Untrack

Read signals without creating dependencies.

The Problem

By default, any signal read inside an effect creates a dependency:

let count = signal(0)
let multiplier = signal(2)

effect(fn() {
  // This effect re-runs when EITHER signal changes
  println((count.get() * multiplier.get()).to_string())
})

The Solution

Use untrack to read without tracking:

using @luna { signal, effect, untrack }

let count = signal(0)
let multiplier = signal(2)

effect(fn() {
  // Only re-runs when count changes
  let mult = untrack(fn() { multiplier.get() })
  println((count.get() * mult).to_string())
})

Using peek()

Signals also have a .peek() method:

let count = signal(0)
let multiplier = signal(2)

effect(fn() {
  // Only re-runs when count changes
  println((count.get() * multiplier.peek()).to_string())
})

untrack vs peek

// Use peek for single signal
let value = count.peek()

// Use untrack for multiple signals
let result = untrack(fn() {
  a.get() + b.get() + c.get()
})

Common Use Cases

Logging Without Re-triggering

effect(fn() {
  let value = count.get()

  // Log debug info without tracking
  let debug = untrack(fn() {
    "Count=" + count.peek().to_string() +
    " at " + timestamp.peek()
  })
  println(debug)
})

Conditional Logic

effect(fn() {
  let value = count.get()

  // Check config without tracking it
  if untrack(fn() { config.get().verbose }) {
    println("Verbose: " + value.to_string())
  }
})

Comparison with Previous Value

let count = signal(0)
let prev = signal(0)

effect(fn() {
  let current = count.get()
  let previous = prev.peek()  // Don't track prev

  if current != previous {
    println("Changed from " + previous.to_string() + " to " + current.to_string())
    prev.set(current)
  }
})

Caution

Overusing untrack can lead to stale data:

// Dangerous: display won't update when user changes
effect(fn() {
  let id = user_id.get()
  let user = untrack(fn() { users.get()[id] })  // Won't update!
  display_user(user)
})

API Summary

Try It

Create an effect that:

1. Tracks a query signal for search

2. Uses limit without tracking (config value)

3. Logs results

Next

Learn about Nested Effects →