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Overview

The Daily Image feature displays NASA’s Astronomy Picture of the Day (APOD), showcasing stunning space imagery with detailed explanations. Users can view today’s image or select any specific date to explore past astronomical wonders.

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User Experience

1

View Today's Image

Launch the Daily Image screen to see today’s featured astronomy picture with its title, date, and full explanation.
2

Select a Date

Choose any date to view the APOD from that specific day, allowing exploration of NASA’s historical image archive.
3

Save to Favorites

Tap the favorite icon to save images to your personal collection for easy access later.
The Daily Image feature uses NASA’s official APOD API, which has been providing daily space images since June 16, 1995.

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API Integration

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NASA API Service

The NasaApiService defines the API endpoint for fetching daily images:
app/src/main/java/com/ccandeladev/nasaexplorer/data/api/NasaApiService.kt
interface NasaApiService {

    @GET("planetary/apod")
    suspend fun getImageOfTheDay(
        @Query("api_key") apiKey: String,
        @Query("date") date: String? = null
    ): NasaResponse
}
Parameters:
  • api_key: Your NASA API key for authentication
  • date (optional): Date in YYYY-MM-DD format. If not provided, returns today’s image

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Repository Layer

The NasaRepository acts as an intermediary between the API and UI:
app/src/main/java/com/ccandeladev/nasaexplorer/data/api/NasaRepository.kt
class NasaRepository @Inject constructor(private val nasaApiService: NasaApiService) {

    companion object {
        private const val API_KEY = BuildConfig.NASA_API_KEY
    }

    suspend fun getImageOfTheDay(date: String? = null): NasaModel {
        val response = nasaApiService.getImageOfTheDay(apiKey = API_KEY, date = date)
        return response.toNasaModel()
    }
}

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Data Models

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NasaResponse

The raw API response model:
app/src/main/java/com/ccandeladev/nasaexplorer/data/api/NasaResponse.kt
data class NasaResponse(
    @SerializedName("title") val title: String,
    @SerializedName("url") val url: String,
    @SerializedName("date") val date: String,
    @SerializedName("explanation") val explanation: String,
    @SerializedName("media_type") val mediaType: String?,
    @SerializedName("hdurl") val hdUrl: String?,
    @SerializedName("service_version") val serviceVersion: String?,
    @SerializedName("copyright") val copyright: String?
)

fun NasaResponse.toNasaModel(): NasaModel {
    return NasaModel(
        title = this.title,
        url = this.url,
        date = this.date,
        explanation = this.explanation
    )
}

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NasaModel

The domain model used in the UI layer:
app/src/main/java/com/ccandeladev/nasaexplorer/domain/NasaModel.kt
data class NasaModel(
    val title: String,
    val url: String,
    val date: String,
    val explanation: String
)
The app transforms the full API response into a simplified domain model, keeping only the essential fields needed for the UI.

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ViewModel Architecture

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DailyImageViewModel

Handles business logic and state management for the daily image screen:
app/src/main/java/com/ccandeladev/nasaexplorer/ui/dailyimagescreen/DailyImageViewModel.kt
@HiltViewModel
class DailyImageViewModel @Inject constructor(
    private val nasaRepository: NasaRepository,
    private val firebaseAuth: FirebaseAuth,
    private val firebaseDatabase: FirebaseDatabase
) : ViewModel() {

    private val _dailyImage = MutableStateFlow<NasaModel?>(null)
    val dailyImage: StateFlow<NasaModel?> = _dailyImage

    private val _errorMessage = MutableStateFlow<String?>(null)
    val errorMessage: StateFlow<String?> = _errorMessage

    private val _isLoading = MutableStateFlow(false)
    val isLoading: StateFlow<Boolean> = _isLoading

    private val _isFavorite = MutableStateFlow<Boolean>(false)
    val isFavorite: StateFlow<Boolean> = _isFavorite

    /**
     * Load the daily image
     * @param date Optional parameter to load image from a specific date
     */
    fun loadDailyImage(date: String? = null) {
        viewModelScope.launch {
            _isLoading.value = true
            try {
                val result = nasaRepository.getImageOfTheDay(date = date)
                _dailyImage.value = result
                _errorMessage.value = null
            } catch (e: Exception) {
                _errorMessage.value = "Sin conexión a internet. Conéctate a una red Wi-Fi o habilita datos móviles para ver las imágenes"
                _dailyImage.value = null
            } finally {
                _isLoading.value = false
            }
        }
    }
}

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State Management

The ViewModel exposes four key states:

dailyImage

Contains the current image data (title, URL, date, explanation) or null if not loaded.

isLoading

Boolean flag to show/hide loading indicators during API calls.

errorMessage

Error message string for displaying connection or API errors.

isFavorite

Tracks whether the current image is saved in the user’s favorites.

