C＃WPF MVVM Light无法执行

Question

i am learning WPF and MVVM at the moment and having a little problem.

Im using MVVM Light and i want some Buttons to be dis/enabled after a validation, but its not using the functions.

ViewModelMain:

using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.Entity;

namespace EF_MVVM_Test
{
    public class ViewModelMain : ViewModelBase
    {
        public RelayCommand AddAuthorCommand { get; private set; }
        public RelayCommand DeleteAuthorCommand { get; private set; }
        public RelayCommand RefreshCommand { get; private set; }
        public RelayCommand SaveCommand { get; private set; }

        public ObservableCollection<Author> Authors { get; set; }
        private LibraryContext db;

        private Author _SelectedAuthor;
        public Author SelectedAuthor
        {
            get { return _SelectedAuthor; }
            set { Set("SelectedAuthor", ref _SelectedAuthor, value); }
        }

        private Author _NewAuthor;
        public Author NewAuthor
        {
            get { return _NewAuthor; }
            set { Set("NewAuthor", ref _NewAuthor, value); }
        }

        public ViewModelMain()
        {
            db = new LibraryContext();
            db.Author.Load();
            Authors = db.Author.Local;

            AddAuthorCommand = new RelayCommand(ExecuteAddAuthorCommand, CanExecuteAddAuthorCommand);
            DeleteAuthorCommand = new RelayCommand(ExecuteDeleteAuthorCommand, CanExecuteDeleteAuthorCommand);
            RefreshCommand = new RelayCommand(ExecuteRefreshListCommand);
            SaveCommand = new RelayCommand(ExecuteSaveCommand);

            NewAuthor = new Author();
        }

        private void ExecuteAddAuthorCommand()
        {
            db.Author.Add(_NewAuthor);
            NewAuthor = new Author();
        }
        private void ExecuteDeleteAuthorCommand()
        {
            db.Author.Remove(SelectedAuthor);
        }
        private void ExecuteRefreshListCommand()
        {
            db.Author.Load();
            Authors = db.Author.Local;
        }
        private void ExecuteSaveCommand()
        {
            db.SaveChanges();
        }

        private bool CanExecuteAddAuthorCommand()
        {
            return (!string.IsNullOrEmpty(NewAuthor.Name) && NewAuthor.Birthday != null);
        }
        private bool CanExecuteDeleteAuthorCommand()
        {
            return SelectedAuthor != null;
        }
    }
}

XAML:

<StackPanel>
        <DataGrid ItemsSource="{Binding Authors}" SelectedItem="{Binding SelectedAuthor}" AutoGenerateColumns="False" Height="200" Margin="5" CanUserAddRows="False" CanUserDeleteRows="False">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Name" Binding="{Binding Name}"/>
                <DataGridTextColumn Header="Birthday" Binding="{Binding Birthday}"/>
            </DataGrid.Columns>
        </DataGrid>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Name" Margin="5" VerticalAlignment="Center"/>
            <TextBox Height="25" Width="70" Margin="5" Text="{Binding NewAuthor.Name}"/>
            <TextBlock Text="Geburtstag" Margin="5" VerticalAlignment="Center"/>
            <DatePicker Height="25" Width="130" Margin="5" SelectedDate="{Binding NewAuthor.Birthday}"/>
            <Button Margin="5" Height="25" Width="80" Content="Hinzufügen" HorizontalAlignment="Left" Command="{Binding AddAuthorCommand}"/>
            <Button Margin="5" Height="25" Width="80" Content="Löschen" HorizontalAlignment="Left" Command="{Binding DeleteAuthorCommand}"/>
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <Button Width="120" Height="25" Content="Speichern" HorizontalAlignment="Left" Margin="5" Command="{Binding SaveCommand}"/>
            <!--Button Width="120" Height="25" Content="Aktualisieren" HorizontalAlignment="Left" Margin="5" Command="{Binding RefreshCommand}"/-->
        </StackPanel>
</StackPanel>

any ideas why the buttons aren't using the canexecute functions?

Answer 1

Your problem is the namespace you're using:

using GalaSoft.MvvmLight.Command;

Replace it with:

using GalaSoft.MvvmLight.CommandWpf;

Answer 2

WPF's CommandManager calls CanExecute when it detects a UI changes? What constitutes a UI change? Adding and removing elements, some binding updates, visual state changes. Basically, whenever WPF feels like it. It is unreliable.

How can you skirt this? When you need to refresh a RelayCommand , call RelayCommand.RaiseCanExecuteChanged() . This method raises an event that notifies WPF that it should re-evaluate the status of the command.

Answer 3

Looks like you have the same problem as me a few days ago.

ICommand doesn't work

ICommand.CanExecute is not calling, because of course you don't say the button: "The ability of the button click may change when I insert something in the TextBox ". So your button doesn't realize that he can be enable if textbox text changes. You must tell the button explicitly that it should check ICommand.CanExecute when the text changes, by raising the ICommand.CanExecuteChanged event when textbox tex has changed.

Answer 4

我认为，不幸的是，您必须在分配SelectedAuthor并拦截NewAuthor对象的属性更改后，调用RelayCommand对象的RaiseCanExecuteChanged方法。