How do you build a Singleton in Dart

Question

The singleton pattern ensures only one instance of a class is ever created  How do I build this in Dart

User · Answer

This is how I implement singleton in my projects

Inspired from flutter firebase => FirebaseFirestore.instance.collection('collectionName')

class FooAPI {
  foo() {
    // some async func to api
  }
}

class SingletonService {
  FooAPI _fooAPI;

  static final SingletonService _instance = SingletonService._internal();

  static SingletonService instance = SingletonService();

  factory SingletonService() {
    return _instance;
  }

  SingletonService._internal() {
    // TODO: add init logic if needed
    // FOR EXAMPLE API parameters
  }

  void foo() async {
    await _fooAPI.foo();
  }
}

void main(){
  SingletonService.instance.foo();
}

example from my project

class FirebaseLessonRepository implements LessonRepository {
  FirebaseLessonRepository._internal();

  static final _instance = FirebaseLessonRepository._internal();

  static final instance = FirebaseLessonRepository();

  factory FirebaseLessonRepository() => _instance;

  var lessonsCollection = fb.firestore().collection('lessons');
  
  // ... other code for crud etc ...
}

// then in my widgets
FirebaseLessonRepository.instance.someMethod(someParams);

User · Answer

Thanks to Dart s factory constructors  it s easy to build a singleton   class Singleton     static final Singleton  singleton   Singleton  internal       factory Singleton         return  singleton         Singleton  internal        You can construct it like this  main       var s1   Singleton      var s2   Singleton      print identical s1  s2        true   print s1    s2                true

User · Answer

This should work   class GlobalStore       static GlobalStore  instance      static GlobalStore get instance          if  instance    null              instance   new GlobalStore               return  instance                             factory GlobalStore    gt  instance

User · Answer

Dart singleton by const constructor  amp  factory  class Singleton     factory Singleton     gt      Singleton  internal       Singleton  internal           void main       print new Singleton      new Singleton       print identical new Singleton     new Singleton

User · Answer

Here s a concise example that combines the other solutions  Accessing the singleton can be done by    Using a singleton global variable that points to the instance  The common Singleton instance pattern  Using the default constructor  which is a factory that returns the instance    Note  You should implement only one of the three options so that code using the singleton is consistent   Singleton get singleton   gt  Singleton instance  ComplexSingleton get complexSingleton   gt  ComplexSingleton  instance   class Singleton     static final Singleton instance   Singleton  private      Singleton  private      factory Singleton     gt  instance     class ComplexSingleton     static ComplexSingleton  instance    static ComplexSingleton get instance   gt   instance    static void init arg    gt   instance     ComplexSingleton  init arg      final property    ComplexSingleton  init this property     factory ComplexSingleton     gt   instance      If you need to do complex initialization  you ll just have to do so before using the instance later in the program   Example  void main       print identical singleton  Singleton instance              true   print identical singleton  Singleton                       true   print complexSingleton    null                             true   ComplexSingleton init 0      print complexSingleton    null                             false   print identical complexSingleton  ComplexSingleton         true

User · Answer

Here is another possible way     void main       var s1   Singleton instance    s1 somedata   123    var s2   Singleton instance    print s2 somedata      123   print identical s1  s2        true   print s1    s2      true     var s3   new Singleton      produces a warning re missing default constructor and breaks on execution    class Singleton     static final Singleton  singleton   new Singleton  internal      Singleton  internal      static Singleton get instance   gt   singleton    var somedata

User · Answer

I don t find it very intuitive reading new Singleton    You have to read the docs to know that new isn t actually creating a new instance  as it normally would   Here s another way to do singletons  Basically what Andrew said above    lib thing dart    library thing   final Thing thing   new Thing  private     class Thing      Thing  private     print   2         foo          print   3             main dart  import  package thing thing dart    main       print   1      thing foo        Note that the singleton doesn t get created until the first time the getter is called due to Dart s lazy initialization   If you prefer you can also implement singletons as static getter on the singleton class  i e  Thing singleton  instead of a top level getter   Also read Bob Nystrom s take on singletons from his Game programming patterns book

User · Answer

As I m not very fond of using the new keyword or other constructor like calls on singletons  I would prefer to use a static getter called inst for example      the singleton class class Dao          singleton boilerplate         Dao  internal              static final Dao  singleton   new Dao  internal            static get inst   gt   singleton          business logic         void greet     gt  print  Hello from singleton        example usage   Dao inst greet             call a method     Dao x   new Dao         compiler error  Method not found   Dao      verify that there only exists one and only one instance assert identical Dao inst  Dao inst

