Question

如何通过传入CanExecute参数来调用RelayCommand。该对象被绑定为视图中的命令参数。我有以下代码，其中说'Delegate Func不接受0参数'。请帮忙。 OnPrintAllChartsInCurrentView和IsPrintAndExportEnabled的定义如下。

     RelayCommand m_PrintAllChartsCommand;
                public ICommand PrintAllChartsCommand
                {
                    get
                    {
                        return m_PrintAllChartsCommand ?? (m_PrintAllChartsCommand = new RelayCommand<object>(
                            OnPrintAllChartsInCurrentView,
                            () => IsPrintAndExportEnabled() //This line there is a compiler error
                            ));
                    }
                }

private void OnPrintAllChartsInCurrentView(object obj)
        {
//do something. The obj is bound as command parameter from the view.
        }

    private bool IsPrintAndExportEnabled()
    {
    //I will do some operation here and change to return true or false later
    return false;
    }

这是我试图调用的RelayCommand类

namespace GalaSoft.MvvmLight.Command
{

    public class RelayCommand<T> : ICommand
    {
        //
        // Summary:
        //     Initializes a new instance of the RelayCommand class that can always execute.
        //
        // Parameters:
        //   execute:
        //     The execution logic.
        //
        // Exceptions:
        //   T:System.ArgumentNullException:
        //     If the execute argument is null.
        public RelayCommand(Action<T> execute);
        //
        // Summary:
        //     Initializes a new instance of the RelayCommand class.
        //
        // Parameters:
        //   execute:
        //     The execution logic.
        //
        //   canExecute:
        //     The execution status logic.
        //
        // Exceptions:
        //   T:System.ArgumentNullException:
        //     If the execute argument is null.
        public RelayCommand(Action<T> execute, Func<T, bool> canExecute);

        //
        // Summary:
        //     Occurs when changes occur that affect whether the command should execute.
        public event EventHandler CanExecuteChanged;

        //
        // Summary:
        //     Defines the method that determines whether the command can execute in its current
        //     state.
        //
        // Parameters:
        //   parameter:
        //     Data used by the command. If the command does not require data to be passed,
        //     this object can be set to a null reference
        //
        // Returns:
        //     true if this command can be executed; otherwise, false.
        public bool CanExecute(object parameter);
        //
        // Summary:
        //     Defines the method to be called when the command is invoked.
        //
        // Parameters:
        //   parameter:
        //     Data used by the command. If the command does not require data to be passed,
        //     this object can be set to a null reference
        public virtual void Execute(object parameter);
        //
        // Summary:
        //     Raises the GalaSoft.MvvmLight.Command.RelayCommand`1.CanExecuteChanged event.
        public void RaiseCanExecuteChanged();
    }
}

Answer 1

如果要获取类型为T的命令参数（您声明为对象），则RelayCommand CanExecute需要一个类型为T的参数并返回bool。你传入一个匿名函数，它不带参数并返回一个bool。你可以简单地替换

() => IsPrintAndExportEnabled();

与

arg => { return IsPrintAndExportEnabled(); }

如果您不打算对传递给CanExecute的对象执行任何操作。

如果您不需要命令参数，则无需将RelayCommand声明为

RealyCommand<object>(execute, canExecute);

它可以简单地

RelayCommand(execute, canExecute);

在这种情况下，Execute将不接受任何参数并返回void，而CanExecute将不接受任何参数并返回bool。

Answer 2

应该是这样的。请注意，RelayCommand canExecute参数为Func<T,bool>，这意味着您传递具有相同签名的方法（如下所示）。有关Func<T,TResult>的更多信息，请参阅this。

 RelayCommand m_PrintAllChartsCommand;
 public ICommand PrintAllChartsCommand
 {
       get
       {
             return m_PrintAllChartsCommand ?? (m_PrintAllChartsCommand = new RelayCommand<object>(
                        OnPrintAllChartsInCurrentView,
                        IsPrintAndExportEnabled
                        ));
        }
  }

 private void OnPrintAllChartsInCurrentView(object arg)
 {

 }

 private bool IsPrintAndExportEnabled(object arg)
 {
      return false;
 }

Answer 3

这对我有用（IsRealShunter是一个布尔属性）：

RaiseAlarmClickCommand = new RelayCommand<Guid>(RaiseAlarmClick, x => IsRealShunter);

RaiseAlarmClick是一种方法：

private void RaiseAlarmClick(Guid idPrinter) {...}

一个小注意事项，如果您的视图模型中有更新内容，并且您想确保您的RelayCommand反映更改，请从发生更改的地方调用此内容：

RaiseAlarmClickCommand.RaiseCanExecuteChanged();

Answer 4

这对我有用。如果它可以帮助任何人。首先看看RelayCommand，我声明的方式是错误的。它应该是RelayCommand<object>这是完整的声明和用法。

   RelayCommand<object> m_PrintAllChartsCommand;
        public ICommand PrintAllChartsCommand
        {
            get
            {

                return m_PrintAllChartsCommand ?? (m_PrintAllChartsCommand = new RelayCommand<object>(
                    OnPrintAllChartsInCurrentView, IsPrintAndExportEnabled));

            }
        }
private void OnPrintAllChartsInCurrentView(object arg)
 {

 }

 private bool IsPrintAndExportEnabled(object arg)
 {
 }

使用CanExecute参数调用RelayCommand <t>

4 个答案: