Building a Singleton Object using Providers about objection HOT 7 CLOSED

Hi Thomas,

The object will only act as a singleton if Objection is responsible for constructing it. If you manually allocate it then it will not act as a singleton.

However, if you use a provider with Objection you will get a singleton -- I have verified this.

  [self bindBlock:^(JSObjectionInjector *context) {
    Car *car = [context getObject:[ManualCar class]];
    car.engine = (id)myEngine;
    return (id)car;    
  } toClass:[Car class]];

from objection.

Hi Justin,

this only works if your class has a default constructor:

[[ManualCar alloc] init];

But what if your class has to be constructed like this:

[[ManualCar alloc] initWithEngine:myEngine];

~Thomas

from objection.

Objection uses the default initializer only. You can use the awakeFromObjection method or set the properties after the fact to setup other things.

from objection.

Hi Justin,

I've tested your suggestion above but I'm not sure if this will work for me.
Objection will call the provider each time I'm asking for the singleton. Thus all my dependencies also need to be singletons.
What I'm looking for is something like that:

[self bindProvider:[[[CarProvider alloc] init] autorelease] toClass:[Car class]
    lifeCycle:JSObjectionInstantiationRuleSingleton];

~Thomas

from objection.

Tziem,

If you are creating your own provider there isn't any reason you couldn't make the object it builds a singleton. The provider can create and hold a reference to an instance of whatever object you are trying to create. It can then return that same object anytime it is called on to provide an object.

The provider mechanism is built for flexibility, so you can control whatever it is you like within the context of the provider.

from objection.

But this means that there are more than one managed contexts. Objection holds a global context and a provider can also hold a context.
In Java I'm using Google Guice for DI.
There I can do something like that:

bind(TransactionLog.class).to(InMemoryTransactionLog.class).in(Singleton.class);

Neither TransactionLog nor InMemoryTransactionLog knows anything about singletons.
Providers can also have dependencies which will be managed by the injector.

~Thomas

from objection.

Indeed,

If you need use of a custom initializer with custom dependencies you are outside of Objection's normal workflow anyhow. Objection is delegating the construction of an object to a provider, much like it delegates to an Objection Module. There isn't any reason, however, that you couldn't create your own custom provider (singleton vs normal) and objection module class (new binding method that specifies lifecycle) that does this for you.

Objection does not initialize objects using 'constructors' (initializer methods), but instead uses property injection via Key-Value coding. Hence, it is awkward to try and use Objection with customer initializers.

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