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Mastering Kotlin Coroutines

Hot Channel and Cold Flow

Discover how to distinguish and utilize hot and cold data streams in Kotlin coroutines. Learn about hot channels that produce elements independently and cold flows that generate data on demand with terminal operations. Understand the key differences to effectively apply these concepts in your Android applications.

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Returning to coroutines, the most typical way to create a flow is using a builder, similar to the produce function, called flow.

Kotlin 1.5
val channel = produce {
   while (true) {
       val x = computeNextValue()
       send(x)
   }
}
val flow = flow {
   while (true) {
       val x = computeNextValue()
       emit(x)
   }
}

These builders are conceptually equivalent, but since the behavior of channels and flow is very different, there are also significant differences between these two functions.

Channel example

Take a look at the example below. Channels are hot, so they immediately start calculating the values. This calculation starts in a separate coroutine. This is why produce needs to be a coroutine builder defined as an extension function on CoroutineScope. The ...