如何在MVVM中实现模型

Question

I am currently studying the MVVM pattern. So thus far I developed a simple demo programm which contains a view and a viewmodel with commands etc. Now I want to implement a Model but I am not quite sure how to do so. My demo contains a view with a textbox and a button. When the Button gets pressed a Command is launched. The text from the textbox should be written in a textfile with upper cased letters. This functionality should be part of my model. How do i call this functionality from my viewmodel? Should the viemodel contain a instance of the model class and call a methode in the command execute? And how does the viewmodel get data from a model?

Thank you very much for your help!

Answer 1

Usually for data storage and retrieval, I create a separate class called repository. Your view model has an instance of the repository (or better: an interface of it). In the repository class, you can do your file access.

By the way: If your view model just knows the interface of the repository, you could replace it later with a database access, and the view model would not be affected.

The view model can then interact with the repository ie call it methods when the command code in the view model executes.

You write...

"The text from the textbox should be written in a textfile with upper cased letters. This functionality should be part of my model."

The model usually is just data, so a model class does not have functionality but only properties. Like I said: Do your data access in the view model or in the repository class.

In case of MVVM it would be good if the properties implement INotifyPropertyChanged , like the properties of your view model.

If you just want to write the content of a single text box, then your model would be a class with just one property.

Answer 2

Yes. You could instantiate a model object in the viewmodel and have it save the text in a textfile (Or whatever you want your application to do)

class ViewModelDefault : INotifyPropertyChanged
{
    // Bound to your textbox
    public string TextboxProperty { get; set;}

    // Instantiate modellayer in viewmodel
    private ModelClass _modelClass = new ModelClass();

    // RelayCommand property -> bound to button on viewmodel
    // Will execute method "ExecuteCommand" that contains a call to a method in the ModelClass
    public ICommand ExecuteModelMethod 
    { 
        get {
            RelayCommand relayCommand = new RelayCommand(ExecuteCommand); 
            return relayCommand; 
        } 
    }

    // Method that the RelayCommand will execute.
    private void ExecuteCommand() 
    {
        _modelClass.SaveTextInTextfile(TextboxProperty);
    }

    ...
}

In the code above I've made an example of how this could be done using RelayCommand.

RelayCommand is a class that makes use of delegates such as Action and Func. This means you can pass a method into the RelayCommand object and have it execute it.

What RelayCommand allows you to do is basicly binding a method through delegate to UI control in the view layer.

Read up on Delegates if you wish to study further on that topic. Delegates (C# Programming Guide)