Mobile apps (iOS and Android)

import UIKit

enum FirstMRR {
    private static let endpoint = URL(string: "https://firstmrr.app/app/api/collect")!
    private static let key = "pk_0123456789abcdef01234567" // your public key
    private static let defaults = UserDefaults.standard

    /// A random id created at the first launch: never the IDFA or identifierForVendor.
    static var installId: String {
        if let id = defaults.string(forKey: "firstmrr.installId") { return id }
        let id = UUID().uuidString
        defaults.set(id, forKey: "firstmrr.installId")
        return id
    }

    /// The "Audience measurement" setting of your app.
    static var enabled: Bool {
        get { !defaults.bool(forKey: "firstmrr.optOut") }
        set { defaults.set(!newValue, forKey: "firstmrr.optOut") }
    }

    static func track(_ event: String, screen: String = "/", extra: [String: Any] = [:]) {
        guard enabled else { return }
        var body: [String: Any] = ["k": key, "e": event, "a": installId, "p": screen, "w": Int(UIScreen.main.bounds.width)]
        body.merge(extra) { _, new in new }
        guard let data = try? JSONSerialization.data(withJSONObject: body) else { return }
        var request = URLRequest(url: endpoint, timeoutInterval: 5)
        request.httpMethod = "POST"
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.httpBody = data
        URLSession.shared.dataTask(with: request).resume() // fire and forget: no retry, no queue
    }
}

// FirstMRR.track("open", screen: "/home")
// FirstMRR.track("payment", extra: ["v": 9.99, "c": "USD", "i": transaction.id])