从不同的ViewModel访问数据

Question

我想在一个视图中访问不同ViewModel的数据。但DataContext是一个完全不同的（MainView中的MainViewModel）。是否可以为每个窗口控件设置相应的ViewModel？或者仅在MainViewModel中创建和引用ObservableCollection<Student> Students更好？

目前我想将ViewModel Students中的属性StudentViewModel分配给此ComboBox。

MainViewModel（将ApplicationViewModel设置为CurrentViewModel）

public class MainViewModel : ViewModelBase
    {
        public MainViewModel()
        {
            CurrentViewModel = ApplicationViewModel;            
            ShowStudentViewCommand = new RelayCommand(ShowStudentView);
        }


        public ViewModelBase CurrentViewModel
        {
            get => _currentViewModel;

            set
            {
                if (_currentViewModel == value) return;
                _currentViewModel = value;
                RaisePropertyChanged("CurrentViewModel");
            }
        }


        private ViewModelBase _currentViewModel;
        private static readonly ApplicationViewModel ApplicationViewModel = new ApplicationViewModel();
        private static readonly StudentViewModel StudentViewModel = new StudentViewModel();



        public ICommand ShowStudentViewCommand { get; }        
        public ICommand ShowApplicationViewCommand { get; }


        private void ShowStudentView()
        {
            CurrentViewModel = StudentViewModel;
        }


        private void ShowApplicationView()
        {
            CurrentViewModel = ApplicationViewModel;
        }     

    }

ApplicationViewModel和StudentViewModel（加载数据并创建ObservableCollection）

public class ApplicationViewModel : ViewModelBase
    {
        public ApplicationViewModel()
        {

        }
    }


 public class StudentViewModel : ViewModelBase
 {
        private ObservableCollection<Student> _students;

        public StudentViewModel()
        {
            DataStudentService dataService = new DataStudentService();
            Students = new ObservableCollection<Student>(dataService.GetAllStudents());
        }

        public ObservableCollection<Student> Students
        {
            get => _students;

            private set
            {
                if (_students == value) return;
                _students = value;
                RaisePropertyChanged("Students");
            }
        }
    }

MainView.xaml（设置要显示的CurrentViewModel）

<Window x:Class="Test.View.MainView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="Test" Height="750" Width="700" 

        DataContext="{Binding Main, Source={StaticResource Locator}}">

    <Grid>
        <ContentControl Grid.Row="1" Content="{Binding CurrentViewModel}" />
    </Grid>
</Window>

ApplicationView.xaml（当前显示）

<UserControl x:Class="Test.View.ApplicationView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:viewModel="clr-namespace:Test.ViewModel"
             mc:Ignorable="d">
<Grid >


    <ComboBox
        TextSearch.TextPath=""
        ItemsSource="{Binding Path=Students}"
        DisplayMemberPath="Name"
        SelectedValuePath="Name"
        SelectedValue="{Binding Path=Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
        SelectedItem="{Binding Path=SelectedStudent, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
        SelectedIndex="{Binding Path=SelectedStudentIndex, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
        IsSynchronizedWithCurrentItem="True"
        IsEditable="True"
        Width="200"
        Margin="0,0,20,0"
        VerticalContentAlignment="Center" />



    </Grid>
</UserControl>

Answer 1

最直接的解决方案是通过创建集合并使用您已经提到的其他类中的数据填充它来坚持MVVM模式。这很容易，每个人都知道发生了什么。此外，这样，数据的上下文保持在相同的视图模型中 - 如果您更改了某些内容 - 它也会保留在该上下文中。在你的情况下，这意味着你必须创建一个ObservableCollection<Student> Students并在MainViewModel中填充其内容。

---

如果必须从整个应用程序访问该数据，您应该考虑实现DataManager（例如，单例）而不是在每个视图模型中复制数据。

Answer 2

找到了解决方案：

MainViewModel

public class MainViewModel : ViewModelBase
{
    public StudentViewModel StudentViewModel { get; set; }

    public MainViewModel()
    {
        StudentViewModel = new StudentViewModel();
    }
}

的MainView

<ComboBox
DataContext="{Binding StudentViewModel}"
ItemsSource="{Binding Path=Students, UpdateSourceTrigger=PropertyChanged}"
/>

StudentViewModel

private ObservableCollection<Student> _students;

public ObservableCollection<Student> Students
    {
        get => _students;
        set
            {
                if (_students == value) return;
                _students = value;
                RaisePropertyChanged("Students");
            }
        }