Coroutine Tips and Tricks

This section presents techniques to use coroutines efficiently and avoid common pitfalls.

Switching Between the Background Threads and EDT

Avoid the invokeLater-style often found in Java:

launch(Dispatchers.EDT) {
  val uiData = collectUiData()
  // switch to Default:
  launch(Dispatchers.Default) {
    val result = compute(uiData)
    // switch to EDT again:
    launch(Dispatchers.EDT) {
      applyUiData(result)
    }
  }
}

The recommended approach:

launch(Dispatchers.EDT) {
  val uiData = collectUiData()
  // switch to Default:
  val result = withContext(Dispatchers.Default) {
    compute(uiData)
  }
  // this will be resumed on EDT automatically:
  applyUiData(result)
}

Dispatching to the End of a Queue

In some cases, it is required to exit the current EDT event and continue after all events in the queue are processed. In a non-coroutine context, it could be implemented like in the following snippet:

invokeLater {
  step1()
  invokeLater {
    step2()
    invokeLater {
      step3()
    }
  }
}

In a coroutine context, use the following approach:

withContext(Dispatchers.EDT) {
  step1()
  yield() // suspends here, dispatches the following block again on EDT
  step2()
  yield()
  step3()
}

This approach works with any sequential dispatcher, e.g., created with Dispatchers.Default.limitedParallelism(1).

The same applies to runBlockingCancellable:

runBlockingCancellable {
  println(1)
  launch {
    print(2)
    yield()
    print(3)
  }
  print(4)
  yield()
  print(5)
  yield()
  print(6)
}
// Output: 142536

Scheduling Tasks With a Fixed Delay

There is no scheduleWithFixedDelay() in coroutines, because it can be easily implemented with the following snippet:

val job = coroutineScope.launch {
  delay(initialDelayMs)
  while (true) {
    action() // can be suspending as well
    delay(delayMs)
  }
}

When the job is no longer needed, remember to cancel the launched coroutine:

job.cancel()

or the whole scope:

coroutineScope.cancel()

Limiting Dispatcher Parallelism

Each call of limitedParallelism() creates a new independent dispatcher instance, effectively not limiting the parallelism:

suspend fun doSomething() {
  withContext(Dispatchers.Default.limitedParallelism(3)) {
    // ...
  }
}

Instead, store the dispatcher instance into a static property and use it as a context:

private val myDispatcher = Dispatchers.Default.limitedParallelism(3)

suspend fun doSomething() {
  withContext(myDispatcher) {
    // ...
  }
}

Changing Modality State

Avoid changing modality state in the middle of a running coroutine:

cs.launch {
  // ...
  withContext(Dispatchers.EDT + ModalityState.any().asContextElement()) {
    // ...
  }
}

Add the modality state to the context when launching a coroutine:

cs.launch(ModalityState.current().asContextElement()) {
  // ...
  withContext(Dispatchers.EDT) {
    // ...
  }
}

It is possible that the coroutine is launched as a response to a user event from EDT, where ModalityState.current() is available.

If the coroutine is launched from a background thread, then it should not be invoked on top of an unrelated dialog anyway. The absence of the context modality state is effectively equivalent to specifying ModalityState.nonModal().

Last modified: 21 February 2025