# createLens \[A two-way view into part of an Atom] `createLens` makes a **Lens**: a focused, two-way view into an `Atom` — read a slice out, then write it back. Change detection uses `lodash.isEqual`. ## Why? A Lens **is an `Atom`** with the same `get()` / `set()` / `modify()` surface, but it points at *part* of another Atom instead of owning state. So anything that takes an Atom (`useAtom`, another `createLens`) takes a Lens too. * **Immutable focus.** `get` reads a slice out; `set` returns a **new** source with the slice replaced and leaves the original alone. Compose lenses to reach nested fields. * **Two-way and live.** Reads flow down, writes flow up, and the view re-derives when the source changes elsewhere. * **Dedupe both ways.** Inbound sync and write-back skip no-op updates, so writing an unchanged value won't echo back. Reach for a Lens when a consumer needs one field of a larger entity without knowing about the rest. ```ts import { createAtom, createLens, keyLens } from "rxfy"; const user = createAtom({ id: "1", name: "Alice" }); const name = createLens(user, keyLens("name")); name.get(); // "Alice" name.set("Bob"); user.get(); // { id: "1", name: "Bob" } ``` ## Signatures ```ts function createLens(source$: IAtom, lens: ILens): Lens; function keyLens(key: K): ILens; ``` The signatures name two different shapes. `ILens` is the **optic** you pass in, a pure view/edit pair that `keyLens` builds for you. `Lens` is what `createLens` returns: a reactive cell implementing [`IAtom`](/rxfy/create-atom), so it drops in anywhere an `Atom` is expected. ```ts type ILens = { get(source: S): T; // read the focused slice out of the source set(current: T, source: S): S; // return a NEW source with the slice replaced (immutable) }; // createLens(...) returns a Lens, which implements IAtom: type Lens = IAtom; // get(): T; set(val: T): void; modify(fn: (val: T) => T): void ``` To bind a form input to one field of a stored entity, compose a `keyLens` over a [`ModelStore.get(id)`](/rxfy/create-model#modelstore-get) handle and drive it with [`useAtom`](/react/use-atom). Edits flow back into the model store and reach every subscriber of that entity.