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方法将命令绑定到按钮，该按钮应该提示我的 ViewModel 中的异步函数，该函数应该启动调用并能够取消调用。我看了 Stephen Cleary 的教程，并尝试将它们转换为我的需求，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，所以我们开始了。我有一个 ViewModel，它应该运行一个异步函数，并且应该通过单击按钮来运行？有没有办法在不编写新库的情况下完成此操作？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，如果您需要它告诉我，我会尽快发送他的 git 代码！ :D

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

Answer 1

我不明白为什么你需要创建RelayCommand的异步版本。您只需在执行侦听取消令牌的命令时运行异步方法即可。像这样：

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

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

以及DoSomething方法的示例：

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，您可以使用以下简单RelayCommand：

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();
}

我确实建议使用像MVVM Light这样的适当库。希望这有帮助。