Demonstration of fetching data from React Native AsyncLocalStorage

AsyncLocalStorage.swift

// Since AsyncLocalStorage API is not exposed to native code, we have to grab the data ourselves.
// If you have been able to get Cocoapods use_frameworks! to work, you can `import React`
// Else make sure to include the following in your brindging header:
// #import <React/RCTAsyncLocalStorage.h>

import Foundation

class CacheManager {

    let storageLocation = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent("RCTAsyncLocalStorage_V1")

    /// Smaller values are stored in the manifest file.
    /// Larger values get written to a separate file.
    lazy var manifest: [String: String]? = {
        do {
            let manifestLocation = storageLocation.appendingPathComponent("manifest.json")
            let manifestString = try String(contentsOf: manifestLocation)
            let manifestData = manifestString.data(using: .utf8)!
            let manifestJSON =  try JSONSerialization.jsonObject(with: manifestData, options: .allowFragments)
            return manifestJSON as? [String: String]
        } catch {
            print(error)
            return nil
        }
    }()

    /// Grabs data given the cacheKey.
    /// Looks in manifest first, then looks to the filesystem.
    /// If we find a string, we attempt to convert it to JSON.
    func fetch(cacheKey: String) -> Any? {
        var jsonString: String?
        if let value = manifest?[cacheKey] {
            jsonString = value
        } else {
            let fileName = RCTMD5Hash(cacheKey)
            let fileLocation = storageLocation.appendingPathComponent(fileName)
            jsonString = try? String(contentsOf: fileLocation)
        }

        guard jsonString != nil else {
            return nil
        }

        do {
            let jsonData = jsonString!.data(using: .utf8)!
            let json = try JSONSerialization.jsonObject(with: jsonData, options: .allowFragments)
            return json
        } catch {
            print(error)
            return jsonString
        }
    }
}