Dagger 2 - what is the purpose of a @Singleton annotation class

Solution 1:

@Singleton (and any other scope annotation) makes your class a single instance in your dependencies graph (it means that this instance will be "singleton" as long as Component object exists).

In short - everytime you're injecting @Singleton annotated class (with @Inject annotation) it will be the same instance as long as you inject it from the same Component.

For more I'm referring my blog post about how @Singleton and other scopes annotations works in Dagger 2: http://frogermcs.github.io/dependency-injection-with-dagger-2-custom-scopes/

Solution 2:

@Singleton does not really create a Singleton, it is just a Scope, it is advised to not use @Singleton as it is misleading, it gives the impression that we are infact getting a Singleton, but we are not.

Let's say you annotate your database dependency with @Singleton and link with a Component, now let's say that you initialise this Component in Activities A and B, you will have different instances of your database in your two Activities which is something most people don't want.

How do you overcome this?

Initialise your Component once in your Application class and access it statically in other places like Activities or Fragments, now this could soon get out of hand if you have more than 20 Component's as you cannot initialise all of them in your Application class, doing so will also slow down your app launch time.

The best solution according to me is to create a real Singleton, either double checked or of other variants and use this statically as getInstance() and use this under @Provides in your Module.

I know it breaks my heart too, but please understand that @Singleton is not really a Singleton, it's a Scope.

Solution 3:

What is singleton?

Singleton Pattern in android

A single instance of a class providing a global point of access to itself during the entire application's lifetime.

@Singleton annotation in Dagger

A single instance of a class which is unique to a specific component, its access is limited to the scope of the component.

Purpose of singleton

To provide a single instance of a class within the dependency graph(component). a component is usually initialized in the application level as it executes only ones for the entire applications lifetime and is accessible by all activities and fragment.

Let's take an example:

CoffeeComponent.kt

@Singleton
@Component
interface CoffeeComponent {

  fun getCoffeeMaker():CoffeeMaker
  fun inject(activityA: ActivityA)
  fun inject(activityB: ActivityB)
}

CoffeeMaker.kt

@Singleton
class CoffeeMaker @Inject constructor()

CoffeeAplication.kt

class CoffeeApplication : Application() {

  private val component by lazy {
    DaggerCoffeeComponent.builder().build()
  }

  fun getAppComponent(): CoffeeComponent = component
}

Recommended Practise

Always go for lazy initialization of your component.
Scenario: say, you are team decided to add an onboarding/tutorial screen or incorporate some other design which doesn't require the component during the initial screens, it will help minimize the startup delay. Always remember, component initialization is expensive.

ActivityA.kt

import dagger.Lazy

class ActivityA: AppCompatActivity() {

  @Inject
  lateinit var coffeeMaker1:Lazy<CoffeeMaker>
  @Inject
  lateinit var coffeeMaker2:Lazy<CoffeeMaker>
  private val component by lazy {
    (application as CoffeeApplication).getAppComponent()
}
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    btn_activityB.setOnClickListener { startActivity(Intent(this, NewActivity::class.java)) }

    component.inject(this)

    println("Activity A CoffeeMaker 1 - ${coffeeMaker1.get()}")
    println("Activity A CoffeeMaker 2 - ${coffeeMaker2.get()}")
  }
}

If your class is expensive to construct, use the dagger's Lazy initialization, please don't confuse it with kotlin's Lazy. You have to import

import dagger.Lazy

@Inject
lateinit var coffeeMaker1:Lazy<CoffeeMaker>

ActivityB.kt

class ActivityB: AppCompatActivity() {

@Inject
lateinit var coffeeMaker1:Lazy<CoffeeMaker>
@Inject
lateinit var coffeeMaker2:Lazy<CoffeeMaker>
private val component by lazy { 
(application as CoffeeApplication).getAppComponent() }

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_new)
    component.inject(this)

    println("Activity B CoffeeMaker 1 - ${coffeeMaker1.get()}")
    println("Activity B CoffeeMaker 2 - ${coffeeMaker2.get()}")
}
}

you will get the Log output as

Note:

If you want to share a singleton instance between activities, lazily initialize them in the application level, if you initialize them in an activity you will end up with different instance as the components are different