# Fosutilities API Catalog > Discover FOSUtilities APIs before writing helper code. Use when reaching for JSON/Codable encoding-decoding, wire-format date formatting, URLSession/HTTP fetching, WebSockets, network mocking in tests, collection grouping or throttled iteration, string casing/hashing/obfuscation/CSV parsing, hex or rounded-number formatting, semantic version comparison, semaphores or async-from-sync bridging, typed model identifiers, runtime environment/bundle-version checks (simulator/TestFlight detection), ViewModel declaration and localization, form fields and validation, ServerRequests/CRUD, SwiftUI ViewModel binding, Vapor boot wiring/routes/Fluent/Leaf/middleware, ViewModel or ServerRequest or UI testing, or PDF generation — in any project that imports FOSFoundation, FOSMVVM, FOSMVVMVapor, FOSTesting, or FOSReporting. - Skill: `foscomputerservices/fosutilities-api-catalog` (Agent Skill) - Install (CLI): `npx skillmds@latest add foscomputerservices/fosutilities-api-catalog` - Raw SKILL.md: https://api.skillmd.com/api/skills/foscomputerservices/fosutilities-api-catalog/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Data & Analytics - Author: foscomputerservices (https://skillmd.com/u/foscomputerservices) - Updated: 2026-09-10 - Page: https://skillmd.com/skills/foscomputerservices/fosutilities-api-catalog --- # FOSUtilities API Catalog Before hand-writing a helper in a project that imports these libraries, check whether it already exists. Fifteen-plus years of accumulated API lives here; the most common failure is reinventing it. ## How to use this skill 1. Find your reach in the index below. 2. Read **only** the catalog file(s) it points to, from `../shared/api-catalog/`. 3. Prefer the catalogued API over hand-rolled code; use it as the entry shows (each entry's example is the idiomatic form). If your reach isn't in the index, scan the catalog file for the library you're importing — and if the capability exists but the index line was missing, add the line via the `fosutilities-api-catalog-update` skill. ## Reach-for index - Reaching for `JSONEncoder`/`JSONDecoder`, writing Codable glue → `FOSFoundation.md § Coding` - Reaching for `DateFormatter`/`ISO8601DateFormatter` for wire or JSON dates → `FOSFoundation.md § Coding` - Building stub/test instances of Codable types → `FOSFoundation.md § Coding`, `FOSTesting.md § FOSTesting` - Reaching for `URLSession`/`URLRequest`, fetching or posting Codable data → `FOSFoundation.md § Networking` - Reaching for `URLSessionWebSocketTask` → `FOSFoundation.md § Networking` - Mocking network calls in tests → `FOSFoundation.md § Networking`, `FOSTesting.md § FOSTesting` - Grouping an array into a dictionary, or rate-limiting (throttling) iteration → `FOSFoundation.md § Collections` - Converting string casing (camel/snake), trimming prefixes/suffixes, generating random strings → `FOSFoundation.md § String` - Hashing (SHA-256/HMAC), obfuscating strings, parsing hex strings or CSV files → `FOSFoundation.md § String` - Rounding doubles, or formatting integers as hex → `FOSFoundation.md § Numbers` - Comparing or parsing app/API versions (semver) → `FOSFoundation.md § Versioning` - Reaching for semaphores in async code, or calling async code from a sync context → `FOSFoundation.md § Async` - Detecting simulator/TestFlight installs, reading the app bundle's version → `FOSFoundation.md § Extensions` - Typing a model identifier (never a raw `UUID`/`String` field) → `FOSFoundation.md § Data` - Declaring a ViewModel or versioning its factory (`@ViewModel`, `@VersionedFactory`) → `FOSMVVM.md § Macros` - Localizing properties from YAML (`@LocalizedString`), substituting values into localized text → `FOSMVVM.md § Localization` - Encoding a ViewModel with its localizations resolved → `FOSMVVM.md § Extensions` - Describing form fields — control type, keyboard, input constraints, value binding → `FOSMVVM.md § Forms` - Validating user input, reporting and aggregating validation outcomes → `FOSMVVM.md § Validation` - Binding a screen to server data — ViewModel requests, CRUD writes, factories → `FOSMVVM.md § Protocols` - Identifying *which* entity a model is (opaque `ModelIdentity`) — keying refresh or authorization by it → `FOSMVVM.md § Protocols` - Container-scoped authorization — declaring containers, grant verbs, who may touch which records → `FOSMVVM.md § Protocols` - Client-chosen sort or pagination on a request → `FOSMVVM.md § Protocols` - Live-updating screens that refresh when server data changes (`@ViewModel(options: [.live])`), or replacing the invalidation transport → `FOSMVVM.md § Protocols`, `FOSMVVMVapor.md § Live Invalidation` - Attaching auth headers (bearer token, API key) to every client request, rotation-safe — or recovering when the server refuses one (refresh the credential and retry the request once) → `FOSMVVM.md § Protocols` - Declaring the data a server-rendered body needs — composable factory, load requirements, rooted scopes → `FOSMVVM.md § Protocols` - Rendering a ViewModel in SwiftUI — app setup, view binding, previews, form views → `FOSMVVM.md § SwiftUI Support` - A Button whose action is `async throws` — error routing, re-entry refusal, running state, tap-to-cancel → `FOSMVVM.md § SwiftUI Support` - A view-lifetime (or value-keyed) load that can throw — `.task`-style error routing into the screen binding → `FOSMVVM.md § SwiftUI Support` - Showing errors to the user — localized error messages on error types, one alert fed by a shared error binding → `FOSMVVM.md § Protocols`, `§ SwiftUI Support` - Versioning ViewModel properties, choosing deployment URLs, negotiating versions over HTTP → `FOSMVVM.md § Versioning` - Booting a Vapor server for MVVM — YAML localization store, environment, locale, Leaf rendering → `FOSMVVMVapor.md § Extensions` - Registering request routes (reads and CRUD writes) — including mounting one behind a credential/middleware group — or serving a request outside the guarded verbs → `FOSMVVMVapor.md § Vapor Support` - Projecting loaded records into a response body, or reading them through the projection context → `FOSMVVMVapor.md § Containment`, `§ Protocols` - Declaring Fluent containers and their relations, or mapping sort meanings to database columns → `FOSMVVMVapor.md § Containment` - Registering the container authorization provider, apex resolver, per-request app state, or a container migration → `FOSMVVMVapor.md § Containment`, `§ Extensions` - Filtering (narrowing) a large container load by the request's query → `FOSMVVMVapor.md § Containment` - Enabling server-pushed refresh at boot, or transactional writes that notify live clients → `FOSMVVMVapor.md § Live Invalidation` - Refreshing live screens whose data isn't Fluent-persisted — nudging from an `Application`-hosted actor or computed aggregate, or registering a dependency the load plan can't see → `FOSMVVMVapor.md § Live Invalidation` - The server-side write path — candidate set, field application, authorization provider → `FOSMVVMVapor.md § Protocols` - Projecting the database into ViewModels — resolvable requests, Fluent `DataModel` → `FOSMVVMVapor.md § Protocols` - Serving typed/localized errors, gating routes on client app version → `FOSMVVMVapor.md § Middleware` - Verifying a caller's bearer token / protecting route groups with app-owned credential rules → `FOSMVVMVapor.md § Middleware` - Testing ViewModels — Codable round-trip, version stability, translation coverage → `FOSTesting.md § FOSTesting` - UI-testing SwiftUI ViewModel views (XCUITest hosting, operations, identifiers, keyboard dismissal) → `FOSTesting.md § FOSTestingUI`, `FOSMVVM.md § SwiftUI Support` - Testing Vapor ServerRequests end to end, or Fluent-backed code against a fresh in-memory database → `FOSTesting.md § FOSTestingVapor` - Testing a streaming / SSE endpoint over a real socket → `FOSTesting.md § FOSTestingVapor` - Generating PDFs from SwiftUI views, choosing page size/orientation → `FOSReporting.md § PDF Rendering`