craft-libs/crates/sketch/README.md

craft-sketch

A clean-room reader for the .sketch container, for files saved by Sketch 48 and newer. No unsafe.

This crate is the container layer. It opens the archive, applies the version cut, and hands out document.json, meta.json and the pages as the JSON the file holds (serde_json::Value, every object with its kind in _class) and the images as bytes. What the layers mean, and how they map onto an application's document, stays with the caller.

Provenance

Written by FigCraft's contributors (the .sketch import is Jindřich Šaršon's work), from the public description of the format only, and files built for each case, checked against Sketch's own renders and real files (Sketch 50, 63 and 2026). No code of Sketch or of any other product was copied, and no GPL, LGPL or AGPL code was consulted. Moved here from figcraft/crates/format/src/sketch.rs (the archive half).

The format

• A .sketch file is a ZIP archive: document.json (the pages in order as _refs to pages/<id>, shared styles, swatches, library symbols copied into the document), meta.json (appVersion, the format version), user.json, one pages/<id>.json per page (the layer tree), images/<name> and, in newer files, fonts/.
• Before version 43 a Sketch file was a SQLite database; until 47 text was a binary property list inside the JSON. Both are refused as too old (Error::TooOld, with a message saying to save the file again in a newer Sketch).
• meta.json versions above 139 (Sketch 83, the newest checked against real files) open with a warning.

Reading

if craft_sketch::is_sketch(&bytes) {
    let archive = craft_sketch::Archive::open(&bytes)?;
    for (id, page) in &archive.pages {                       // in the document's order
        let layers = page.get("layers");
    }
    let styles = archive.document.get("layerStyles");
    let image = archive.image("images/0a1b….png");            // Option<Vec<u8>>
    let fonts = craft_sketch::embedded_font_families(&archive.document);
    for warning in &archive.warnings { /* pages skipped, a newer format than tested */ }
}

A document.json that can't be read (damaged, nested too deep) loses the shared styles, not the pages: they are then taken in archive order, with a warning. A page that can't be read is skipped with a warning; the others open.

Limits

Input is hostile. Nothing panics; every limit has a test.

Limit	Value
meta.json, document.json (MAX_JSON_MEMBER)	512 MiB
A page (Limits::max_page)	512 MiB
JSON read in all (MAX_JSON_TOTAL)	2 GiB (512 MiB on 32-bit targets)
Unpacked size per compressed byte (MAX_RATIO), past a 16 MiB floor (MIN_ALLOWANCE)	100
Pages (MAX_PAGES)	1000
An image (Limits::max_image)	256 MiB
Image bytes in all (Limits::max_image_total)	1 GiB (256 MiB on 32-bit targets)
A page id or image name	128 characters of letters, digits, - and _ (an image may have one extension)
JSON nesting	serde_json's 128 levels

A member is refused before unpacking when its declared size is past its allowance, and again if it unpacks past it. A read that fails is charged in full, and a page listed twice is read once, so a page list can't repeat an unpacking within the budget.

ZIP

The archive is read with the zip crate directly, with the bounds above applied to each member. A shared bounded ZIP reader is planned (craft-libs #126); this crate moves to it when it lands.

The layers as REST file JSON (rest)

rest::to_rest maps the pages and layers onto the REST API file JSON that craft-figma-rest models: artboards and frames, groups, shape paths and shape groups with their boolean operations, text with its attributed runs, bitmaps, symbol masters (components) and instances (expanded from their master with the instance's overrides, including those in nested instances and swaps), stacks (auto layout), shared styles, swatches (colour variables), prototype links, export options and layout grids. Paths come as vector networks beside the JSON (vectorNetworkRef); stars and polygons whose points are still the generated ones carry pointCount and innerRadius. What isn't understood is skipped with a warning.

let archive = craft_sketch::Archive::open(&bytes)?;
let mapped = craft_sketch::rest::to_rest(&archive, "Untitled", craft_sketch::rest::Options::default());
if mapped.too_old { /* text stored the pre-48 way */ }

Options::fonts finds the installed face a PostScript name belongs to; without it, families are read from the names.

Limit	Value
Layers (Options::max_nodes)	200 000 by default
Layer nesting, symbol expansion included	128 levels
Work spent expanding symbols (one unit per JSON value copied)	60 million (15 million on 32-bit targets)
Points of one path	100 000
Distinct warnings	200

The walk recurses once per level of nesting, in small frames (each layer's own work is done in functions that return before its children are mapped). An unoptimised build maps about 140 levels of nested instances on a 512 KiB stack; the limit of 128 leaves twice the room on the 1 MiB stack of a WebAssembly build, and a test at the limit runs on a 1 MiB thread. FigCraft followed 200 levels.

The mapping was written in FigCraft (crates/format/src/sketch), mostly by Jindřich Šaršon, from the public description of the format and files built for each case.

Not here yet

• A typed model of the Sketch document itself (_class objects): the mapping reads the JSON.
• Writing.