Effect

An effect is a function that automatically re-runs whenever its tracked signal dependencies change. Effects are useful for side effects — logging to the console, making API calls, or synchronizing state with non-reactive systems.

Creating an Effect

let (count, set_count) = @reactive.create_signal(0)

@reactive.create_effect(fn(_prev) {
  println("Count changed to: \{count.get()}")
})

set_count.set(1)  // triggers the effect

The effect runs immediately on creation, then again whenever count changes.

What Gets Tracked

During an effect’s execution, any signal you read via .get() is automatically registered as a dependency. If a changes, but not b, the effect still re-runs because it read both:

@reactive.create_effect(fn(_) {
  println("Sum: \{a.get() + b.get()}")
})

Cleanup

Effects can register cleanup to run before the next execution or when the owning scope is disposed. This prevents stale callbacks and memory leaks:

@reactive.create_effect(fn(_) {
  let id = @ffi.set_timeout(fn() {
    println("Delayed log")
  }, 1000)

  @reactive.on_cleanup(fn() {
    @ffi.clear_timeout(id)
  })
})

Batching

When updating multiple signals, wrap them in batch so effects run only once after all updates:

@reactive.batch(fn() {
  set_a.set(100)
  set_b.set(200)
  set_c.set(300)
})