Csharp WPF如何获取有关从一个视图模型更改另一个视图模型的信息

您已将您的问题标记为"MVVM"，但您没有执行它.在MVVM中，有Model负责数据持久化.MVVM并不意味着要把所有的东西都塞进View Model个班级.Model分加上Model个班级，应该会让这种流动更加明显.

You could handle the Frameworkelement.Unloaded event to get notified about the current view being unloaded. However, the graceful solution would be to manage page related persistence where you are already in full control of page management: your MainVM class.
Code should be cohesive and similar or related tasks should not be spread across the application.

MainVM已经能够准确地识别页面切换，而不需要额外的努力.由于MainVM应该具有对一个或多个Model个类的引用，以便例如通过初始化页面视图模型类来获取它将expose 给视图的数据，因此它还能够调用相关的模型类以用于持久性.

Please note, that in proper clean OO programming you must care about type and type member accessibility (access modifiers like public or private). It's a very fundamental technique to make your code robust and APIs well defined.
Do not make everything public. Instead make everything private and change it to a less restrictive visibility e.g. public only when necessary. This also includes property accessors. For example, declare a private set; and public get;. Class internals must be only visible to the defining class or subclasses. See Data Hiding.

修复/改进的代码(包括正确的MVVM struct 和访问修饰符用法)如下所示:

PageId.cs

public enum PageId
{
  Default = 0,
  FirstPage,
  SecondPage,
}

MainVM.cs个

public class MainVM : ObservableObject
{
  // Make code extensible by using a single navigation command 
  // which depends on the command parameter to identify the navigation target.
  public RelayCommand NextPageCommand { get; }

  // TODO::Choose a better name as the value is not a view, 
  // but a model for the view (e.g. CurrentViewData)
  private ObservableObject _currentView;
  public ObservableObject CurrentView
  {
    get => _currentView;
    private set => Set(ref _currentView, value);
  }

  // A Model reference
  private Repository Repository { get; }

  private Dictionary<PageId, ObservableObject> PageViewModels { get; }

  public MainVM()
  {
    this.PageViewModels = new Dictionary<PageId, ObservableObject>
    {
      { PageId.FirstPage, new FirstVM() },
      { PageId.SecondPage), new SecondVM() },
    };    

    this.NextPageCommand = new RelayCommand(ExecuteNextPageCommand);

    // Reference the model
    this.Repository = new Repository();

    // Initialize start page
    LoadPage(PageId.FirstPage);
  }
 
  private void ExecuteNextPageCommand(object commandParameter)
  { 
    if (commandParameter is not PageId pageId)
    {
      throw new ArgumentException($"Unexpected parameter type. ICommand.CommandParameter must be of type {typeof(PageId)}", nameof(commandParameter));
    }

    LoadPage(pageId);
  }

  private void LoadPage(PageId pageId)
  {
    ObservableObject oldPage = this.CurrentView;
    if (this.PageViewModels.TryGetValue(pageId, out ObservableObject pageViewModel))
    {
      this.CurrentView = pageViewModel;

      if (oldPage is not null)
      {
        SavePageData(oldPage);
      }
    }
  }

  /* Create overloads for individual page view model types */
  private void SavePageData(FirstVM pageViewModel)
  {
    // Delegate data persistence to the Model
    this.Repository.SaveDetails(pageViewModel.DetailsScheme);
    this.Repository.SaveMaterials(pageViewModel.Materials);
  }

  private void SavePageData(SecondVM pageViewModel)
  {
    // TODO::Delegate data persistence to the Model
    throw new NotImplementedException();
  }
}

按如下方式使用MainVM.NextPageCommand:

<Button Command="{Binding NextPageCommand}"
        CommandParameter="{x:Static local:PageId.FirstPage}" />
<Button Command="{Binding NextPageCommand}"
        CommandParameter="{x:Static local:PageId.SecondPage}" />

实现持久化的Model个类详细说明:

Repository.cs个

public class Repository
{
  // Details about how persistence is implemented (file, database, web service etc.)
  // must be hidden from the View Model. 
  // Therefore, the Repository API must be kept simple.
  // Such details are encapsulated in a "low level" class,
  // in this case the type JsonRepository.
  private JsonRepository JsonRepository { get; }

  public Repository()
  {
    this.JsonRepository = new JsonRepository();
  }

  public void SaveMaterials(IEnumerable<Material> materials)
  {
    this.JsonRepository.SaveMaterials(materials);
  }

  public void SaveDetails(IEnumerable<DetailScheme> details)
  {
    this.JsonRepository.SaveDetails(details);
  }
}

JsonRepository.cs个

internal class JsonRepository
{
  internal JsonRepository()
  {
  }

  internal void SaveMaterials(IEnumerable<Material> materials)
  {
    SaveMaterials(materials, "D:/Materials.json");
  }

  internal void SaveDetails(IEnumerable<DetailScheme> details)
  {
    SaveDetails(details, "D:/Details.json");
  }

  private void SaveMaterials(IEnumerable<Material> materials, string destinationPath) 
    => throw new NotImplementedException();

  private void SaveDetails(IEnumerable<DetailScheme> details, string destinationPath) 
    => throw new NotImplementedException();
}