once

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

target = once(source)

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,
});

target = once({ source, reset })

Motivation

This overload may receive reset in addition to source. If reset is fired, target will be allowed to trigger one more time, when source is called.

Formulae

target = once({ source, reset });

• When source is triggered, launch target with data from source, but only once.
• When reset is triggered, let once be reset to the initial state and allow target to be triggered again upon source being called.

Arguments

• source (Event<T> | Effect<T> | Store<T>) — Source unit, data from this unit is used by target.
• reset (Event | Effect | Store) — A unit that resets once to the initial state, allowing target to be triggered again.

Returns

• target Event<T> — The event that will be triggered once after source is triggered, until once is reset by calling reset.

Example

import { createGate } from 'effector-react';

const PageGate = createGate();

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