Xamarin Forms MVVM 中的级联选择器

Question

我有一个问题，如何制作一个级联选择器来链接所选城市中的所有州？

此代码检索所有城市和州，但我想不通的一件事是如何获取选定的城市值并将其与州同步。

这是视图模型

public class MyViewModel : BaseViewModel
    {  
public async void GetCities()
        {
            using (var client = new HttpClient())
            {
                var uri = Constants.BaseApiAddress + "api/GetCities";
                var result = await client.GetStringAsync(uri);
                var CitiesList= JsonConvert.DeserializeObject<List<CityModel>>(result);
                Cities = new ObservableCollection<CityModel>(CitiesList);
            }
        }

        public async void GetStates()
        {
            using (var client = new HttpClient())
            {
                var uri = Constants.BaseApiAddress + "api/GetStates";
                var result = await client.GetStringAsync(uri);
                var StatesList= JsonConvert.DeserializeObject<List<StateModel>>(result);
                States = new ObservableCollection<StateModel>(StatesList);
            }
        }

StateModel _selectedState;
        public StateModel SelectedState
        {
            get
            {
                return _selectedState;
            }
            set
            {
                if (SelectedState!= value)
                {
                    _selectedState = value;
                    OnPropertyChanged();
                }
            }
        }
CityModel _selectedCity;
        public CityModel SelectedCity
        {
            get
            {
                return _selectedCity;
            }
            set
            {
                if (SelectedCity != value)
                {
                    _selectedCity = value;

                    OnPropertyChanged();
                }
            }
        }
 }

这是 XAML

<Picker x:Name="CityPicker" Title="Select City"
                    ItemsSource="{Binding Cities}" 
                    ItemDisplayBinding="{Binding City}" 
                    SelectedItem="{Binding SelectedCity}" />

            <Picker x:Name="StatePicker" Title="Select State"
                    ItemsSource="{Binding States}" 
                    ItemDisplayBinding="{Binding State}" 
                    SelectedItem="{Binding SelectedState}" />

Answer 1

只需更新SelectedState setter 中的Cities 属性：

        public StateModel SelectedState
        {
            get
            {
                return _selectedState;
            }
            set
            {
                if (SelectedState!= value)
                {
                    _selectedState = value;
                    OnPropertyChanged();
                   if (value != null)
                        //Cities =  RetrieveCitiesByState((CityModel) value); //update Cities according to the new selected State (value)
                }
            }
        }