⚠️ This may appear complex but in reality this issue is quite simple. We tried to ensure it's thoroughly documented for ease of contribution ⚠️

Problem Overview

There is a discrepancy in model initialization between Flutter and React Native (RN): Flutter's CactusContext.init automatically downloads models if modelUrl is provided, while RN requires manual handling of local paths. To unify, we need to remove auto-download from Flutter's init, making both platforms require a local path for consistency.

This keeps init lean and avoids forcing dependencies in RN (since RN does not ship with file management modules).

Note: This is part 2 of a two-part unification effort (remove auto-download from Flutter). See part 1 for removing auto-download from Flutter: #96

Task

Flutter (flutter/lib/utils/download.dart):

• Create a new file and implement Future<String> downloadModel(String url, {String? filename, Function(double, String)? onProgress}).
• Use Dart's built-in http package: Check if file exists in app docs dir; download if not; report progress via callback (e.g., percentage and status like "Downloading...").
• Return the local path on success.

React Native (src/utils/download.ts):

• Create a new file and implement the same function signature using react-native-fs (treat as optional peer dependency and throw clear error like "Install react-native-fs to use downloadModel" if not found).

• Check if file exists; download to DocumentDirectoryPath; support progress callback.

• In src/index.ts, export it with a try-catch for missing rn-fs (e.g., export { downloadModel } from './utils/download';).

• For both: Handle basics like custom filenames, retries on failure (simple 3-try logic), and caching (skip if file exists and is valid/non-zero size).

• Update docs and examples in both platforms to show downloadModel usage before init (e.g., const path = await downloadModel(url); await init({ model: path });).

Contributor Checklist

• Install required deps if needed (Flutter: none; RN: yarn add react-native-fs for testing).
• Test on device/simulator: Download a small test file; verify progress callback works; check error handling (e.g., bad URL, no network).
• Keep in mind: No forced deps in RN core: utility should fail gracefully if rn-fs missing. Support large files (progress essential).
• Align signatures exactly between platforms (same params, return local path string).
• Update README.md with examples; ensure no changes to core init functions here (that's part 1).
• Run linter/tests; in PR, bonus points for demo with screenshots/logs of download working :) .

PR Checklist

• Functions return local path; progress callback fires correctly (e.g., 0-1.0 scale).
• RN version doesn't break if rn-fs not installed (throws helpful error).
• Examples show end-to-end: download + init.
• Good for beginners with file/network handling; cross-links to part 1.