mattrob33 / handlebar

Handlebar - Generates typesafe access to SavedStateHandle

Handlebar

Typesafe SavedStateHandle in Android ViewModels.

Description

Android ViewModels typically interact with SavedStateHandle through a key-value basis. SavedStateHandle allows for ViewModel state to survive process death, but it comes at the cost of having state that is essentially stringly typed.

class LoginViewModel(
    api: API,
    private val stateHandle: SavedStateHandle
) : ViewModel() {

    private val _status = stateHandle.getLiveData<ProgressStatus>("status")
    val status: LiveData<ProgressStatus> = _status
    
    private val _username = stateHandle.getLiveData<String>("username")
    val username: LiveData<String> = _username
    
    private val _password = stateHandle.getLiveData<String>("password")
    val password: LiveData<String> = _password
    
    suspend fun login() {
        _status.value = ProgressStatus.IN_PROGRESS
        
        viewModelScope.launch {
            api.login(username.value, password.value)
            _status.value = ProgressStatus.COMPLETE
        }
    }
}

Handlebar allows Android developers to capitalize on SavedStateHandle's benefits without sacrificing the safety and readability that type safety provides.

Usage

Create an interface that describes your state properties, marked with the @TypedState annotation. Then create a property on your ViewModel to represent your state, using the extension function SavedStateHandle.asType.

class LoginViewModel(
    api: API,
    stateHandle: SavedStateHandle
) : ViewModel() {

    @TypedState interface LoginState {
        var username: LiveData<String>
        var email: LiveData<String>
        var status: LiveData<ProgressStatus>
    }
    private val state = stateHandle.asType<LoginStateContract>()
    
    // Expose status to observers
    val status: LiveData<ProgressStatus> = state.status
    
    val username: LiveData<String> = state.username
    val password: LiveData<String> = state.password
    
    suspend fun login() {
        state.updateStatus(ProgressStatus.IN_PROGRESS)
        
        viewModelScope.launch {
            api.login(username.value, password.value)        
            state.updateStatus(ProgressStatus.COMPLETE)
        }
    }
}

That's it. Handlebar will generate a new interface [TypedState interface name]Contract using the properties from your @TypedState interface, as well as a concrete implementation connecting this interface to the SavedStateHandle on your ViewModel, so all changes you make on your state property will be backed by the SavedStateHandle.