WPF MVVM 命令可以执行启用/禁用按钮

Question

我想在文本框属性文本不为空时启用 RibbonButton。当文本框属性文本为空时禁用 RibbonButton。我想在 ICommand 中使用 CanExecute 方法。我该怎么做？

查看：

 <Custom:RibbonButton
                        LargeImageSource="..\Shared\img\save_diskete.png"
                        Label="Save"
                        Command="{Binding ButtonCommand}">
                    </Custom:RibbonButton>

视图模型

class KomentarViewModel:BaseViewModel
    {
        #region Data
        private ICommand m_ButtonCommand;
        public ICommand ButtonCommand
        {
            get
            {
                return m_ButtonCommand;
            }
            set
            {
                m_ButtonCommand = value;
            }
        }
        private string textKomentar;
        public string TextKomentar
        {
            get
            {
                return this.textKomentar;
            }
            set
            {
                // Implement with property changed handling for INotifyPropertyChanged
                if (!string.Equals(this.textKomentar, value))
                {
                    textKomentar = value;
                    OnPropertyChanged("TextKomentar");
                }
            }
        }        
        private ObservableCollection<Komentar> allCommentsInc;
        public ObservableCollection<Komentar> AllCommentsInc
        {
            get
            {
                return allCommentsInc;
            }
            set
            {
                allCommentsInc = value;
                OnPropertyChanged("AllCommentsInc");
            }
        }

        public int idIncident { get; private set; }
        public Incident incident { get; private set; }
        #endregion

        #region Constructor
        public KomentarViewModel(int id)
        {
            CC_RK2Entities context = new CC_RK2Entities();
            this.idIncident = id;

            AllCommentsInc = new ObservableCollection<Komentar>(context.Komentar.Where(a => a.Incident_id == idIncident));
            incident = context.Incident.Where(a => a.id == idIncident).First();

            //ButtonCommand = new RelayCommand(new Action<object>(ShowMessage));
        }
        #endregion        

        #region Methods
        //ukaz napsany text
        public void ShowMessage(object obj)
        {
            //MessageBox.Show(obj.ToString());
            MessageBox.Show(this.TextKomentar);
        }
}

中继命令

namespace Admin.Shared.Commands
{
    class RelayCommand : ICommand
    {
        private Action<object> _action;

        public RelayCommand(Action<object> action)
        {
            _action = action;
        }

        #region ICommand Members

        public bool CanExecute(object parameter)
        {
            return true;            
        }

        public event EventHandler CanExecuteChanged;

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

        #endregion
    }
}

Answer 1

你需要像这样修改你的 RelayCommand 类

  class RelayCommand : ICommand
{
    private Action<object> _action;
    private Func<bool> _func;  

    public RelayCommand(Action<object> action,Func<bool> func)
    {
        _action = action;
        _func = func;
    }

    public void RaiseCanExecuteChanged()
    {
        if(CanExecuteChanged!=null)
            CanExecuteChanged(this,new EventArgs());
    }

    #region ICommand Members

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



    public event EventHandler CanExecuteChanged;

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

    #endregion
}

将 ButtonCommand 初始化为

ButtonCommand = new RelayCommand((s) => ShowMessage(s),()=>!string.IsNullOrEmpty(TextKomentar));

从 Text 属性的设置器中引发RaiseCanExcuteChanged

        public string TextKomentar
    {
        get
        {
            return this.textKomentar;
        }
        set
        {
            // Implement with property changed handling for INotifyPropertyChanged
            if (!string.Equals(this.textKomentar, value))
            {
                textKomentar = value;
                OnPropertyChanged("TextKomentar");
            }
            ButtonCommand.RaiseCanExecuteChanged();
        }
    }

Answer 2

为 canexecute 实现这个：

public bool CanExecute(object parameter)
        {if(thistext available)
            return true; 
         else
            return false;  
        }

因为，当 ICommand 的 CanExecute 方法发生更改时，会引发 CanExecuteChanged。当某些可以更改canexecute 的命令时调用它。 并且可以执行更改应该改成这样：

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

编辑

在您的视图模型构造函数中：

m_ButtonCommand= new RelayCommand(Submit, CanSubmit);

now method for this submit:
 private bool CanSubmit(object obj)
        {
            if(thistext available)
                return true; 
             else
                return false;  

