Relay Command 的全面实施——是否适用于所有情况？

Question

我正在查看可以找到的中继命令的完整实现 here

我听说 RelayCommand 背后的想法是让一种“universal remote control”用于您的所有命令。

如果是这样，我有 2 个实施问题：

1) 如果我不想为某些控件传递参数，会发生什么情况？我一定要吗？我是否需要相应地更改我的 execute/can-execute 函数以支持这些参数？

2) 如果我不想在 XAML 中传递 CommandParameter 怎么办？如果我想通过使用属性更改或我的代码中的其他一些方法来影响控件的更改。我可以在不传递 XAML 中的 CommandParameter 的情况下影响 CanExecute 或 CanExecuteChanged 吗？

到目前为止，我主要实现了部分 RelayCommands，其中 CanExecute 始终返回 true，并且我只是将控件 IsEnabled 绑定到视图模型中的额外属性。这工作得很好，但我想知道 - 完整的实施能为我做什么？

(能否给出一个完整的工作示例？)

Answer 1

1) ICommand 只有包含参数的方法。如果在 XAML 中没有指定参数，则使用 null。

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

2) 是的，您可以在没有 CommandParameter 的情况下影响 CanExecute。见下文，它在 CanExecute.

中使用了视图模型的字符串属性 "MyData"

MainWindow.xaml

<Window x:Class="WpfApplication8.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication8"
        mc:Ignorable="d"
        FocusManager.FocusedElement="{Binding ElementName=tb}"
        SizeToContent="Height"
        Title="MainWindow" Width="525">
    <Window.DataContext>
        <local:MainWindowViewModel />
    </Window.DataContext>
    <StackPanel>
        <Label Content="MyData (CanExecute returns false if this is whitespace)" />
        <TextBox Name="tb" Text="{Binding MyData, UpdateSourceTrigger=PropertyChanged}" Margin="5" />
        <Button Content="Without XAML CommandParameter" Margin="5" Command="{Binding Command1}" />
        <Button Content="With XAML CommandParameter" Margin="5" Command="{Binding Command1}" CommandParameter="{Binding MyData}" />
    </StackPanel>
</Window>

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<object>(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<T> : ICommand
    {
        #region Fields

        readonly Action<T> _execute = null;
        readonly Predicate<T> _canExecute = null;

        #endregion

        #region Constructors

        /// <summary>
        /// Initializes a new instance of <see cref="DelegateCommand{T}"/>.
        /// </summary>
        /// <param name="execute">Delegate to execute when Execute is called on the command.  This can be null to just hook up a CanExecute delegate.</param>
        /// <remarks><seealso cref="CanExecute"/> will always return true.</remarks>
        public RelayCommand(Action<T> execute)
            : this(execute, null)
        {
        }

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

            _execute = execute;
            _canExecute = canExecute;
        }

        #endregion

        #region ICommand Members

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

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

        ///<summary>
        ///Defines the method to be called when the command is invoked.
        ///</summary>
        ///<param name="parameter">Data used by the command. If the command does not require data to be passed, this object can be set to <see langword="null" />.</param>
        public void Execute(object parameter)
        {
            _execute((T)parameter);
        }

        #endregion
    }
}

Relay Command 的全面实施——是否适用于所有情况？

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

c#

wpf

relaycommand