c# - 使用MVVM按下按钮时如何调用异步方法？

Question

我现在一直在寻找一种将命令绑定到按钮的方法，该按钮应该在我的 ViewModel 中提示一个异步函数，并且应该启动调用并能够取消调用。我查看了 Stephen Cleary 的教程并尝试将它们转换为我的需要，尽管命令管理器在 AsyncCommandBase 的当前上下文中不存在，当您查看他的 git 项目代码时，它与他的教程中的完全不同...我不知道从哪里继续得到我的答案，所以我们开始吧。我有一个 ViewModel 应该运行一个异步的函数并且应该通过单击一个按钮来运行？有没有办法在不编写新库的情况下完成这项工作？我做了一个看起来像这样的界面......

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Input;
using System.Threading.Tasks; 

    namespace Data
    {
        public interface IAsyncCommand : ICommand
        {
            Task ExcecuteAsync(Object parameter); 
        }
    }

和一个看起来像这样的基本命令类

using System;
using System.Threading.Tasks;
using System.Windows.Input;

namespace Data
{
    /// <summary>
    /// An async command that implements <see cref="ICommand"/>, forwarding <see cref="ICommand.Execute(object)"/> to <see cref="IAsyncCommand.ExecuteAsync(object)"/>.
    /// </summary>
    public abstract class AsyncCommandBase : IAsyncCommand
    {
        /// <summary>
        /// The local implementation of <see cref="ICommand.CanExecuteChanged"/>.
        /// </summary>
        private readonly ICanExecuteChanged _canExecuteChanged;

        /// <summary>
        /// Creates an instance with its own implementation of <see cref="ICommand.CanExecuteChanged"/>.
        /// </summary>
        protected AsyncCommandBase(Func<object, ICanExecuteChanged> canExecuteChangedFactory)
        {
            _canExecuteChanged = canExecuteChangedFactory(this);
        }

        /// <summary>
        /// Executes the command asynchronously.
        /// </summary>
        /// <param name="parameter">The parameter for the command.</param>
        public abstract Task ExecuteAsync(object parameter);

        /// <summary>
        /// The implementation of <see cref="ICommand.CanExecute(object)"/>.
        /// </summary>
        /// <param name="parameter">The parameter for the command.</param>
        protected abstract bool CanExecute(object parameter);

        /// <summary>
        /// Raises <see cref="ICommand.CanExecuteChanged"/>.
        /// </summary>
        protected void OnCanExecuteChanged()
        {
            _canExecuteChanged.OnCanExecuteChanged();
        }

        event EventHandler ICommand.CanExecuteChanged
        {
            add { _canExecuteChanged.CanExecuteChanged += value; }
            remove { _canExecuteChanged.CanExecuteChanged -= value; }
        }

        bool ICommand.CanExecute(object parameter)
        {
            return CanExecute(parameter);
        }

        async void ICommand.Execute(object parameter)
        {
            await ExecuteAsync(parameter);
        }
    }
}

该链接指向他的教程，如果您需要它，请告诉我，我会尽快发送他的 git 代码！:D

https://docs.microsoft.com/en-us/archive/msdn-magazine/2014/april/async-programming-patterns-for-asynchronous-mvvm-applications-commands

标签： c#mvvm

Answer 1

我不明白你为什么需要创建一个异步版本的RelayCommand. 您可以在执行侦听取消令牌的命令时简单地运行异步方法。就像是：

你的命令：public ICommand DoSomethingCommand { get; set; }

在某处实例化您的命令：DoSomethingCommand = new RelayCommand(DoSomething);

以及该方法的一个示例DoSomething：

private async void DoSomething()
{
    using (var cancellationToken = new CancellationTokenSource())
    {
        await Task.Run(() => 
        {
            for(i = 0; i < 100; i++)
            {
                if (cancellationToken.IsCancellationRequested) break;
                //Here's where something is done.
            }
        });
    }
}

正常绑定：<Button Content="Do Something" Command="{Binding DoSomethingCommand}"/>

以防万一您需要它，RelayCommand您可以使用以下简单方法：

public class RelayCommand : ICommand
    {
        public RelayCommand(Action execute, Func<bool> canExecute)
        {
            CommandManager.RequerySuggested += (s, e) => CanExecuteChanged(s, e);
            CanExecuteDelegate = canExecute;
            ExecuteDelegate = execute;
        }
        public RelayCommand(Action execute) : this(execute, () => true) { }
        public event EventHandler CanExecuteChanged = delegate { };
        public Func<bool> CanExecuteDelegate { get; set; }
        public Action ExecuteDelegate { get; set; }
        public bool CanExecute(object parameter) => CanExecuteDelegate();
        public void Execute(object parameter) => ExecuteDelegate();
    }

我确实建议使用适当的库，例如 MVVM Light。希望这可以帮助。