UUID in Mobile Apps — iOS, Android, and React Native

Mobile apps face a unique identifier challenge: records are often created offline — no network, no server, no sequence counter. UUIDs are the natural solution. The question is which version to use and how to store them efficiently on device.

iOS — Swift

UUID v4 (standard library):

import Foundation

let id = UUID() // UUID v4
let str = id.uuidString // "550E8400-E29B-41D4-A716-446655440000"

Note: Apple’s UUID type returns uppercase hex — most other ecosystems use lowercase. Both are valid per RFC 9562, but be consistent when comparing IDs across platforms.

Storing UUIDs in Core Data:

// Core Data entity attribute type: UUID
// In NSManagedObject subclass:
@NSManaged var id: UUID

Core Data stores UUID as a 16-byte BLOB internally (efficient). In SwiftUI previews and tests, generate deterministic UUIDs:

let testId = UUID(uuidString: "00000000-0000-0000-0000-000000000001")!

SQLite on iOS (GRDB.swift):

GRDB.swift is the most popular type-safe SQLite library for Swift:

import GRDB

struct Order: Codable, FetchableRecord, PersistableRecord {
    var id: UUID = UUID() // v4; swap for v7 generator when available
    var status: String
}

Android — Kotlin

UUID v4:

import java.util.UUID

val id = UUID.randomUUID()
val str = id.toString() // "550e8400-e29b-41d4-a716-446655440000"

UUID v7 with java-uuid-generator:

// build.gradle.kts
dependencies {
    implementation("com.fasterxml.uuid:java-uuid-generator:5.0.0")
}

import com.fasterxml.uuid.Generators

val id = Generators.timeBasedEpochGenerator().generate() // UUID v7

Room database:

@Entity(tableName = "orders")
data class Order(
    @PrimaryKey val id: UUID = Generators.timeBasedEpochGenerator().generate(),
    val status: String,
)

// In AppDatabase:
@TypeConverters(UuidConverter::class)
abstract class AppDatabase : RoomDatabase()

class UuidConverter {
    @TypeConverter fun fromUuid(uuid: UUID): String = uuid.toString()
    @TypeConverter fun toUuid(str: String): UUID = UUID.fromString(str)
}

Room stores the UUID as TEXT by default via the type converter. For BLOB storage (more efficient), convert to/from ByteArray.

React Native

React Native runs JavaScript in a Hermes or JavaScriptCore engine. crypto.randomUUID() is available in React Native 0.73+ (Hermes with the Web Crypto API polyfill). For UUID v7:

import { v7 as uuidv7 } from 'uuid';
// uuid npm package works in React Native via the react-native-get-random-values polyfill

import 'react-native-get-random-values'; // must be imported before uuid
import { v7 as uuidv7 } from 'uuid';

const id = uuidv7();

react-native-get-random-values bridges React Native’s Math.random() environment to the native CSPRNG, providing the entropy source the uuid package requires.

Offline-First Sync Patterns

Mobile apps create records offline, then sync to the server. UUID v7 simplifies sync because:

1. The client generates the UUID before network connectivity is available
2. The UUID’s timestamp indicates when the record was created offline
3. The server can detect records created far in the past (large timestamp gap) and apply conflict resolution rules
4. Sorted display by primary key gives approximate creation order — useful for showing “recently created” lists

For conflict resolution in collaborative apps, pair UUIDs with a vector clock or last-write-wins timestamp rather than relying on UUID ordering alone.

Related Resources

• Apple Foundation UUID documentation
• GRDB.swift — SQLite library for iOS
• java-uuid-generator (JUG) — UUID v7 for Android/JVM
• react-native-get-random-values — CSPRNG polyfill for React Native
• Android Room database documentation