当ViewModel SelectedItem中的值发生更改时，不会更新选取器Xamarin.Forms

private object _selectedItem;
public object SelectedItem
{
    get { return _selectedItem; }
    set
    {
        if (_selectedItem != value)
        {
            _selectedItem = value;
            OnPropertyChanged(nameof(SelectedItem));
        }
    }
}

public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

<Picker SelectedItem="{Binding SelectedItem}">
    <!-- 选项列表 -->
</Picker>

public MyPage()
{
    InitializeComponent();
    BindingContext = new MyViewModel();

    // 订阅SelectedItem属性的更改事件
    (BindingContext as MyViewModel).PropertyChanged += OnSelectedItemChanged;
}

private void OnSelectedItemChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == nameof(MyViewModel.SelectedItem))
    {
        // 更新选取器的选中项
        // 可以通过Picker的SelectedIndex或者SelectedValue属性来更新选中项
    }
}