User · Answer

I use this simple pattern on dart and previously on Swift  I like that it s terse and only one way of using it  class Singleton     static Singleton shared   Singleton  init      Singleton  init            init work here        void doSomething            Singleton shared doSomething

User · Answer

This is also a way to create a Singleton class class Singleton    Singleton        static final Singleton db   Singleton

User · Answer

Modified  Seth Ladd answer for who s prefer Swift style of singleton like  shared   class Auth        singleton   static final Auth  singleton   Auth  internal      factory Auth     gt   singleton    Auth  internal      static Auth get shared   gt   singleton        variables   String username    String password      Sample   Auth shared username    abc

User · Answer

What about just using a global variable within your library  like so     single dart   library singleton   var Singleton   new Impl     class Impl     int i      main dart   import  single dart    void main       var a   Singleton    var b   Singleton    a i   2    print b i       Or is this frowned upon   The singleton pattern is necessary in Java where the concept of globals doesn t exist  but it seems like you shouldn t need to go the long way around in Dart

User · Answer

After reading all the alternatives I came up with this  which reminds me a  classic singleton    class AccountService     static final  instance   AccountService  internal       AccountService  internal       static AccountService getInstance         return  instance

User · Answer

In this example I do other things that are also necessary when wanting to use a Singleton  For instance   pass a value to the singleton s constructor initialize a value inside the constructor itself set a value to a Singleton s variable be able to access and access those values   Like this  class MySingleton      static final MySingleton  singleton   MySingleton  internal       String  valueToBeSet    String  valueAlreadyInSingleton    String  passedValueInContructor     get getValueToBeSet   gt   valueToBeSet     get getValueAlreadyInSingleton   gt   valueAlreadyInSingleton     get getPassedValueInConstructor   gt   passedValueInContructor     void setValue newValue         valueToBeSet   newValue         factory MySingleton String passedString         singleton  valueAlreadyInSingleton    quot foo quot        singleton  passedValueInContructor   passedString       return  singleton         MySingleton  internal       Usage of MySingleton  void main      MySingleton mySingleton    MySingleton  quot passedString quot    mySingleton setValue  quot setValue quot    print mySingleton getPassedValueInConstructor   print mySingleton getValueToBeSet   print mySingleton getValueAlreadyInSingleton

User · Answer

Singleton that can t change the object after the instantiation class User     final int age    final String name       User       this name      this age              static User  instance       static User getInstance  name  age          if  instance    null            instance   User name  name  age  age          return  instance             return  instance           print User getInstance name   quot baidu quot   age  24  age     24      print User getInstance name   quot baidu 2 quot   name      is not changed   baidu    print User getInstance         name   quot baidu quot   age 24

User · Answer

Here is a comparison of several different ways to create a singleton in Dart  1  Factory constructor class SingletonOne      SingletonOne  privateConstructor       static final SingletonOne  instance   SingletonOne  privateConstructor       factory SingletonOne         return  instance          2  Static field with getter class SingletonTwo      SingletonTwo  privateConstructor       static final SingletonTwo  instance   SingletonTwo  privateConstructor       static SingletonTwo get instance   gt   instance        3  Static field class SingletonThree      SingletonThree  privateConstructor       static final SingletonThree instance   SingletonThree  privateConstructor          How to instantiate The above singletons are instantiated like this  SingletonOne one   SingletonOne    SingletonTwo two   SingletonTwo instance  SingletonThree three   SingletonThree instance   Note  I originally asked this as a question  but discovered that all of the methods above are valid and the choice largely depends on personal preference

User · Answer

Here is a simple answer  class Singleton     static Singleton  instance     Singleton         static Singleton get getInstance   gt   instance     Singleton

User · Answer

Hello what about something like this  Very simple implementation  Injector itself is singleton and also added classes into it  Of course can be extended very easily  If you are looking for something more sophisticated check this package  https   pub dartlang org packages flutter simple dependency injection  void main         Injector injector   Injector      injector add      gt  Person  Filip       injector add      gt  City  New York        Person person    injector get lt Person gt        City city    injector get lt City gt        print person name     print city name      class Person     String name     Person this name      class City     String name     City this name       typedef T CreateInstanceFn lt T gt      class Injector     static final Injector  singleton    Injector  internal      final  factories   Map lt String  dynamic gt        factory Injector         return  singleton         Injector  internal       String  generateKey lt T gt  T type        return    type toString    instance          void add lt T gt  CreateInstanceFn lt T gt  createInstance        final typeKey    generateKey T        factories typeKey    createInstance           T get lt T gt          final typeKey    generateKey T       T instance    factories typeKey       if  instance    null          print  Cannot find instance for type  typeKey               return instance

[dart] How do you build a Singleton in Dart?

example from my project

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