Question

我使用MVVM架构设置了我的应用。我要解决的问题是基于两个查询参数（在本例中为起点和终点）触发api调用，此参数是动态的并实时更新。当更新这两个值中的任何一个时，都应检查两个值都不为null，然后根据这两个值进行api调用。

我一直在和MediatorLiveData和Transformations.switchMap()一起玩，但是还没有找到一个行之有效的解决方案。尝试使用MediatorLiveData：

class MyViewModel(val repository: AppRepository) : ViewModel() {

  val origin = MutableLiveData<Location>()
  val destination = MutableLiveData<Location>()

  val directions = MediatorLiveData<DrivingDirections>()

  init {
    directions.addSource(origin) { origin ->
       if (origin != null && destination.value != null) {
          directions.value = // problem here
          // I want to make a call to something like 
          // repository.getDirections(origin, destination), but this comes
          // back as LiveData<Directions> so *can't* set it to directions.value
          // I, in effect, want to make directions = repository.getDirections(origin, destination),
          // but then I lose the Mediator functionality
       }
    }
    directions.addSource(destination) {
      // as above
    }
  }

}

因此，尝试使用switchMap，我创建了一个粗略的OriginAndDestination对象，然后观察到对此的更改。

class myViewModel(val repository: AppRepository) : ViewModel() {

  val originAndDestination = MutableLiveData<OriginAndDestination>()
  val directions: LiveData<Directions>

  init {
    directions = Transformations.switchMap(originAndDestination) { originAndDestination -> 
    // probably should do some checks that both origin and destination are not null, 
    // so this switch map could return null? How do I guard against null in a switch map?

      repository.getDirections(originAndDestination.origin, originAndDestination.destination)
    }
  }

  fun setOrigin(location: Location) {
    // bit of problem code here... need to retrieve the current value of 
    // originAndDestination.value, then update the 'origin' property,
    // then set it to the liveData value again to trigger the switchMap, above... while checking that the value isn't null in the first place...
    //  something like:
    val tempValue = originAndDestination.value
    if (tempValue != null) {
      // update tempValue.origin
    } else {
      // create a new OriginAndDestination object?
    }
    // just feels really messy
  }

  fun setDestination(location: Location) {
    // As above
  }

}

很抱歉，所有评论都在这里突出显示了一些痛点和挫折感。我会以错误的方式处理吗？ Origin和Destination是在用户界面字段中设置的。

Answer 1

这是比MVVM更多的MVI，但这是我的处理方式：

data class myViewState(
  val origin: Location,
  val destination: Location,
  val directions: DrivingDirections
)

class myViewModel(val repository: AppRepository) : ViewModel() {
  private val _states = MediatorLiveData<myViewState>()
  val states: LiveData<myViewState> = _states
  private val lastSource: LiveData<myViewState>? = null

  fun updateLocations(origin: Location, destination: Location) {
    lastSource?.let { _states.removeSource(lastSource) }
    lastSource = repository.getDirections(origin, destination)

    _states.addSource(lastSource) { directions ->
      _states.value = myViewState(origin, destination, directions)
    }
  }
}

（幸运的是，我们可以摆脱lastSource sometime in the future）

您的UI层然后观察states并更新其UI的位置和方向。

这可能不适合您的体系结构，但可能会给您一些想法。

如何设置LiveData以根据多个参数执行搜索？

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