Search documentation

Search by topic, API name, or import path.

GitHub ↗

variable / @typed/fx

Fx.keyed

Efficiently transforms a list of values into a list of Fx streams, using keys to track identity.

This is crucial for performance when rendering lists or managing collections of stateful entities. When the input list changes:

  • New keys cause onValue to be called.
  • Existing keys have their RefSubject updated with the new value.
  • Removed keys close the supplied child Scope and clean resources registered through it; the onValue run fiber remains owned by the outer parent Scope.
Package version
2.0.0-beta.13
Category
Keyed work
Since
1.0.0
Member of
Fx

Follow one execution

Efficiently transforms a list of values into a list of Fx streams, using keys to track identity. This is crucial for performance when rendering lists or managing collections of stateful entities. When the input list changes: - New keys cause onValue to be called. - Existing keys have their RefSubject updated with the new value. - Removed keys close the supplied child Scope and clean resources registered through it; the onValue run fiber remains owned by the outer parent Scope.

Fx timelinekeyed reuses b, closes removed a, and creates c under separate child scopeskeyed
Operatorkeyed({ getKey, onValue })
Read this diagram

Follow each lane from left to right. Events stacked vertically share a tick; the green cursor marks the current time across every lane.

  • A value
    The text inside the pill is the emitted value.
  • Work starts
    The raised chevron starts an inner run (^ in the source).
  • The run returns
    The vertical bar ends this lane’s run.
  • A cause is delivered
    The exclamation mark belongs to this lane.
  • Work is interrupted
    The cross marks cancellation of this run.
  • Current time
    The line and diamond move together across all lanes.
  • Happening now
    A highlighted event is at the current tick.
  • Still ahead
    Muted, dashed values have not happened yet.
  • Time continues
    The lane’s arrow is not a return marker. An empty stretch can be quiet work that is still running.

Illustrated ticks start at 0. At 1×, one illustrated tick takes one second; captions specify real durations when timing matters. A cause or interruption belongs to its lane, and other work may continue. Scroll horizontally to inspect the rest of a long timeline.

Each key receives its own RefSubject and child Scope. Reusing a key updates that subject in place. Removing a key closes the child Scope and therefore the resources registered through its supplied Scope service. The onValue run fiber itself is forked in the outer parent Scope, so child-Scope closure does not guarantee interruption of an arbitrary Fx that ignores Scope. Interrupting the outer run closes the parent and remaining children. Source and onValue failures, services, and Scope requirements remain visible in the return type.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

import { Fx } from "@typed/fx";

Access Fx.keyed through the imported Fx export. Its declaration below describes the member.

This public exposure is a re-export. Its import path is supported; declaration documentation is shared with the other public exposures below.

Signatures

export declare const keyed: {
    <A, B extends PropertyKey, C, E2, R2>(options: KeyedOptions<A, B, C, E2, R2>): <E, R>(fx: Fx<ReadonlyArray<A>, E, R>) => Fx<ReadonlyArray<C>, E | E2 | Cause.IllegalArgumentError, R | R2 | Scope.Scope>;
    <A, E, R, B extends PropertyKey, C, E2, R2>(fx: Fx<ReadonlyArray<A>, E, R>, options: KeyedOptions<A, B, C, E2, R2>): Fx<ReadonlyArray<C>, E | E2 | Cause.IllegalArgumentError, R | R2 | Scope.Scope>;
};

Why

A stable key separates identity from position. Consumers can move or update the exact result already associated with a value instead of recreating every result when a collection changes. This is the state-preserving substrate used by keyed template rendering, but the combinator itself is renderer-agnostic.

Ownership and lifetime

Each key receives its own RefSubject and child Scope. Reusing a key updates that subject in place. Removing a key closes the child Scope and therefore the resources registered through its supplied Scope service. The onValue run fiber itself is forked in the outer parent Scope, so child-Scope closure does not guarantee interruption of an arbitrary Fx that ignores Scope. Interrupting the outer run closes the parent and remaining children. Source and onValue failures, services, and Scope requirements remain visible in the return type.

Ordering and failures

Each source array produces a unique-key diff. Added keys start one child Fx; reused keys update their existing RefSubject, including moves; removed keys have their child Scope closed. A child may emit repeatedly, replacing only that key’s latest output. Arrays follow current source order and contain the contiguous ready prefix: a later key is withheld until every earlier key has emitted at least once. Debounce coalesces scheduled arrays, not child Fx values. Duplicate keys fail with Cause.IllegalArgumentError rather than sharing a child lifetime.

Examples

import { Fx } from "@typed/fx"

const rows = Fx.keyed(Fx.succeed([{ id: "a", value: 1 }]), {
  getKey: (row) => row.id,
  onValue: (row) => Fx.map(row, ({ value }) => value)
})

Other public imports

These import paths expose the same declaration. Each page retains its own public name and signature.

Source