WPF / MVVM / Model does not update viewmodel

Question

Context:

• I had a problem with binding property in my MVVM app. Now I've made a little project to test this case. It includes just window, viewmodel, model, BindableObject(abstract class with INotifyProperyChanged) and Command classes. All classes are in same namespace, datacontext is set in view, model and vm have INotifyProperyChanged, text in view is binded to vm property that is binded to model property. Constructor sets model property and it affects property in viewmodel and view.

Problem:

• When I change property in model it does not change property in viewmodel and view.

Here are BaseModel and Command classes:

    abstract class BindableObject : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropertieChanged ([CallerMemberName]string name = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }
    }


    class Command : ICommand
    {
        private Action<object> execute;
        private Func<object, bool> canExecute;

        public Command(Action<object> execute, Func<object, bool> canExecute = null)
        {
            this.execute    = execute;
            this.canExecute = canExecute;
        }

        public event EventHandler CanExecuteChanged
        {
            add     { CommandManager.RequerySuggested += value; }
            remove  { CommandManager.RequerySuggested -= value; }
        }

        public bool CanExecute(object parameter)
        {
            return this.canExecute == null || this.canExecute(parameter);
        }

        public void Execute(object parameter)
        {
            this.execute(parameter);
        }
    }

Model:

    public class Model : BindableObject
    {
        private int modelValue;
        public int ModelValue { get => modelValue; set { modelValue = value; OnPropertieChanged(); } }

        public Model ()
        {
            ModelValue = 111;
        }

        public void ChangeValue ()
        {
            ModelValue = 777;
        }

    }

ViewModel:

    class MainVM : BindableObject
    {
        private int myValue;
        public int MyValue { get => myValue; set { myValue = value; OnPropertieChanged(); } }

        public ICommand Command1 { get; set; }

        public MainVM()
        {
            var model = new Model();
            MyValue  = model.ModelValue;
            Command1 = new Command( (obj) => model.ChangeValue() );
        }

    }

View:

    <Window>
    ...
        <Window.DataContext>
            <local:MainVM/>
        </Window.DataContext>

        <StackPanel>
            <TextBlock Text="{Binding MyValue}"/>
            <Button Command="{Binding Command1}"/>
        </StackPanel>

    </Window>

Answer 1

The problem is your command in your view model is invoking a method on an instance of your model, but your xaml is binding to a separate property in your view model of type int (MyValue). That's why MyValue is getting set once in the vm constructor but never getting updated with the command.

Make the instance of your Model class a property in the view model:

private Model _myModel;

public Model MyModel
{
    get
    {
        return _myModel;
    }
    set
    {
        _myModel = value;
        OnPropertieChanged();
    }
}

public ICommand Command1 { get; set; }

public MainVM()
{
    MyModel = new Model();           
    Command1 = new Command((obj) => MyModel.ChangeValue());
}

And bind to it's ModelValue property in the xaml.

<StackPanel>
    <TextBlock Text="{Binding MyModel.ModelValue}"/>
    <Button Height="50" Width="100" Command="{Binding Command1}"/>
</StackPanel>

In general, though, a model class doesn't typically perform logic; rather, logic is either performed on instances of your model in a service class, or client-side in the view model. But that's a separate discussion.