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In an Android project, there is a facade implemented as the singleton. I thought it is an better idea converting it to DI with HILT SingletonComponent.

@Module
@InstallIn(SingletonComponent::class)
object MyManagerModule {
    @Provides
    fun provide(@ApplicationContext context: Context): MyManager {
        return MyManager(context)
    }
}

class MyManager @Inject constructor(@ApplicationContext private val ctx: Context){
    //
}

From a few callers, I get the instance of the above MyManager using HILT field injection, i.e.,

@AndroidEntryPoint
class MyCallerFragment : Fragment() {
    @Inject lateinit var myManager: MyManager
// ...

In the debugger, I observed the DI instances are actually NOT the same instance (assuming these fragments are in the same activity lifecycle). I think I must have misunderstand the Hilt DI :-( and would love to hear any explanation if you see my blindspots.

Sean
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2 Answers2

16

TL;DR

You need to use the annotation @Singleton. This will tell Hilt to use the same instance of MyManager throughout your app.

Scope of Bindings

According to the documentation:

By default, all bindings in Hilt are unscoped. This means that each time your app requests the binding, Hilt creates a new instance of the needed type.

and

However, Hilt also allows a binding to be scoped to a particular component. Hilt only creates a scoped binding once per instance of the component that the binding is scoped to, and all requests for that binding share the same instance.

The @Singleton annotation scopes your Hilt binding to the application component. (including all children, which are all components) So Hilt will inject the same instance of your object throughout your entire app.

There is an example in this guide by Google.

The @InstallIn annotation

The annotation @InstallIn() tells Hilt in which component MyManager objects will be injected. In your case of @InstallIn(SingletonComponent::class), Hilt will make MyManager available for injection in the application component and all children of this component, which doesn't mean that Hilt will supply the same instance though. Since any default component is a child of the application component, MyManager is currently accessible for injection in any component. (according to the documentation)

TomS
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  • So if we need a single instance of the class, we need to remove InstallIn and add Singleton ?? – Mohsin Falak Sep 12 '22 at 09:17
  • If I'm correct, what he means with adding `@Singleton` is to keep the `InstallIn(SingletonComponent)` and add th `@Singleton` label in the function that you want to be unique so Hilt doesn't make another one when you are calling it in other places. – Esteban Nov 05 '22 at 14:02
-1

I had kinda the same question, Scope vs Component in Hilt, and this is what I got:

When you annotate a class with @InstalIn, actually you are defining the lifetime of this dependencies' module. For example, if you use SingletonComponent all dependencies from this class(Module) are stay as long as the application is up.

And for Scope, when you provide a dependency in a module, each time that module is called, a new instance is created. this is why you observed in debugger generated instances are not the same. by using Scopes like @Singleton you are changing the way of instantiation of dagger-hilt on called dependencies.

so generally speaking, Component is for instance lifetime and Scope is for the way of dependency instantiation on each call.

don't miss this article: https://medium.com/digigeek/hilt-components-and-scopes-in-android-b96546cb07df

Yasin Hajilou
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