Android Core Concept

Android Interview Huh?

If your interviewer asks " Explain Dagger Hilt? "

Here is how you do it:

What is Dagger Hilt?

• Dagger Hilt is another library that allows us to implement Dependency Injection

• Dagger Hilt is easier than Dagger 2

• The hilt is just a wrapper around Dagger 2

• For now, it has become a standard way to implement DI

• Behind the scenes, it generates Dagger code

• Knowing about Dagger 2 will make it easier to understand, But you don't necessarily need it

Why is it needed?

• Simply because Dagger 2 has a lot of methods and implementations

• Hilt just streamlines it

  

To implement Hilt inside your app

• You need to add dependencies and some plugins like Kotlin-Kapt

• Kapt -> Kotlin Annotation Processing Tool

• Dagger works similarly to Dagger 2 with Annotations

How does Dagger Hilt streamline Dagger 2?

• Annotations provided by Hilt are standardized

• You don't need to implement your own annotations like in the case of Dagger 2

• These standard annotations form a layer on top of other annotations

• Therefore one annotation acts as the parent of another and it becomes its sub-annotations that inherit from the parent

• To understand more pls refer to the documentation

What is @ HiltAndroidApp ?

• This annotation is always placed in the application class (application Context)

• Other annotations take the application context from this annotation

• It simply generates the Dagger code behind the scenes

  

What is @ AndroidEntryPoint?

• This annotation will be on top of your main activity

• It tells Dagger about the Entry point of the injections

• It simply tells us where the dependency needs to be injected

• An important thing to remember is if you use @ AndroidEntryPoint on a class

• And if other classes depend on that class

• Those classes also need to have @ AndroidEntryPoint

What is @ Inject?

• In order to give a dependency to AndroidEntryPoint (of Repository, services etc.)

• We use @ Inject

• It simply means in order to construct this object please run his constructor

• 

What is @ Module?

• Suppose you want to access an object that is using a third-party service (Like calling API from Retrofit)

• Hilt does not know how to create that object because it does not have any @ Inject constructor

• Here comes the usage of @ Module

• It tells us how to create that third-party object

• We use @ Provides to create a third-party object (Like this retrofit call)

  

Now what is InstallIn(SingletonCompoenent::Class)?

• Here is where Hilt shines over Dagger 2

• Remember there were many components in Dagger 2

• Now these components are layered on top of each other making it easier to access

  

• We need to provide Singleton because we need to tell Dagger to please not make the same object again and again

• When activities are created or destroyed

• Because it will become expensive (especially in the case of Retrofit call)

• Therefore we use Singleton which allows us to restrict object creation

• Singelton Component is applicable in the Application Context

• We can also create an Activitycompoenent or fragment Component if we need to

How does the hilt work in this case?

• First, it checks if is there any @ Inject constructor to build the object - NO

• Secondly goes to the activity component is there any @ provides

  here - NO

• Thirdly goes to the activity retained component are there any @ provides - No

  

• Fourthly goes to the Singelton component - Here he finds the @ provides

• Constructs the object and injects it where it is needed

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