How to make a Swift Class Singleton instance thread safe?

Question

I have a singleton class as so:

class Database {
   static let instance:Database = Database()
   private var db: Connection?

   private init(){
      do {
        db = try Connection("\(path)/SalesPresenterDatabase.sqlite3")
        }catch{print(error)}
   }
}

Now I access this class using Database.instance.xxxxxx to perform a function within the class. However when I access the instance from another thread it throws bizarre results as if its trying to create another instance. Should I be referencing the instance in the same thread?

To clarify the bizarre results show database I/o errors because of two instances trying to access the db at once

Update
please see this question for more info on the database code: Using transactions to insert is throwing errors Sqlite.swift

Swift singleton creation is thread safe. This doesn't meant that the functions you call in the singleton instance will be magically thread safe. You need to use something like a semaphore, a serial dispatch queue or an operation queue if you need to make functions thread safe — Paulw11, Apr 13 '17 at 13:33
`However when I access the instance from another thread it throws bizarre results as if its trying to create another instance` I don't think so, given the current definition of your `Database` class, you **cannot** create 2 instances. However maybe there's more code into that class that you are not showing. BTW: one last refinement to your class: you should declare it as `final` to prevent subclassing. — Luca Angeletti, Apr 13 '17 at 13:38
The additional code in the class shows an **addRow** function. This function gets called in an alamofire completion block which then throws the error where it can't insert data. Interestingly though if I remove all calls to my Database.instance. singleton and let the problematic bit of code run it first, it will work fine... — Richard Thompson, Apr 13 '17 at 13:43
please see this question for more info on the database code: http://stackoverflow.com/questions/43388443/using-transactions-to-insert-is-throwing-errors-sqlite-swift — Richard Thompson, Apr 13 '17 at 13:44
@RichardThompson you can use FMDB and use his `FMDatabaseQueue` for thread safe access to your db — Reinier Melian, Apr 13 '17 at 13:58
Thanks for the advise I'll look into this now. I've managed to find a work around by just creating a new instance of the class and referencing the **addRows** function. Not ideal though.... — Richard Thompson, Apr 13 '17 at 14:01
You can also use [GRDB](https://github.com/groue/GRDB.swift). it has robust threading support. It will provide as many guarantees as FMDB, but without Objective-C. — Gwendal Roué, Apr 14 '17 at 03:16

Dhananjay Patil · Answer 1 · 2017-04-14T09:34:33.690

6

class var shareInstance: ClassName {

    get {
        struct Static {
            static var instance: ClassName? = nil
            static var token: dispatch_once_t = 0
        }
        dispatch_once(&Static.token, {
            Static.instance = ClassName()
        })
        return Static.instance!
    }
}

USE: let object:ClassName = ClassName.shareInstance

Swift 3.0

class ClassName {
  static let sharedInstance: ClassName = { ClassName()} ()
}

USE: let object:ClassName = ClassName.shareInstance

edited Apr 14 '17 at 09:34

answered Apr 14 '17 at 04:36

Dhananjay Patil

442
3
10

I'm using swift3 so the dispatch_once_t option isn't available to me.... – Richard Thompson Apr 14 '17 at 09:28

score 3 · Answer 2 · answered Jun 07 '17 at 15:33

3

In Swift 3.0 add a private init to prevent others from using the default () initializer.

class ClassName {
  static let sharedInstance = ClassName()
  private init() {} //This prevents others from using the default '()' initializer for this class.
}

answered Jun 07 '17 at 15:33

Richard Hope

251
3
6

But, do you think that creating sharedInstance is thread safe ? – Neeraj Khede Feb 11 '19 at 07:46

score 1 · Answer 3 · answered Mar 22 '21 at 13:45

Singleton thread class.

final public class SettingsThreadSafe {

   public static let shared = SettingsThreadSafe()

   private let concurrentQueue = DispatchQueue(label: "com.appname.typeOfQueueAndUse", attributes: .concurrent)
   private var settings: [String: Any] = ["Theme": "Dark",
                                       "MaxConsurrentDownloads": 4]

   private init() {}

   public func string(forKey key: String) -> String? {
       var result: String?
       concurrentQueue.sync {
           result = self.settings[key] as? String
       }
       return result
   }

   public func int(forKey key: String) -> Int? {
       var result: Int?
       concurrentQueue.sync {
           result = self.settings[key] as? Int
       }
       return result
   }

   public func set(value: Any, forKey key: String) {
       concurrentQueue.async( flags: .barrier ) {
           self.settings[key] = value
       }
   }
}

Unit to test the singleton class.

func testConcurrentUsage() {
    let concurrentQueue = DispatchQueue(label: "concurrentQueue", attributes: .concurrent)
    
    let expect = expectation(description: "Using SettingsThreadSafe.shared from multiple threads shall succeed")
    
    let callCount = 300
    for callIndex in 1...callCount {
        concurrentQueue.async {
            SettingsThreadSafe.shared.set(value: callIndex, forKey: String(callIndex))
        }
    }
    
    while SettingsThreadSafe.shared.int(forKey: String(callCount)) != callCount {
        // nop
    }
    
    expect.fulfill()
    waitForExpectations(timeout: 5) { (error) in
        XCTAssertNil(error, "Test expectation failed")
    }
}

How to make a Swift Class Singleton instance thread safe?

3 Answers3