once

import { once } from 'patronum';
// or
import { once } from 'patronum/once';

Motivation

The method allows to do something only on the first ever trigger of source. It is useful to trigger effects or other logic only once per application's lifetime.

Formulae

target = once(source);

• When source is triggered, launch target with data from source, but only once.

Arguments

• source (Event<T> | Effect<T> | Store<T>) — Source unit, data from this unit is used by target.

Returns

• target Event<T> — The event that will be triggered exactly once after source is triggered.

Example

const messageReceived = createEvent<string>();
const firstMessageReceived = once(messageReceived);

firstMessageReceived.watch((message) =>
  console.log('First message received:', message),
);

messageReceived('Hello'); // First message received: Hello
messageReceived('World');

Alternative

import { createGate } from 'effector-react';

const PageGate = createGate();

sample({
  source: once(PageGate.open),
  target: fetchDataFx,
});