        }
 public void Submit(object _)
  {//... code}

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

这样做。

Answer 3

简单来说，您需要以下内容：

1. 让我们首先创建自己的delegate 命令：

public class DelegateCommand : DelegateCommandBase
{
    private Action _executeMethod;
    private Func<bool> _canExecute;
    
    public DelegateCommand(Action executeMethod)
        : this(executeMethod, () => true) {}
   
    public DelegateCommand(Action executeMethod, Func<bool> _canExecute): base()
    {
        if (executeMethod == null || _canExecute == null) {         
              throw new ArgumentNullException(nameof(executeMethod),      
                              Resources.DelegateCommandDelegatesCannotBeNull);
        }

        _executeMethod = executeMethod;
        _canExecute = _canExecute;
    }

    public void Execute() => _executeMethod();  
    public bool CanExecute() => _canExecute();
    
    protected override void Execute(object parameter) => Execute();
    protected override bool CanExecute(object parameter) => CanExecute();

    public DelegateCommand ObservesProperty<T>(Expression<Func<T>> propertyExpression)
    {
        ObservesPropertyInternal(propertyExpression);
        return this;
    }
    
    public DelegateCommand ObservesCanExecute(Expression<Func<bool>> canExecuteExpression)
    {
        _canExecute = canExecuteExpression.Compile();
        ObservesPropertyInternal(canExecuteExpression);
        return this;
    }
}

这里，DelegateCommandBase 实际上来自 Prism.Commands 命名空间。

如果您不使用 Prism 作为 WPF 的 MVVM 框架，您可以创建自己的 DelegateCommandBase 副本（寻找解决方案 here）。

1. 在您的视图模型中，创建一个类型为 DelegateCommand 的成员并在构造函数中对其进行初始化：

public class MyViewModel
{
    private DelegateCommand _okCommand;
    public DelegateCommand OkCommand
    {
        get => _okCommand;
        set => SetProperty(ref _okCommand, value);
    }
        
    public MyViewModel() 
    {
        OkCommand = new PrismCommands.DelegateCommand(OkCommandHandler,
                                                      OkCanExecuteCommandHandler);
    }
    
    private void OkCommandHandler()
    {
        // ...
    } 
    
    // This is important part: need to return true/false based
    // on the need to enable or disable item
    private bool OkCanExecuteCommandHandler() =>
        return some_condition_to_enable_disable_item;   
}

注意：确保在每次发生可能影响some_condition_to_enable_disable_item 条件行为的更改时引发执行更改事件。

例如，在Prism 的情况下，一旦发生与条件相关的更改（在我们的情况下为OkCommand.RaiseCanExecuteChanged();），您就可以调用RaiseCanExecuteChanged 方法。

小提示：对于 Telerik WPF Controls，您需要调用 InvalidateCanExecute() 而不是 RaiseCanExecuteChanged()。

---

最后，我们的XAML 将如下所示：

<Button x:Name="btnOk"
        Content="Ok"
        Command="{Binding OkCommand}"/>

Answer 4

上次我使用来自Microsoft.Practices.Prism.dll 的Microsoft.Practices.Prism.Commands 命名空间。 DelegateCommand 类有自己的 RaiseCanExecuteChanged() 方法。所以好处是您不必编写自己的ICommand 实现。

XAML：

<StackPanel>
    <CheckBox IsChecked="{Binding IsCanDoExportChecked}" />
    <Button Command="{Binding ExportCommand}" Content="Export" />
</StackPanel>

视图模型：

public class ViewModel
{
    public DelegateCommand ExportCommand { get; }

    public ViewModel()
    {
        ExportCommand = new DelegateCommand(Export, CanDoExptor);
    }

    private void Export()
    {
        //logic
    }

    private bool _isCanDoExportChecked;

    public bool IsCanDoExportChecked
    {
        get { return _isCanDoExportChecked; }
        set
        {
            if (_isCanDoExportChecked == value) return;

            _isCanDoExportChecked = value;
            ExportCommand.RaiseCanExecuteChanged();
        }
    }

    private bool CanDoExptor()
    {
        return IsCanDoExportChecked;
    }
}