Handling Errors in Kotlin Flow

In this lesson, you’ll learn about handling errors in Kotlin Flow. Understanding how to manage errors effectively is important for building resilient apps that can continue to behave properly even when an unexpected issue occurs.

The catch Operator

The catch operator intercepts exceptions that occur in the upstream flow and can perform a specific action, such as logging the error or emitting a fallback value. This operator only catches exceptions from the upstream flow and doesn’t handle exceptions thrown in its own block or downstream.

Imagine you have a flow that processes various types of carrots, but sometimes the process might fail due to an invalid type:

fun carrotTypes(): Flow<String> = flow {
  emit("Nantes")
  emit("Imperator")
  if ((0..5).random() == 0)
    throw RuntimeException("Unexpected carrot type") // Simulate an error
  emit("Chantenay")
}.catch { e ->
  println("Error processing carrots: ${e.message}")
  emit("Fallback Carrot")
}

fun processCarrots() = runBlocking {
  carrotTypes().collect { type ->
    println("Processing carrot: $type")
  }
}

In this example, when the exception occurs, the catch block catches it, logs the error, and then emits a “Fallback Carrot.” The important thing is that the flow continues to operate after handling the error.

The onCompletion Operator

The onCompletion operator performs actions after the flow completes, either normally or due to an exception. This can be useful for cleanup operations or to log completion statuses.

fun carrotTypes(): Flow<String> = flow {
  emit("Nantes")
  emit("Imperator")
  if ((0..5).random() == 0)
    throw RuntimeException("Error in processing") // Simulate an error
  emit("Chantenay")
}.onCompletion { cause ->
  if (cause != null)
    println("Flow completed with error: ${cause.message}")
  else
    println("Flow completed successfully")
}

fun processCarrots() = runBlocking {
  carrotTypes().collect { type ->
    println("Collected carrot: $type")
  }
}

In this scenario, onCompletion checks if the flow completed normally or with an error. It then logs the appropriate message based on the completion status.

Retry Mechanisms

Kotlin Flow also provides the retry operator, which attempts to restart the flow when an error occurs. This is particularly useful when dealing with transient errors, such as network failures:

fun carrotTypes(): Flow<String> = flow {
  emit("Nantes")
  emit("Imperator")
  if ((0..5).random() == 0)
    throw RuntimeException("Error in processing") // Simulate an error
  emit("Chantenay")
}.retry(3) { e ->
  println("Attempting to retry due to: ${cause.message}")
  true
}

fun processCarrots() = runBlocking {
  carrotTypes().collect { type ->
    println("Collected carrot: $type")
  }
}

Here, retry attempts to run the flow up to three times if a RuntimeException occurs. After the retries, if the error persists, it propagates downstream.

Wrap-Up

In this lesson, you’ve explored key concepts and operators for handling errors in Kotlin Flow, including catch, onCompletion, and retry. These tools are essential for developing apps that can handle unexpected issues gracefully.

Now, proceed to the demo, where you’ll use some of these concepts.