c# - 如何实现异步命令

Question

虽然我以某种方式理解了使用 c# 进行异步编程，但仍然不明白为什么使用 void 进行异步不是更好的解决方案，然后当我想改进我的 Xamarin Forms 代码时，我发现许多 MVVM 框架使用 AsyncCommand 来避免使用 async 进行 void “不同于事件”，如下所示:

public class AsyncCommand : Command {
    public AsyncCommand(Func<Task> execute) : base(() => execute()) { }
    public AsyncCommand(Func<object, Task> execute) : base((arg) => execute(arg)) { }
}

但我不知道为什么 async if 命令本身不是异步的，以及如何使用带有操作的异步命令并运行这样的任务:

public AsyncCommand(Action execute) : this(() => Task.Run(execute))
public AsyncCommand(Action<object> execute) : this((arg) => Task.Run(() => execute(arg)))

Answer 1

这是我为此 NuGet 包创建的 AsyncCommand 的实现:AsyncAwaitBestPractices.MVVM .

此实现的灵感来自 @John Thiriet's博文，"Going Async With AsyncCommand" .

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

namespace AsyncAwaitBestPractices.MVVM
{
    /// <summary>
    /// An implmentation of IAsyncCommand. Allows Commands to safely be used asynchronously with Task.
    /// </summary>
    public sealed class AsyncCommand<T> : IAsyncCommand<T>
    {
        #region Constant Fields
        readonly Func<T, Task> _execute;
        readonly Func<object, bool> _canExecute;
        readonly Action<Exception> _onException;
        readonly bool _continueOnCapturedContext;
        readonly WeakEventManager _weakEventManager = new WeakEventManager();
        #endregion

        #region Constructors
        /// <summary>
        /// Initializes a new instance of the <see cref="T:TaskExtensions.MVVM.AsyncCommand`1"/> class.
        /// </summary>
        /// <param name="execute">The Function executed when Execute or ExecuteAysnc is called. This does not check canExecute before executing and will execute even if canExecute is false</param>
        /// <param name="canExecute">The Function that verifies whether or not AsyncCommand should execute.</param>
        /// <param name="onException">If an exception is thrown in the Task, <c>onException</c> will execute. If onException is null, the exception will be re-thrown</param>
        /// <param name="continueOnCapturedContext">If set to <c>true</c> continue on captured context; this will ensure that the Synchronization Context returns to the calling thread. If set to <c>false</c> continue on a different context; this will allow the Synchronization Context to continue on a different thread</param>
        public AsyncCommand(Func<T, Task> execute,
                            Func<object, bool> canExecute = null,
                            Action<Exception> onException = null,
                            bool continueOnCapturedContext = true)
        {
            _execute = execute ?? throw new ArgumentNullException(nameof(execute), $"{nameof(execute)} cannot be null");
            _canExecute = canExecute ?? (_ => true);
            _onException = onException;
            _continueOnCapturedContext = continueOnCapturedContext;
        }
        #endregion

        #region Events
        /// <summary>
        /// Occurs when changes occur that affect whether or not the command should execute
        /// </summary>
        public event EventHandler CanExecuteChanged
        {
            add => _weakEventManager.AddEventHandler(value);
            remove => _weakEventManager.RemoveEventHandler(value);
        }
        #endregion

        #region Methods
        /// <summary>
        /// Determines whether the command can execute in its current state
        /// </summary>
        /// <returns><c>true</c>, if this command can be executed; otherwise, <c>false</c>.</returns>
        /// <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>
        public bool CanExecute(object parameter) => _canExecute(parameter);

        /// <summary>
        /// Raises the CanExecuteChanged event.
        /// </summary>
        public void RaiseCanExecuteChanged() => _weakEventManager.HandleEvent(this, EventArgs.Empty, nameof(CanExecuteChanged));

        /// <summary>
        /// Executes the Command as a Task
        /// </summary>
        /// <returns>The executed Task</returns>
        /// <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>
        public Task ExecuteAsync(T parameter) => _execute(parameter);

        void ICommand.Execute(object parameter)
        {
            if (parameter is T validParameter)
                ExecuteAsync(validParameter).SafeFireAndForget(_continueOnCapturedContext, _onException);
            else if (parameter is null && !typeof(T).IsValueType)
                ExecuteAsync((T)parameter).SafeFireAndForget(_continueOnCapturedContext, _onException);
            else
                throw new InvalidCommandParameterException(typeof(T), parameter.GetType());
        }
        #endregion
    }

    /// <summary>
    /// An implmentation of IAsyncCommand. Allows Commands to safely be used asynchronously with Task.
    /// </summary>
    public sealed class AsyncCommand : IAsyncCommand
    {
        #region Constant Fields
        readonly Func<Task> _execute;
        readonly Func<object, bool> _canExecute;
        readonly Action<Exception> _onException;
        readonly bool _continueOnCapturedContext;
        readonly WeakEventManager _weakEventManager = new WeakEventManager();
        #endregion

        #region Constructors
        /// <summary>
        /// Initializes a new instance of the <see cref="T:TaskExtensions.MVVM.AsyncCommand`1"/> class.
        /// </summary>
        /// <param name="execute">The Function executed when Execute or ExecuteAysnc is called. This does not check canExecute before executing and will execute even if canExecute is false</param>
        /// <param name="canExecute">The Function that verifies whether or not AsyncCommand should execute.</param>
        /// <param name="onException">If an exception is thrown in the Task, <c>onException</c> will execute. If onException is null, the exception will be re-thrown</param>
        /// <param name="continueOnCapturedContext">If set to <c>true</c> continue on captured context; this will ensure that the Synchronization Context returns to the calling thread. If set to <c>false</c> continue on a different context; this will allow the Synchronization Context to continue on a different thread</param>
        public AsyncCommand(Func<Task> execute,
                            Func<object, bool> canExecute = null,
                            Action<Exception> onException = null,
                            bool continueOnCapturedContext = true)
        {
            _execute = execute ?? throw new ArgumentNullException(nameof(execute), $"{nameof(execute)} cannot be null");
            _canExecute = canExecute ?? (_ => true);
            _onException = onException;
            _continueOnCapturedContext = continueOnCapturedContext;
        }
        #endregion

