MVVM WPF - 更改视图会创建新的视图实例

Question

I've decided to move away from WinForms as I wanted to be more flexible in creating my UIs. That said I'm just starting with WPF and the MVVM structure. I'm having a blast learning all the new stuff and trying to learn the "best practices" right away.

One thing I'm wondering is that with my current implementation of MVVM and navigation through my views it would always create a new instance of the View when changing my CurrentView of my MainViewModel of my MainWindow .

Is that the intended way? As I notice quite a bit of lagging when changing them which feels... wrong? As a sidenote I'm currently not on my normal workstation but on a older surface which is getting stressed easily with VS running etc

A pretty 'standard' MainViewModel

    public class MainViewModel : ObservableObject
    {

        public MainViewModel()
        {
            this.PageViewModels.Add(new EventsViewModel());
            this.PageViewModels.Add(new SettingsViewModel());
            this.PageViewModels.Add(new AnotherViewModel());

            this.CurrentView = this.PageViewModels[0];
        }

        public ViewModelBase CurrentView
        {
            get
            {
                return this._currentVIew;
            }

            set
            {
                this._currentVIew = value;
                this.OnPropertyChanged("CurrentView");
            }
        }


        public ICommand ChangePageCommand
        {
            get
            {
                if (this.changePageCommand == null)
                {
                    this.changePageCommand = new RelayCommand(
                        p => this.ChangeViewModel(p),
                        p => true);
                }

                return this.changePageCommand;
            }
        }

        private void ChangeViewModel(object viewModel)
        {
            var t = (Type)viewModel;
            foreach (var v in this.PageViewModels)
            {
                if (v.GetType() == t)
                {
                    this.CurrentView = v;
                    return;
                }
            }
        }
}

'Normal' View Data Templating

            <DataTemplate DataType="{x:Type viewModel:EventsViewModel}">
                <view:EventView/>
            </DataTemplate>
            
            <DataTemplate DataType="{x:Type viewModel:SettingsViewModel}">
                <view:SettingsView/>
            </DataTemplate>
            
            <DataTemplate DataType="{x:Type viewModel:AnotherViewModel}">
                <view:AnotherView/>
            </DataTemplate>

The way the Command is beeing called, just with different x:Type parameter

Command="{Binding DataContext.ChangePageCommand, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}"
CommandParameter="{x:Type viewModel:SettingsViewModel}"      

Answer 1

First on the general theory. All "MV*" patterns separate the functionality of the View and Model into different layers. An additional third layer (Controler, Presenter, ViewModel, etc.) performs the function of matching between the Model and the View. Therefore, it is meaningless to consider View and ViewMode in isolation. VM is (in fact) a proxy reflecting the Model in its properties. A common mistake is when ANY Data Context is called a ViewModel. This is done out of habit. And you need to understand that such a name does not always correctly reflect the functionality of the Data Context. You also need to understand that the Data Context is a typical, but not mandatory, storage location for the ViewModel. VM can be: in resources (this is quite often used); in a static member (for example, Singlton is sometimes used); received from some service, locator; and many other options.

I hope it is already clear that the VM is a reflection of the Model. And its connection with the View is "weak". The view uses the ViewModel. But the ViewModel doesn't "know" anything about the View. In relation to navigation, this means that you first need to understand how your navigation changes or depends on the state of the Model. In most cases, the implementation of navigation - no way.

And judging by the code you showed, this is your case. And in this case, all navigation is implemented at the presentation layer. That is, your MainViewModel class is not really a ViewModel, but a View layer class. And if so, then you may well instead of ViewModel instances (EventsViewModel, SettingsViewModel, AnotherViewModel) - create and use View instances (EventsView, SettingsView, AnotherView). Instances will be created once and will exist for the duration of the application session.