Jetpack Compose

Oct 11 2022 Kotlin 1.7.10, Android 13, Android Studio Chipmunk

Part 1: Jetpack Compose Basics

4. Add State to Composables

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Previous episode: 3. Use Basic Compose Building Blocks Next episode: 5. Decouple Composables

The student materials have been reviewed and are updated as of September 2022.

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Demo

So far you’ve been getting to know the Jetpack Compose framework, and it’s been going great! :]

private val _genresState = MutableLiveData(emptyList<Genre>())

override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContent { AddBookContent() }

  loadGenres()
}

private fun loadGenres() {
  lifecycleScope.launch {
    _genresState.value = repository.getGenres()
  }
}

val genres = _genresState.value ?: emptyList()
val isGenresPickerOpen = remember { mutableStateOf(false) }

val bookNameState = remember { mutableStateOf("") }
val bookDescriptionState = remember { mutableStateOf("") }
val selectedGenreName =
  genres.firstOrNull { it.id == _addBookState.value?.genreId }?.name ?: "None"

OutlinedTextField(
value = bookNameState.value, // new
onValueChange = { newValue -> // new
  bookNameState.value = newValue 
  _addBookState.value = _addBookState.value?.copy(name = newValue)
},
label = { Text(text = stringResource(id = R.string.book_title_hint)) })

OutlinedTextField(
value = bookDescriptionState.value, // new
onValueChange = { newValue -> // new
  bookDescriptionState.value = newValue
  _addBookState.value =_addBookState.value?.copy(description = newValue)
},
label = { Text(text = stringResource(id = R.string.book_description_hint)) })

Row {
  TextButton(
    onClick = { isGenresPickerOpen.value = true }, // new
    content = { Text(text = stringResource(id =  R.string.genre_select)) })

  DropdownMenu(
    expanded = isGenresPickerOpen.value, // new
    onDismissRequest = { isGenresPickerOpen.value = false }) { // new

    for (genre in genres) { // new
      DropdownMenuItem(onClick = {
        _addBookState.value = _addBookState.value?.copy(genreId = genre.id)
        isGenresPickerOpen.value = false
      }) {
        Text(text = genre.name)
      }
    }
  }
}

Row(verticalAlignment = Alignment.CenterVertically) { // alignment
  ...

  Text(text = selectedGenreName) // text element
}