I'm trying to create a class with some fields of a type that should only be constructed when those f...

# getting-started

[Rob Elliot]

I'm trying to create a class with some fields of a type that should only be constructed when those fields are initialised, but I can't work out how to make the type system do it. Code in thread.

[Rob Elliot]

This does not compile, because the FeatureFlag constructor is private:

class FeatureFlags(
  private val featureFlagsClient: FeatureFlagsClient,
) {

  val myNewFeature = FeatureFlag(key = "my-new-feature", defaultValue = false)

  inner class FeatureFlag private constructor(val key: String, val defaultValue: Boolean) {
    fun isEnabled(): Boolean = featureFlagsClient.isEnabled(this)
  }
}

interface FeatureFlagsClient {
  fun isEnabled(flag: FeatureFlags.FeatureFlag): Boolean
}

[Rob Elliot]

However, if I make it internal then it is possible to construct a FeatureFlag from outside the body of `FeatureFlags`:

FeatureFlags(featureFlagsClient).FeatureFlag("escaped-feature-flag", true)

[Rob Elliot]

Is there a way to restrict the scope of the constructor to the file?

[Rob Elliot]

OK, worked it out. Extract an interface and then the inner class can be private

[Rob Elliot]

package mocklab.system.featureflags

interface FeatureFlag {
  fun isEnabled(): Boolean
}

class FeatureFlags(
  private val featureFlagsClient: FeatureFlagsClient,
) {

  val myNewFeature: FeatureFlag = ClientBackedFeatureFlag(key = "my-new-feature", defaultValue = false)

  private inner class ClientBackedFeatureFlag(
    override val key: String,
    override val defaultValue: Boolean,
  ) : FeatureFlag, FeatureFlagDefinition {
    override fun isEnabled(): Boolean = featureFlagsClient.isEnabled(this)
  }
}

interface FeatureFlagDefinition {
  val key: String
  val defaultValue: Boolean
}

interface FeatureFlagsClient {
  fun isEnabled(flag: FeatureFlagDefinition): Boolean
}

[Sam]

I think I would still try to get rid of the circular dependency though… if you change it to

interface FeatureFlagsClient {
  fun isEnabled(key: String): Boolean?
}

it'll be much easier to test 😄

[Sam]

At the moment the feature flags depend on the client and the client depends on the feature flags, it's quite exciting

[Sam]

Inner classes allow all kinds of fun like that

[Rob Elliot]

I pulled out the FeatureFlagDefinition interface to get round that - I don't think there's a circular dep any more

[Sam]

Oh I see, I missed that change

[Rob Elliot]

Oh, maybe there is...

[Sam]

No I think you got it

[Sam]

At least the bit I was worried about

[Rob Elliot]

I think it's OK now - yes, the circular dep in the original was bothering me too!