Dispatchers

Learn contexts with Dispatchers.IO, Default and Main.

What are Dispatchers?

Dispatchers decide which thread pool a coroutine runs on. Dispatchers.IO is for network and disk, Dispatchers.Default for CPU heavy work, and Dispatchers.Main for the UI thread.

You pass a dispatcher when starting a coroutine: launch(Dispatchers.IO) { ... }. You can switch a running coroutine's dispatcher temporarily with withContext. Choosing the right dispatcher matters for performance.

IO vs Default vs Main

Dispatchers.IO is a pool that can grow up to 64 threads; it is ideal for waiting work like network and files. Dispatchers.Default offers as many threads as CPU cores; it suits computation.

Dispatchers.Main is platform specific; on Android it is the UI thread. UI updates happen here. With withContext(Dispatchers.Default) { ... } you move CPU work to Default temporarily, then return to Main. A wrong dispatcher freezes the UI.

Exercise Switch dispatcher with withContext

Inside runBlocking, open a block with withContext(Dispatchers.Default). Print println("Computed") inside it. After the block, print println("Done").

Show example solution

fun main() = runBlocking {
    withContext(Dispatchers.Default) {
        println("Computed")
    }
    println("Done")
}

Test yourself

Pick the dispatcher for a network call

launch(Dispatchers.____) { fetchUsers() }

Show answer

launch(Dispatchers.IO) { fetchUsers() }

Match dispatchers to their use

Dispatchers.IO

Network and disk
Dispatchers.Default

CPU heavy work
Dispatchers.Main

UI thread
withContext

Temporary switch
Which dispatcher should run a CPU heavy computation?
- Dispatchers.Main
- Dispatchers.Default
- Dispatchers.IO
- Dispatchers.Unconfined
Show answer

Dispatchers.Default — CPU work runs on Dispatchers.Default.

Next lesson: Job

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