MVI 架构更佳实践：支持 LiveData 属性监听

// 1. 包含页面所有状态的data class
data class CounterState(val count: Int = 0) : MavericksState
// 2.负责处理业务逻辑的ViewModel,易于单元测试
class CounterViewModel(initialState: CounterState) : MavericksViewModel<CounterState>(initialState) {
    // 通过setState更新页面状态
    fun incrementCount() = setState { copy(count = count + 1) }
}
// 3. View层，必须实现MavericksView接口    
class CounterFragment : Fragment(R.layout.counter_fragment), MavericksView {
    private val viewModel: CounterViewModel by fragmentViewModel()
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        counterText.setOnClickListener {
            viewModel.incrementCount()
        }
    }
    //4. 页面刷新回调，每当状态刷新时会回调这里
    override fun invalidate() = withState(viewModel) { state ->
        counterText.text = "Count: ${state.count}"
    }
}

data class UserState(
    val score: Int = 0,
    val previousHighScore: Int = 150,
    val livesLeft: Int = 99,
) : MavericksState {
    val pointsUntilHighScore = (previousHighScore - score).coerceAtLeast(0)
    val isHighScore = score >= previousHighScore
}
class CounterFragment : Fragment(R.layout.counter_fragment), MavericksView {
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
     //直接监听State的属性,并且支持设置监听模式  
        viewModel.onEach(UserState::pointsUntilHighScore,deliveryMode = uniqueOnly()) {
            //..
        }
  viewModel.onEach(UserState::score) {  
            //...
        }    
    }
}

fun <VM : MavericksViewModel<S>, S : MavericksState, A> VM._internal1(
    owner: LifecycleOwner?,
    prop1: KProperty1<S, A>,
    deliveryMode: DeliveryMode = RedeliverOnStart,
    action: suspend (A) -> Unit
) = stateFlow
    // 通过对象取出属性的值
    .map { MavericksTuple1(prop1.get(it)) }
    // 值发生变化了才会触发回调  
    .distinctUntilChanged()
    .resolveSubscription(owner, deliveryMode.appendPropertiesToId(prop1)) { (a) ->
        action(a)
    }

//监听一个属性
fun <T, A> LiveData<T>.observeState(
    lifecycleOwner: LifecycleOwner,
    prop1: KProperty1<T, A>,
    action: (A) -> Unit
) {
    this.map {
        StateTuple1(prop1.get(it))
    }.distinctUntilChanged().observe(lifecycleOwner) { (a) ->
        action.invoke(a)
    }
}
//监听两个属性
fun <T, A, B> LiveData<T>.observeState(
    lifecycleOwner: LifecycleOwner,
    prop1: KProperty1<T, A>,
    prop2: KProperty1<T, B>,
    action: (A, B) -> Unit
) {
    this.map {
        StateTuple2(prop1.get(it), prop2.get(it))
    }.distinctUntilChanged().observe(lifecycleOwner) { (a, b) ->
        action.invoke(a, b)
    }
}
internal data class StateTuple1<A>(val a: A)
internal data class StateTuple2<A, B>(val a: A, val b: B)
//更新State
fun <T> MutableLiveData<T>.setState(reducer: T.() -> T) {
    this.value = this.value?.reducer()
}

//页面状态，需要避免混淆
data class MainViewState(
    val fetchStatus: FetchStatus = FetchStatus.NotFetched,
    val newsList: List<NewsItem> = emptyList()
)
//ViewModel
class MainViewModel : ViewModel() {
    private val _viewStates: MutableLiveData<MainViewState> = MutableLiveData(MainViewState())
    //只需要暴露一个LiveData，包括页面所有状态
    val viewStates = _viewStates.asLiveData()
    private fun fetchNews() {
     //更新页面状态
        _viewStates.setState {
            copy(fetchStatus = FetchStatus.Fetching)
        }
        viewModelScope.launch {
            when (val result = repository.getMockApiResponse()) {
                //...
                is PageState.Success -> {
                    _viewStates.setState {
                        copy(fetchStatus = FetchStatus.Fetched, newsList = result.data)
                    }
                }
            }
        }
    }
}
//View层
class MainActivity : AppCompatActivity() {
 private fun initViewModel() {
        viewModel.viewStates.run {
            //监听newsList
            observeState(this@MainActivity, MainViewState::newsList) {
                newsRvAdapter.submitList(it)
            }
            //监听网络状态
            observeState(this@MainActivity, MainViewState::fetchStatus) {
                //..
            }
        }
    }
}