1 Summary

Android uses UI - ViewModel - Repo stack which is MVVM (Model View ViewModel)

2 Activity

• Jetpack compose means there is only 1 activity

3 Intent and Intent Filters

• Intent filters are set in AndroidManifest.xml
  • Intent are sent to Activities and Activities will only receive intents listed in the AndroidManifest.xml IntentFilter
  • Typically Apps send intent between each other

4 Context

• Context is a bridge between vanilla kotlin and the android ecosystem
  • A kotlin class is useless without the android ecosystem, a class that is passed a context can manipulate the android ecosystem, turn on camera, open files, etc
• You have Activity Context and Application Context
  • Typically use Application Context
  • Activity Context is used when asking for permissions ActivityCompat.requestPermissions()

5 BackStack

• BackStack is just a Stack of Activities
• Task is a Stack of Stacks, which each stack representing a different App which we name a Task
  • [TaskA TaskB] where TaskA = [ScreenX ScreenY ..] and TaskB = [ScreenG ScreenF ..]

Launch Modes

• Standard - Example: inside a browser Activity and you click a link you push a new Browser activity to the backstack
• Single Top - Example: inside a browser Activity and you click a link, nothing is pushed to the backstack
• Single Task - Example: From instagram you click a link that opens a browser, if you go back, you go back to Instagram not the browser
• Single Instance - Example: Typically used for Payment apps and rarely used

6 ViewModel

• This is effectively just the useEffect() and setState() but for android
• The ViewModel gives setters and getters of state for the UI
• Why? Naive state gets destroyed just by rotating the screen while the ViewModel persists state

7 DI

• Dependency injection is parametrization
  • f(x) = x + 2 -> f(x,y) = x + y
  • y is the dependency injected into f
• The dependency is the producer
• The injected is the consumer typically a class

Modern DI libraries goes a step further and does automatic instantiation of the parameter, meaning y is set a value automatically.

8 install library

go to libs.version.toml

[versions]

[libraries]

[plugins]

9 Hilt

• First things first is to:
  • annotation your Application() Class with @HiltAndroidApp (lets us pass android application context in dependency functions)
    • What is Context? Remember a vanilla kotlin class knows nothing about android or its file system. The Context is the bridge between kotlin and Android ecosystem like opening a file or using the camera, etc.
  • annotate your MainActivity with @AndroidEntryPoint

@HiltAndroidApp
class MyApp: Application()

<application>
    android:name=".MyApp"
    ...

@Module
@InstallIn(SingletonComponent::class)
object AppModule {
    
    @Provides
    @Singleton
    fun provideMyApi(): MyApi {
        return Bleh
    }

    @Provides
    @Singleton
    fun provideMyRepository(api: MyApi): MyRepository {
        //notice it passes another Dependency Producer MyApi as argument
        //Since they are both in @Module MyApi will get injected from provideMyApi()
        return api
    }

    @Provides
    @Singleton
    fun provideSomethingElse(app: Application) : SomethingElse {
        //remember @HiltAndroidApp annotation lets us use android application context in dependencies
        return Bleh(app)
    }

}

@HiltViewModel
class MyViewModel @Inject constructor(
    private val repository: MyRepository
): ViewModel() {

}

import dagger.hilt.android.AndroidEntryPoint
import javax.inject.Inject

class MyText @Inject constructor() {
    val text = "Hello from Hilt!"
}
@AndroidEntryPoint
class MainActivity : ComponentActivity() {
    @Inject
    lateinit var myText: MyText
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        enableEdgeToEdge()
        setContent {
            DemoOneTheme {
                Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->
                    Column{

                        Greeting(
                            name = "hiilo",
                            modifier = Modifier.padding(innerPadding)
                        )
                        Log.d("MainActivity", "Injected text: ${myText.text}")
                        Text(myText.text)

                    }
                }
            }
        }
    }
}

@Composable
fun Greeting(name: String, modifier: Modifier = Modifier) {
    Text(
        text = "Hello $name!",
        modifier = modifier
    )
}

@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    DemoOneTheme {
        Greeting("Android")
    }
}