Using Jetpack Compose State delegate syntax with data class-CodePudding

I really like using the by delegate with Compose State as it hides the boiler plate of having "reactive" data ie:

val message: String by remember { mutableStateOf("") }

message = "No boilerplate"

When using props this way, unidirectional data flow (or props down, events up) is clean and easy.

However when wanting to group data together in a data class, or observe a view model prop as state (ie: using observeAsAState()) I have to declare the prop of type State eg:

data class MyUiProps(
  val message: State<String>
)

var props = MyUiProps(viewModel.message.observesAsState(""))
props = MyUiProps(remember { mutableStateOf("") })

props.message.value = "Urgh boilerplate"

Is there a way to still use the by delegate with a data class like the following pseudo code

data class MyUiProps(
  val message: String
)

val props = MyUiProps(
  message = by remember { mutableStateOf("") }
)

props.message = "Yay no bolierplate"

CodePudding user response：

Just use MyUiProps as the state you are exposing in ViewModel. Exposing single fields is tedious and unreadable.

If you have something like this in ViewModel:

// Expose whole state in ViewModel
private val _state = MutableStateFlow(MyUiProps()) // this will set default values in the model
val state: StateFlow<MyUiProps> = _state // observe this as state in compose

// inside ViewModel if you want to update the state
_state.value = _state.value.copy(fieldYouWantToUpdate = newValue)

// in composable
val state by viewModel.state.collectAsState()

CodePudding user response：

You cannot use by inside a constructor. Still, you don't need the so-called "boilerplate" that you are showing. This is how you do it:

data class MyUiProps(
    var message: MutableState<String>
) {

}

val props = MyUiProps(
    message = remember { mutableStateOf("") }
)

props.message.value = "Yay no bolierplate"

You need to be using MutableState and not just State in your data classes parameter. This avoids the need to use observeAsState