This code demonstrates a simple state update to our MutableStateFlow. After creating it with an initial value of “Baeldung Team”, we update the state with emit() to be “Baeldung Team Kotlin”. Additionally, we use runBlocking() to create a parent coroutine context. If the buffer of a Flow is full, emit() will suspend until the buffer has room and the value can be set.

This example showcases how MutableStateFlow enables the asynchronous modification of mutable state.

5. The tryEmit() Function

The tryEmit() function attempts to emit a new value to the MutableStateFlow. It returns a boolean indicating whether the emission was successful. If the emission succeeds, the function returns true, if it fails, it returns false, meaning that a call to the emit() function would suspend until there is buffer space available. This method is thread-safe and can be safely invoked from concurrent threads without external synchronization:

@Test
fun `tryEmit function should update the value`() {
    val mutableStateFlow = MutableStateFlow("Baledung Team Kotlin 2024")
    val emitResult = mutableStateFlow.tryEmit("Baledung Team Kotlin 2024")
    assertTrue(emitResult)
    assertEquals("Baledung Team Kotlin 2024", mutableStateFlow.value)
}

In this example, we create a MutableStateFlow with an initial value, and then we use tryEmit() to update the value without being within a coroutine context. Finally, we checked the return value of tryEmit() to ensure the update was successful.

6. Conclusion

The value property of the MutableStaeFlow provides a straightforward means to access the current state without the need for a subscription, allowing developers to retrieve information efficiently. On the other hand, the emit() function plays a vital role in updating the MutableStateFlow’s value asynchronously, although it must be used within a coroutine context. Finally, we can also publish new values without a coroutine context with tryEmit(), although it may not be able to publish a new value.

As always, the full implementation of these examples is available over on GitHub.

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