Re: Dependency injection in Scala?

> On 2008-02-01 19:18:53 Stepan Koltsov wrote:
> > Hello,
> >
> > Is there any way to write code in Scala as in Java+Spring? I. e. with
> > dependency injection of components.
> >
> > It is hard to write large applications without DI.
> >
> > Should DI be done in pure Scala some way, or there is any framework,
> > or Spring itself should be used?
>
> The code below (using the example from David's link) is a way to very
> closely model Spring-style autowiring using pure, immutable, type-safe
> Scala. I should say that I'm not necessarily recommending this style;
> there are a great many ways to skin this cat. For example the Cake
> pattern uses mixins to combine all the components into a single
> namespace.
>
> First define the services, exactly as usual:
>
>   trait OnOffDevice {
>       def on() : Unit
>       def off() : Unit
>   }
>
>   trait SensorDevice {
>       def isCoffeePresent : Boolean
>   }
>
>   class Heater extends OnOffDevice {
>       def on() {}
>       def off() {}
>   }
>
>   class PotSensor extends SensorDevice {
>       def isCoffeePresent = true
>   }
>
> Now the consumer. The dependencies are defined as a nested trait with
> an abstract instance. You could import it if you like, to avoid
> writing env.foo each time.
>
>   abstract class Warmer {
>       trait Env {
>           val potSensor : SensorDevice
>           val heater : OnOffDevice
>       }
>       protected val env : Env
>
>       def trigger() {
>           if(env.potSensor.isCoffeePresent) env.heater.on()
>           else env.heater.off()
>       }
>   }
>
> To build a configuration, we instantiate the objects:
>
>   val thePotSensor = new PotSensor
>   val theHeater = new Heater
>   val theWarmer = new Warmer { protected val env = new Config with Env }
>
> And name them, so that they will be wired together:
>
>   class Config {
>       lazy val potSensor = thePotSensor
>       lazy val heater = theHeater
>   }
>
> The trick is the 'new Config with Env', which allows us to take a
> single global Config class, and make it implement whatever Env each
> component requests. That's the bit you can't do in Java. Another
> possibility would be to use structural subtyping, which would save a
> few keystrokes for a slight performance hit.
>
> /J
>