按钮上的绑定命令不起作用wpf mvvm

Question

I'm trying to create simple add entity to database form, but binding command doesn't work and I can't figure out why. Here is XAML

<DockPanel Margin="30">
    <StackPanel DockPanel.Dock="Top">
        <Label>Manufacturer</Label>
        <TextBox Text="{Binding Manufacturer, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" />
        <Label>Type</Label>
        <TextBox Text="{Binding Type, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" />
        <Label>Serial number</Label>
        <TextBox Text="{Binding SerialNumber, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" />
        <Button Command="{Binding AddScaleCommand}">Add Scale</Button>
    </StackPanel>
    <ListBox ItemsSource="{Binding Scales}" DockPanel.Dock="Bottom"></ListBox>
</DockPanel>

And here is ScaleViewModel where the command is located

public class ScaleViewModel : ViewModel
{
    public ScaleViewModel()
    {
        Scales = new ObservableCollection<Scale>();
    }

    public ICollection<Scale> Scales { get; private set; }
    public string Manufacturer { get; set; }
    public string Type { get; set; }

    public string SerialNumber { get; set; }

    public bool IsValid
    {
        get
        {
            return !string.IsNullOrWhiteSpace(SerialNumber);
        }
    }

    public ActionCommand AddScaleCommand
    {
        get
        {
            return new ActionCommand(p => AddScale(Manufacturer, Type, SerialNumber), 
                                    p => IsValid);
        }
    }

    private void AddScale(string manufacturer, string type, string serialNumber)
    {
        using (var api = new BusinessContext())
        {
            var scale = new Scale
            {
                Manifacturer = manufacturer,
                Type = type,
                SerialNumber = serialNumber
            };
            try
            {
                api.AddNewScale(scale);
            }
            catch (Exception ex)
            {
                //TODO kasnije
                return;
            }

            Scales.Add(scale);
        };
    }
}

Scale is simple class with 3 properties (Manufacturer, type and serial number), and ViewModel class implements INotifyPropertyChanged and IDataErrorInfo and implemented necessary methods. ActionCommand class implements ICommand and implements ICommand methods.

UPDATE added ActionCommand class

public class ActionCommand : ICommand
{
    private readonly Action<Object> action;
    private readonly Predicate<Object> predicate;

    public ActionCommand(Action<Object> action) : this(action, null)
    {

    }

    public ActionCommand(Action<Object> action, Predicate<Object> predicate)
    {
        if (action == null)
        {
            throw new ArgumentNullException("Action", "Yout must specify an Action<T>");
        }

        this.action = action;
        this.predicate = predicate;
    }

    #region ICommand Members

    public bool CanExecute(object parameter)
    {
        if (predicate == null)
        {
            return true;
        }

        return predicate(parameter);
    }

    public void Execute(object parameter)
    {
        action(parameter);
    }

    public void Execute()
    {
        Execute(null);
    }

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

    #endregion
}

Answer 1

Your ViewModel class needs to implement both IDataErrorInfo and INotifyPropertyChanged for the validation to work.

Also, there doesn't appear any way for the ActionCommand to re-evaluate IsValid() after SerialNumber has changed.

For more detail on data validation in WPF / MVVM using IDataErrorInfo, check out my blog post .

Answer 2

The problem was I didn't add DataContext to MainWindow in App.xaml.cs

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        var window = new MainWindow
        {
            DataContext = new ScaleViewModel()
        };

        window.ShowDialog();
    }
}