UI actions

It is well-known that

The Andoid UI toolkit is not thread-safe. So, you must not manipulate your UI from a worker thread—you must do all manipulation to your user interface from the UI thread.

In order to enforce correct thread usage, Macroid introduces UI actions.

What are they

UI actions are pieces of code that are supposed to be run on the UI thread. A simple UI action can be created like this:

import macroid.Ui

val action = Ui {
  3 + 3

It does not do anything yet and has a type Ui[Int], since it returns an integer. If you know Haskell or scalaz, so far that’s your typical IO monad.

A UI action can be sent to the UI thread to be run:

// or

Running an action of type Ui[A] returns a Future[A] (in this case — Future[Int]), so that you can monitor its completion. If the action fails, you can handle the failure with the standard Future API.

Note that UI actions are not memoized, so running an action will recalculate the result each time.

In some cases you will also want to run it on the current thread (but you have to be 100% sure you are inside the UI thread):

// or


The major advantages of using UI actions are:

  • We can ensure that the UI code is always run on the correct thread;
  • When submitting several UI commands from a background thread, we can combine them and batch in a single Runnable, so that the operation is atomic.

Usage in Macroid APIs

In Macroid all UI-related methods return UI actions. This includes the layout bricks:

val button: Ui[Button] = w[Button]


val action: Ui[Button] = button <~ text("Hi")

and so on. The first case is one of the reasons why Ui.get is needed sometimes, as setContentView expects a View and not a Ui[View]:

setContentView {
  Ui.get {
    w[TextView] <~ text("Hi")


UI actions can be composed in several ways.

  • Using the ~ operator (the type of the last action is preserved)

    val action: Ui[Button] =
      (startButton <~ disable) ~
      (stopButton <~ enable)
  • Using the ~~ operator (in case of Ui[Future]). See Snailing workflows for more details, or Understanding operators, if you are confused.

  • Using Ui.sequence (the sequence type is preserved)

    val action: Ui[Seq[Button]] = Ui.sequence(
      startButton <~ disable,
      stopButton <~ enable
  • If you feel like depending on scalaz, you can define an instance of Monad[Ui] and use Effectful:

    val action: Ui[Button] = effectfully {
      (startButton <~ disable).!
      // some other code, 
      // that will also be run
      // on the UI thread
      (stopButton <~ enable).!

Usage with Futures

The UiFuture extension class provides mapUi, flatMapUi and other xxxUi methods, which allow to use UI actions as callbacks for Futures:

```scala import macroid._ import scala.concurrent.ExecutionContext.Implicits.global

Future { … // happens on background thread } mapUi { x ⇒ Ui(actionBar.setTitle(x)) // happens on the UI thread } ```

Best practices

Since the UI thread has to be cherished, it is a good practice to demarkate the parts of your code that need to use it. Returning UI actions instead of just executing the code allows to do just that. That’s why, for example, setting an event handler in Macroid requires you to provide a UI action. Declaring your methods as UI actions also helps when they should be evaluated each time they are called (which is normally the case with event handlers).

For an example of how to take advantage of UI actions together with Akka actors, take a look at macroid-akka-pingpong Activator template.

Low-level threading utilities

Macroid also provides macroid.UiThreadExecutionContext, which allows to run futures on the UI thread. You probably wouldn’t need to use this directly.