Example of singleton for SQLite using FMDB

FMDBDatabase.swift

class FMDBDatabase {
    
    /*
     * @description defines completion closure for database calls
     */
    public typealias Completion = ((Bool, Error?)-> Void)
    
    public typealias ResultCompletion = ((Bool, FMResultSet?, Error?)-> Void)
    
    /*
     * @description singleton to access the database
     */
    static let sharedDatabase:FMDatabase = {
        let fileURL = try! FileManager.default
            .url(for: .applicationSupportDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
            .appendingPathComponent("test.sqlite")
        let database = FMDatabase(url: fileURL)
        return database
    }()
    
    /*
     * @description singleton to access a serial queue for updating the database
     */
    static let sharedQueue: FMDatabaseQueue = {
        let documents = try! FileManager.default
        .url(for:.applicationSupportDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
        let fileURL = documents.appendingPathComponent("test.sqlite")
        return FMDatabaseQueue(path: fileURL.path)!
    }()
}