How to implement Singleton pattern in Dart using factory constructors?

Question

I'm trying to implement the singleton pattern in a database helper class, but, I can't seem to understand the purpose of a factory constructor and if there's an alternative method to using it.

class DbHelper {

         final String tblName ='';
         final String clmnName ='';
         final String clmnPass='';

         DbHelper._constr();
         static final DbHelper _db = new DbHelper._constr();

         factory DbHelper(){ return _db;}  

         Database _mydb;
         Future<Database> get mydb async{

         initDb() {
          if(_mydb != null)
          {
              return _mydb;
          }
          _mydb = await  initDb();
          return _mydb;
         }

Günter Zöchbauer · Accepted Answer · 2018-12-28 09:23:21Z

6

There is no need to use the factory constructor. The factory constructor was convenient when new was not yet optional because then it new MyClass() worked for classes where the constructor returned a new instance every time or where the class returned a cached instance. It was not the callers responsibility to know how and when the object was actually created.

You can change

factory DbHelper(){ return _db;}

to

DbHelper get singleton { return _db;}

and acquire the instance using

var mySingletonReference = DbHelper.singleton;

instead of

var mySingletonReference = DbHelper();

It's just a matter of preference.

answered Dec 28, 2018 at 9:23

Günter Zöchbauer

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6 Comments

Salma Over a year ago

so it's either use a static singleton property or use a factory pattern. correct?

Günter Zöchbauer Over a year ago

You can also change static final DbHelper _db = new DbHelper._constr(); to static final DbHelper singleton = new DbHelper._constr(); and remove the singleton getter I suggested in my answer. It depends on your use case. You might not be able to use a field initializer if you need additional config values to create the instance. In your example it would be sufficient though.

Suragch Over a year ago

I'm not really understanding. A full example (or a link to one) would be really helpful. I also don't understand how the use cases affect it.

Suragch Over a year ago

Is this what you are talking about? So it is just a matter of preference? Both have the same effect?

Günter Zöchbauer Over a year ago

Yes it's only a matter of preference.

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