when-daggers-singleton-is-created-twice.md

When Dagger's @Singleton is created twice?

Assumptions:

1. DaggerAndroid is used
2. Main Application @Component has @Module with @Subcomponent in it

When you want to inject Fragment dependencies from subcomponent, you have to call for Main Application @Component in order to fetch exposed @Subcomponent.

This can look like this:

override fun onAttach(context: Context) {
    (requireActivity().application as HasFeatureComponent).featureComponent().create().inject(this)
    super.onAttach(context)
}

When Application setup looks like this:

override fun applicationInjector() = DaggerMainComponent.factory().create(this)

override fun featureComponent() = (applicationInjector() as MainComponent).featureComponent()

execution of featureComponent() will cause creating DaggerMainComponent every single call.

This means, that few instances of DaggerMainComponent will be created, which results, among others, with creating @Singleton instance for every created component.

Fix?

We have to create one DaggerMainComponent and reuse it later:

lateinit var component: MainComponent

override fun applicationInjector(): AndroidInjector<out DaggerApplication> {
    component = DaggerMainComponent.factory().create(this)
    return component
}

override fun featureComponent(): FeatureComponent.Factory = component.featureComponent()