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Favorites Integration

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Save to Favorites

Users can save daily images to their personal collection:
app/src/main/java/com/ccandeladev/nasaexplorer/ui/dailyimagescreen/DailyImageViewModel.kt
fun saveToFavorites(nasaModel: NasaModel) {
    val userId = firebaseAuth.currentUser?.uid
    if (userId != null) {
        viewModelScope.launch {
            try {
                val favoriteRef = firebaseDatabase.reference
                    .child("favorites")
                    .child(userId)
                    .push()
                
                val favoriteImage = mapOf(
                    "id" to (favoriteRef.key ?: ""),
                    "title" to nasaModel.title,
                    "url" to nasaModel.url
                )
                
                favoriteRef.setValue(favoriteImage).await()
                _isFavorite.value = true
            } catch (e: Exception) {
                _errorMessage.value = "Error al guardar favorito ${e.message}"
            }
        }
    }
}

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Remove from Favorites

app/src/main/java/com/ccandeladev/nasaexplorer/ui/dailyimagescreen/DailyImageViewModel.kt
fun removeFromFavorites(nasaModel: NasaModel) {
    val userId = firebaseAuth.currentUser?.uid
    if (userId != null) {
        viewModelScope.launch {
            try {
                val favoriteRef = firebaseDatabase.reference
                    .child("favorites")
                    .child(userId)
                
                val snapshot = favoriteRef
                    .orderByChild("url")
                    .equalTo(nasaModel.url)
                    .get()
                    .await()
                
                for (child in snapshot.children) {
                    child.ref.removeValue().await()
                }
                _isFavorite.value = false
            } catch (e: Exception) {
                _errorMessage.value = "Error al buscar favorito ${e.message}"
            }
        }
    }
}

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Check Favorite Status

app/src/main/java/com/ccandeladev/nasaexplorer/ui/dailyimagescreen/DailyImageViewModel.kt
fun checkIsFavorite(url: String) {
    val userId = firebaseAuth.currentUser?.uid
    if (userId != null) {
        viewModelScope.launch {
            try {
                val favoriteRef = firebaseDatabase.reference
                    .child("favorites")
                    .child(userId)
                
                val snapshot = favoriteRef
                    .orderByChild("url")
                    .equalTo(url)
                    .get()
                    .await()
                
                _isFavorite.value = snapshot.exists()
            } catch (e: Exception) {
                _errorMessage.value = "Error al cargar estado de favoritos ${e.message}"
            }
        }
    }
}
The daily image screen doesn’t automatically check favorite status on load, as newly fetched images are never favorites by default.

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Firebase Database Structure

Favorites are stored per user in Firebase Realtime Database: 
favorites/
  └── {userId}/
      └── {pushId}/
          ├── id: "{pushId}"
          ├── title: "Image Title"
          └── url: "https://..."

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Error Handling

The feature gracefully handles common errors:
Displays: “Sin conexión a internet. Conéctate a una red Wi-Fi o habilita datos móviles para ver las imágenes”
Caught in the try-catch block and displayed via errorMessage state.
Shows: “Usuario no autenticado” when trying to save favorites without being logged in.
Displays: “Error al guardar favorito” with the exception message.

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Developer Implementation

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Setting Up the Daily Image Feature

1

Configure NASA API Key

Add your NASA API key to BuildConfig:
android {
    buildTypes {
        debug {
            buildConfigField "String", "NASA_API_KEY", '"YOUR_API_KEY_HERE"'
        }
    }
}
2

Inject Dependencies

Use Hilt to inject the ViewModel in your composable:
@Composable
fun DailyImageScreen(
    dailyImageViewModel: DailyImageViewModel = hiltViewModel()
) {
    // Access ViewModel properties
}
3

Collect State Flows

Observe state changes in your UI:
val dailyImage by dailyImageViewModel.dailyImage.collectAsState()
val isLoading by dailyImageViewModel.isLoading.collectAsState()
val errorMessage by dailyImageViewModel.errorMessage.collectAsState()
4

Load Image

Call loadDailyImage() when the screen launches:
LaunchedEffect(Unit) {
    dailyImageViewModel.loadDailyImage()
}
Always handle the loading and error states in your UI to provide good user experience during network operations.

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