Creating a Model & ViewModel

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Now that we've got a basic view on-screen let's think about how we could display real data from a database or similar in it. The most obvious, and traditional way to do this would be to load the data in the view's constructor and create a CheckBox for each TODO item in code. However doing it this way has a few disadvantages:
We can't use XAML
We have to write code to react to changes in the data and update the display
It's not unit-testable
These considerations may not seem important for a small application, but as your application grows they become bigger and bigger problems: particularly the testability part.
There is a better way! We mentioned the MVVM pattern at the beginning of this tutorial and it's that pattern that we're going to use now.
Create the Model
The first thing to do is to create the model which will represent our data as it would be stored in a database. Our model is going to be pretty simple: each TODO item will consist of a textual description and a boolean value representing whether the item is checked.
Place the following class in the Models directory in your project:
Models/TodoItem.cs
namespace Todo.Models
{
    public class TodoItem
    {
        public string Description { get; set; }
        public bool IsChecked { get; set; }
    }
}
Create a (fake) database
We could use something like Entity Framework to read this data from an SQLite database but because this is a tutorial on Avalonia not Entity Framework, we're not actually going to be using a database. Instead we'll just populate our models from an array.
We'll do this in a service called Database and put this in a Services directory:
Services/Database.cs
using System.Collections.Generic;
using Todo.Models;
​
namespace Todo.Services
{
    public class Database
    {
        public IEnumerable<TodoItem> GetItems() => new[]
        {
            new TodoItem { Description = "Walk the dog" },
            new TodoItem { Description = "Buy some milk" },
            new TodoItem { Description = "Learn Avalonia", IsChecked = true },
        };
    }
}
Create a View Model
Now we're going to need a view model which represents the list. This is the class that will provide the data for our view.
We have already created the view and called it TodoListView and so the associated view model is going to be called TodoListViewModel. Place this class in the ViewModels directory in your project:
ViewModels/TodoListViewModel.cs
using System.Collections.Generic;
using System.Collections.ObjectModel;
using Todo.Models;
​
namespace Todo.ViewModels
{
    public class TodoListViewModel : ViewModelBase
    {
        public TodoListViewModel(IEnumerable<TodoItem> items)
        {
            Items = new ObservableCollection<TodoItem>(items);
        }
​
        public ObservableCollection<TodoItem> Items { get; }
    }
}
Again, our view model is very simple at this stage. It simply takes a collection of TodoItem models in its constructor and puts them into an ObservableCollection which is exposed via an Items property.
One thing to notice is the use of the ViewModelBase class. The importance of this will become clear shortly.
Create an instance of TodoListViewModel
We've seen that our TodoListViewModel requires a collection of TodoItem models to be passed to its constructor, but where do these items come from? Where is TodoListViewModel created?
Well, we now have two views and two view models:
MainWindow (created by the template)
MainWindowViewModel (created by the template)
TodoListView (created by us)
TodoListViewModel (created by us)
If you think back to when we created the views, you'll remember that our views have a parent-child relationship (TodoListView is a child of MainWindow). This might give us a clue: MainWindowViewModel should create the TodoListViewModel!
Edit the MainWindowViewModel.cs file to look like this:
MainWindowViewModel.cs
using Todo.Services;
​
namespace Todo.ViewModels
{
    class MainWindowViewModel : ViewModelBase
    {
        public MainWindowViewModel(Database db)
        {
            List = new TodoListViewModel(db.GetItems());
        }
​
        public TodoListViewModel List { get; }
    }
}
Finally, edit the OnFrameworkInitializationCompleted method in App.axaml.cs to create an instance of Database and pass it to MainWindowViewModel:
public override void OnFrameworkInitializationCompleted()
{
    base.OnFrameworkInitializationCompleted();
​
    if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
    {
        var db = new Database();
​
        desktop.MainWindow = new MainWindow
        {
            DataContext = new MainWindowViewModel(db),
        };
    }
}
Next we're going to wire up the views to read from our view models...
Creating a View
Wiring up the Views
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Outline
Create the Model
Create a (fake) database
Create a View Model
Create an instance of TodoListViewModel