> ## Knowledge Base Index
> Fetch the complete knowledge base index at: https://documentation.celestory.io/sitemap.xml
> Use this file to discover available pages before exploring further.
> Pure-Markdown content can be obtained by appending a '.md' suffix to the content URLs listed in the sitemap (without the trailing slash).

# 🟥 Once

# 🟥 Once

![Once](https://celestory-docs-illustrations.netlify.app/en/Once_EN.svg)

The Once block lets a flow through only once: as soon as a signal has passed through it, it closes itself automatically and blocks any signal that follows, a bit like a single-use lock. Dedicated inputs let you reopen or close it again manually whenever you need to.

This block is ideal for preventing unwanted repetitions (e.g. multiple triggering of a trap) or guaranteeing the uniqueness of an action (e.g. distribution of a reward).

## 📥 Entries

- in (Flow): Input flow to execute if the block is open. Once passed, the block closes automatically.
- open (Control flow): Forces the block to open, allowing the in flow to pass again.
- close (Control flow): Forces the block to close, blocking any further passage.
- toggle (Control flow): Inverts the current state of the block (open → closed / closed → open).
- closed (Boolean): Initial state of the block upon loading (True = closed, False = open by default).

## 📤 Outings

- out (Flow): Output flow activated only if the block was open when in was triggered.

## 💡 Example of use

Scenario: Single-use emergency button

1. A red “Alert” button is placed in a level.
1. Clicking on this button is connected to the in input of the Once block (set to closed = False).
1. The out output triggers an audible alarm and sends a message to the guards.
1. The player presses once: the alarm goes off, and the block closes automatically.
1. The player presses again: nothing happens, because the block is now closed.
1. To reset the button (e.g. after a level reset), a stream is connected to the open input.

## ⚙️ Technical Details

- Internal behavior: The block always remembers whether it's open or closed. When a signal passes through in, the block automatically closes itself right after (if it was already closed, the signal doesn't get through).
- Priority management: The open, close and toggle inputs change the block's state immediately, no matter its current state.

---

*Documented version: v1*