RelayCommand是CommunityToolkit.Mvvm库中非常强大的命令实现，它简化了MVVM模式中命令的创建和使用。本文将详细探讨RelayCommand的各种应用场景。

RelayCommand 的主要作用

简化命令创建
- 提供轻量级的命令实现
- 减少样板代码
- 支持无参数和带参数的命令
解耦用户交互逻辑
- 将界面交互逻辑从视图中分离
- 提高代码的可测试性和可维护性

Nuget 安装包

基本命令实现

同步命令示例

C#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

namespace App06
{
    // ViewModel  
    public partial class UserViewModel : ObservableObject
    {
        // 无参数同步命令  
        [RelayCommand]
        private void SaveUser()
        {
            // 执行保存用户逻辑  
            MessageBox.Show("用户信息已保存");
        }

        // 带条件的同步命令  
        [RelayCommand(CanExecute = nameof(CanSaveUser))]
        private void SaveUserWithCondition()
        {
            MessageBox.Show("带条件的用户保存");
        }

        // 命令是否可执行的条件方法  
        private bool CanSaveUser() => !string.IsNullOrEmpty(UserName);

        // 属性  
        private string _userName;
        public string UserName
        {
            get => _userName;
            set
            {
                // 使用SetProperty更新属性并通知UI  
                SetProperty(ref _userName, value);
                // 触发命令的可执行状态更新  
                SaveUserWithConditionCommand.NotifyCanExecuteChanged();
            }
        }
    }
}

C#
namespace App06
{
    public partial class Form1 : Form
    {
        private UserViewModel _viewModel;
        public Form1()
        {
            InitializeComponent();
            // 初始化ViewModel  
            _viewModel = new UserViewModel();

            // 绑定数据和命令  
            BindViewModel();
        }

        private void BindViewModel()
        {
            // 文本框绑定UserName属性  
            txtUserName.DataBindings.Add(
                new Binding("Text", _viewModel, "UserName", true,
                DataSourceUpdateMode.OnPropertyChanged)
            );

            // 绑定无条件保存按钮  
            btnSaveUser.Click += (s, e) => _viewModel.SaveUserCommand.Execute(null);

            // 绑定带条件保存按钮  
            btnSaveUserWithCondition.Click += (s, e) =>
                _viewModel.SaveUserWithConditionCommand.Execute(null);

            // 初始时禁用带条件的保存按钮  
            btnSaveUserWithCondition.Enabled = false;

            // 监听命令的可执行状态变化  
            _viewModel.SaveUserWithConditionCommand.CanExecuteChanged +=
                (s, e) =>
                {
                    btnSaveUserWithCondition.Enabled =
                    _viewModel.SaveUserWithConditionCommand.CanExecute(null);
                };
        }

    }
}

异步命令实现

C#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

namespace App06
{
    public partial class NetworkViewModel : ObservableObject
    {
        private readonly HttpClient _httpClient;
        private CancellationTokenSource _cts;

        // 数据属性  
        private string _fetchedData;
        public string FetchedData
        {
            get => _fetchedData;
            set => SetProperty(ref _fetchedData, value);
        }

        // 加载状态属性  
        private bool _isLoading;
        public bool IsLoading
        {
            get => _isLoading;
            set => SetProperty(ref _isLoading, value);
        }

        // 错误信息属性  
        private string _errorMessage;
        public string ErrorMessage
        {
            get => _errorMessage;
            set => SetProperty(ref _errorMessage, value);
        }

        public NetworkViewModel()
        {
            // 初始化HttpClient  
            _httpClient = new HttpClient();
        }

        // 异步命令示例  
        [RelayCommand]
        public async Task FetchDataAsync()
        {
            // 取消之前的请求  
            _cts?.Cancel();
            _cts = new CancellationTokenSource();

            try
            {
                IsLoading = true;
                var token = _cts.Token;

                var response = await _httpClient.GetAsync("https://jsonplaceholder.typicode.com/posts/1");
                response.EnsureSuccessStatusCode();

                FetchedData = await response.Content.ReadAsStringAsync();
            }
            catch (OperationCanceledException)
            {
                ErrorMessage = "请求已取消";
            }
            catch (HttpRequestException ex)
            {
                ErrorMessage = $"网络请求失败: {ex.Message}";
            }
            finally
            {
                IsLoading = false;
            }
        }
    }
}

