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Full implementation of Relay Command - can it be applied to all cases?

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梦谈多话
梦谈多话 2021-01-25 21:41

I\'m looking at the full implementation of a Relay Command that can be found here

I heard that the idea behind RelayCommand is to have a sort of \"universal remote contr

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  • 醉话见心
    醉话见心 (楼主)
    2021-01-25 22:14

    1) ICommand only has methods that include a parameter. If you don't specify a parameter in XAML, null is used.

    https://msdn.microsoft.com/en-us/library/system.windows.input.icommand(v=vs.110).aspx

    2) Yes, you can effect CanExecute without a CommandParameter. See below, it uses the viewmodel's string property "MyData" in CanExecute.

    MainWindow.xaml

    MainWindow.xaml.cs

    using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Input;

namespace WpfApplication8
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }

    public class MainWindowViewModel : INotifyPropertyChanged
    {
        private ICommand command1;
        public ICommand Command1 { get { return command1; } set { command1 = value; PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Command1))); } }

        private string myData;
        public string MyData { get { return myData; } set { myData = value; PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(MyData))); } }

        public event PropertyChangedEventHandler PropertyChanged;

        public MainWindowViewModel()
        {
            Command1 = new RelayCommand(Command1Execute, Command1CanExecute);
        }

        private bool Command1CanExecute(object obj)
        {
            // Only allow execute if MyData has data
            return !string.IsNullOrWhiteSpace(MyData);
        }

        private void Command1Execute(object obj)
        {
            MessageBox.Show($"CommandParameter = '{obj}'");
        }
    }

    public class RelayCommand : ICommand
    {
        #region Fields

        readonly Action _execute = null;
        readonly Predicate _canExecute = null;

        #endregion

        #region Constructors

        /// 
        /// Initializes a new instance of .
        /// 
        /// Delegate to execute when Execute is called on the command.  This can be null to just hook up a CanExecute delegate.
        ///  will always return true.
        public RelayCommand(Action execute)
            : this(execute, null)
        {
        }

        /// 
        /// Creates a new command.
        /// 
        /// The execution logic.
        /// The execution status logic.
        public RelayCommand(Action execute, Predicate canExecute)
        {
            if (execute == null)
                throw new ArgumentNullException("execute");

            _execute = execute;
            _canExecute = canExecute;
        }

        #endregion

        #region ICommand Members

        ///
        ///Defines the method that determines whether the command can execute in its current state.
        ///
        ///Data used by the command.  If the command does not require data to be passed, this object can be set to null.
        ///
        ///true if this command can be executed; otherwise, false.
        ///
        public bool CanExecute(object parameter)
        {
            return _canExecute == null ? true : _canExecute((T)parameter);
        }

        ///
        ///Occurs when changes occur that affect whether or not the command should execute.
        ///
        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }

        ///
        ///Defines the method to be called when the command is invoked.
        ///
        ///Data used by the command. If the command does not require data to be passed, this object can be set to .
        public void Execute(object parameter)
        {
            _execute((T)parameter);
        }

        #endregion
    }
}

    
    
             
    
                                                        
    
            
            
    
              
                
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