expose()

expose(name, source) publishes a piece of the part's geometry under a name. It is the outbound half of the part's interface: param() takes values in, connector() hands mate frames out, expose() hands geometry out.

Two consumers read it:

• Another part builds on it as def.features.<name> — extrude a donor's profile, sketch on its face, project its edges.
• A tangent mate touches it as instance.features.<name> — the contact surface of a tangent joint.

In the viewport

expose() is written in code. In the timeline, a part row folds its exposures behind an N exposed toggle; clicking one highlights the published geometry.

The code behind it

flange.part.js

import { part, sketch, circle, extrude, cut, color, plane, select, expose } from 'fluidcad/core';
import { face } from 'fluidcad/filters';

// A pipe flange: Ø60 disc, 6 thick, with a Ø20 bore and four Ø6 bolt holes
// on a 44 mm bolt circle. It publishes the hole pattern and its sealing
// face; consumers read them as `flange.features.<name>`.
export const flange = part('Flange', () => {
    sketch('xy', () => {
        circle([0, 0], 60);
    });
    extrude(6);
    // The bore and the bolt pattern in one sketch. .reusable() keeps it
    // alive after the cut consumes it, so it can be published below.
    const holes = sketch('xy', () => {
        circle([0, 0], 20);
        circle([22, 0], 6);
        circle([0, 22], 6);
        circle([-22, 0], 6);
        circle([0, -22], 6);
    }).reusable();
    cut(-6, holes);   // negative: cut along the sketch normal, up through the disc

    // Publish the hole sketch under the name "holes" …
    expose('holes', holes);
    // … and the sealing face under "seal". A tangent mate in an assembly
    // reads this one as `instance.features.seal`.
    expose('seal', select(face().planar().onPlane('xy', 6)));
});

// A gasket for that flange, in the same file: a 2 mm disc 6 mm below the
// flange with the flange's own hole pattern cut through it.
export const gasket = part('Gasket', () => {
    sketch(plane('xy', { offset: -6 }), () => {
        circle([0, 0], 60);
    });
    extrude(-2);
    color('tomato');
    // The published sketch lies on the flange's plane (z = 0); cut 8 deep
    // from there to reach through the gasket.
    cut(8, flange.features.holes);
});

The hole sketch is marked .reusable() because the cut that follows would otherwise consume it, leaving nothing to publish. The gasket never redraws the bolt circle: it cuts the flange's own pattern, so moving a bolt hole on the flange moves it on the gasket.

Signature

expose(name, source)

Argument	Meaning
name	A plain identifier, unique within the part.
source	Any scene object: a sketch, a select() of faces or edges, a plane, an axis, or a feature.

The exposure is a pass-through: it consumes nothing and adds no shape of its own. Consumers read the source object, so a change to the donor flows through on the next render. A published select() stops rendering its highlight once the statement has run — publishing is not selecting.

Consuming across files

// gasket.part.js
import { part, sketch, circle, extrude, cut, plane } from 'fluidcad/core';
import { flange } from './flange.part.js';

export const gasket = part('Gasket', () => {
    sketch(plane('xy', { offset: -6 }), () => {
        circle([0, 0], 60);
    });
    extrude(-2);
    cut(8, flange.features.holes);   // the published sketch is on the flange's plane, 6 above
});

Reading flange.features materialises the flange if it has not been built yet, in the consumer's unit.

Contact geometry for tangent mates

A tangent mate needs a surface to touch. Expose the face (planar, cylindrical, conical, spherical) and pass the bound exposure to the mate:

// roller.part.js
expose('tread', select(face().cylinder(20)));

// roller-stand.assembly.js
mate('tangent', wheel.features.tread, deck.features.deck);

Raw values cannot be exposed — every exposure must be real geometry, so that it re-derives on every render.

Rules

• Declare exposures directly in the part body, not inside a nested callback.
• Names are unique within a part.
• The callback's return value no longer feeds def.features; use expose().