C#
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using static System.Windows.Forms.VisualStyles.VisualStyleElement;

namespace App06
{
    public partial class Form2 : Form
    {
        private bool InverseBoolean(bool value) => !value;
        private NetworkViewModel _viewModel;
        public Form2()
        {
            InitializeComponent();
            // 初始化ViewModel  
            _viewModel = new NetworkViewModel();

            // 绑定ViewModel  
            BindViewModel();
        }

        private void BindViewModel()
        {
            // 绑定数据显示  
            txtFetchedData.DataBindings.Add(
                new Binding("Text", _viewModel, "FetchedData", true,
                DataSourceUpdateMode.OnPropertyChanged)
            );

            // 绑定错误消息  
            lblErrorMessage.DataBindings.Add(
                new Binding("Text", _viewModel, "ErrorMessage", true,
                DataSourceUpdateMode.OnPropertyChanged)
            );

            // 进度条绑定  
            pbar.DataBindings.Add(
                new Binding("Visible", _viewModel, "IsLoading", true,
                DataSourceUpdateMode.OnPropertyChanged)
            );

        }

        private async void btnFetchData_Click(object sender, EventArgs e)
        {
            try
            {
                btnFetchData.Enabled = false;
                await _viewModel.FetchDataAsync();
            }
            catch (Exception ex)
            {
                MessageBox.Show($"执行失败: {ex.Message}", "错误",
                    MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            finally
            {
                // 始终确保按钮可用  
                btnFetchData.Enabled = true;
            }
        }
    }
}

命令参数传递

C#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace App06
{
    // 用户模型  
    public class User
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}

C#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

namespace App06
{
    // ViewModel  
    public partial class ParameterCommandViewModel : ObservableObject
    {
        // 属性绑定  
        [ObservableProperty]
        private string nameInput;

        [ObservableProperty]
        private int ageInput;

        // 接收字符串参数的命令  
        [RelayCommand]
        private void ProcessName(string name)
        {
            MessageBox.Show($"处理名称: {name}");
        }

        // 接收复杂对象参数的命令  
        [RelayCommand]
        private void UpdateUser(User user)
        {
            MessageBox.Show($"更新用户: {user.Name}, 年龄: {user.Age}");
        }

        // 创建用户的命令  
        [RelayCommand]
        private void CreateUser()
        {
            var newUser = new User
            {
                Name = NameInput,
                Age = AgeInput
            };

            UpdateUser(newUser);
        }
    }
}

C#
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Xml.Linq;

namespace App06
{
    public partial class Form3 : Form
    {
        private ParameterCommandViewModel viewModel;

        public Form3()
        {
            InitializeComponent();
            InitializeViewModel();
            SetupBindings();
        }

        private void InitializeViewModel()
        {
            viewModel = new ParameterCommandViewModel();
        }

        private void SetupBindings()
        {
            // 绑定文本框到ViewModel的属性  
            txtName.DataBindings.Add(
                "Text",
                viewModel,
                nameof(ParameterCommandViewModel.NameInput),
                true,
                DataSourceUpdateMode.OnPropertyChanged
            );

            txtAge.DataBindings.Add(
                "Text",
                viewModel,
                nameof(ParameterCommandViewModel.AgeInput),
                true,
                DataSourceUpdateMode.OnPropertyChanged
            );

            // 绑定按钮到命令  
            btnProcessName.Click += (s, e) =>
                viewModel.ProcessNameCommand.Execute(txtName.Text);

            btnCreateUser.Click += (s, e) =>
                viewModel.CreateUserCommand.Execute(null);
        }
    }
}

注意事项

RelayCommand默认在UI线程执行
异步命令建议添加错误处理
使用部分类和源生成器获得最佳性能

结论

CommunityToolkit.Mvvm的RelayCommand提供了强大且灵活的命令实现方式，极大地简化了MVVM开发中的命令逻辑。

目录

RelayCommand 的主要作用

Nuget 安装包

基本命令实现

同步命令示例

异步命令实现

命令参数传递

注意事项

结论