Overview

Miso.Subscription.Util provides createSub, the building block used by every subscription in Miso.Subscription. It handles the acquire-use-release lifecycle of an external resource (an event listener, an animation-frame callback, etc.) using bracket so the resource is always cleaned up when the Component unmounts, even if an exception is thrown.

Quick start

Use createSub to build a custom subscription from any pair of acquire/release actions. The subscription sleeps between polls and relies entirely on the acquire step to register whatever callbacks deliver events to the sink:

import Miso.Subscription.Util (createSub)
import Miso.Effect (Sub)
import qualified Miso.FFI.Internal as FFI

-- Custom subscription: fire an action whenever the window is resized
resizeSub :: (Int -> action) -> Sub action
resizeSub toAction sink = createSub acquire release sink
  where
    acquire =
      FFI.windowAddEventListener "resize" $ \_ -> do
        w <- FFI.windowInnerWidth
        sink (toAction w)
    release cb =
      FFI.windowRemoveEventListener "resize" cb

How it works

createSub acquire release sink runs:

bracket acquire release (\_ -> forever (threadDelay 10000_000_000))

The forever loop keeps the subscription thread alive by sleeping in very long increments. All actual work is done inside callbacks registered during acquire, which call sink directly. release is guaranteed to run (via bracket) when the component teardown kills the thread.

See also

• Miso.Effect — Sub, Sink
• Miso.FFI.Internal — windowAddEventListener, windowRemoveEventListener
• Miso.Subscription — all built-in subscriptions