        #region Events
        /// <summary>
        /// Occurs when changes occur that affect whether or not the command should execute
        /// </summary>
        public event EventHandler CanExecuteChanged
        {
            add => _weakEventManager.AddEventHandler(value);
            remove => _weakEventManager.RemoveEventHandler(value);
        }
        #endregion

        #region Methods
        /// <summary>
        /// Determines whether the command can execute in its current state
        /// </summary>
        /// <returns><c>true</c>, if this command can be executed; otherwise, <c>false</c>.</returns>
        /// <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>
        public bool CanExecute(object parameter) => _canExecute(parameter);

        /// <summary>
        /// Raises the CanExecuteChanged event.
        /// </summary>
        public void RaiseCanExecuteChanged() => _weakEventManager.HandleEvent(this, EventArgs.Empty, nameof(CanExecuteChanged));

        /// <summary>
        /// Executes the Command as a Task
        /// </summary>
        /// <returns>The executed Task</returns>
        public Task ExecuteAsync() => _execute();

        void ICommand.Execute(object parameter) => _execute().SafeFireAndForget(_continueOnCapturedContext, _onException);
        #endregion
    }

    /// <summary>
    /// Extension methods for System.Threading.Tasks.Task
    /// </summary>
    public static class TaskExtensions
    {
        /// <summary>
        /// Safely execute the Task without waiting for it to complete before moving to the next line of code; commonly known as "Fire And Forget". Inspired by John Thiriet's blog post, "Removing Async Void": https://johnthiriet.com/removing-async-void/.
        /// </summary>
        /// <param name="task">Task.</param>
        /// <param name="continueOnCapturedContext">If set to <c>true</c> continue on captured context; this will ensure that the Synchronization Context returns to the calling thread. If set to <c>false</c> continue on a different context; this will allow the Synchronization Context to continue on a different thread</param>
        /// <param name="onException">If an exception is thrown in the Task, <c>onException</c> will execute. If onException is null, the exception will be re-thrown</param>
        #pragma warning disable RECS0165 // Asynchronous methods should return a Task instead of void
        public static async void SafeFireAndForget(this System.Threading.Tasks.Task task, bool continueOnCapturedContext = true, System.Action<System.Exception> onException = null)
        #pragma warning restore RECS0165 // Asynchronous methods should return a Task instead of void
        {
            try
            {
                await task.ConfigureAwait(continueOnCapturedContext);
            }
            catch (System.Exception ex) when (onException != null)
            {
                onException?.Invoke(ex);
            }
        }
    }

    /// <summary>
    /// Weak event manager that allows for garbage collection when the EventHandler is still subscribed
    /// </summary>
    public class WeakEventManager
    {
        readonly Dictionary<string, List<Subscription>> _eventHandlers = new Dictionary<string, List<Subscription>>();

        /// <summary>
        /// Adds the event handler
        /// </summary>
        /// <param name="handler">Handler</param>
        /// <param name="eventName">Event name</param>
        public void AddEventHandler(Delegate handler, [CallerMemberName] string eventName = "")
    {
            if (IsNullOrWhiteSpace(eventName))
                throw new ArgumentNullException(nameof(eventName));

            if (handler is null)
                throw new ArgumentNullException(nameof(handler));

            EventManagerService.AddEventHandler(eventName, handler.Target, handler.GetMethodInfo(), _eventHandlers);
        }

        /// <summary>
        /// Removes the event handler.
        /// </summary>
        /// <param name="handler">Handler</param>
        /// <param name="eventName">Event name</param>
        public void RemoveEventHandler(Delegate handler, [CallerMemberName] string eventName = "")
        {
            if (IsNullOrWhiteSpace(eventName))
                throw new ArgumentNullException(nameof(eventName));

            if (handler is null)
                throw new ArgumentNullException(nameof(handler));

            EventManagerService.RemoveEventHandler(eventName, handler.Target, handler.GetMethodInfo(), _eventHandlers);
        }

        /// <summary>
        /// Executes the event
        /// </summary>
        /// <param name="sender">Sender</param>
        /// <param name="eventArgs">Event arguments</param>
        /// <param name="eventName">Event name</param>
        public void HandleEvent(object sender, object eventArgs, string eventName) => EventManagerService.HandleEvent(eventName, sender, eventArgs, _eventHandlers);
    }

    /// <summary>
    /// An Async implmentation of ICommand
    /// </summary>
    public interface IAsyncCommand<T> : System.Windows.Input.ICommand
    {
        /// <summary>
        /// Executes the Command as a Task
        /// </summary>
        /// <returns>The executed Task</returns>
        /// <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>
        System.Threading.Tasks.Task ExecuteAsync(T parameter);
    }

    /// <summary>
    /// An Async implmentation of ICommand
    /// </summary>
    public interface IAsyncCommand : System.Windows.Input.ICommand
    {
        /// <summary>
        /// Executes the Command as a Task
        /// </summary>
        /// <returns>The executed Task</returns>
        System.Threading.Tasks.Task ExecuteAsync();
    }
}