How to implement Async Command

Question

Although I have understood async programming with c# somehow, still don\'t get why async with void is not the better solution, then when I want to improve my Xamarin Forms code

Answer 1

There is nothing wrong with async void on a command execute handler providing you handle your exceptions.

So what does an AsyncCommand, offer? Potentially the following

• An error channel to pass back any unhandled exceptions

• Not having to write async void or async lamdas

• IsBusy framework, to stop things like double hits or what ever you can imagine

Answer 2

You should always avoid async void occurrences in your code (exceptions of event handlers).

Please refer this blog from Stephen for more detail

Async void methods have different error-handling semantics. When an exception is thrown out of an async Task or async Task method, that exception is captured and placed on the Task object. With async void methods, there is no Task object, so any exceptions thrown out of an async void method will be raised directly on the SynchronizationContext that was active when the async void method started.

Answer 3

To anyone interested: Brandons solution above does not requery the CanExecute automatically and requires RaiseCanExecuteChanged(). To change this, you can exchange

    public event EventHandler CanExecuteChanged
    {
        add => _weakEventManager.AddEventHandler(value);
        remove => _weakEventManager.RemoveEventHandler(value);
    }

with

    public event EventHandler CanExecuteChanged {
        add => CommandManager.RequerySuggested += value;
        remove => CommandManager.RequerySuggested -= value;
    }

and remove

public void RaiseCanExecuteChanged() => _weakEventManager.HandleEvent(this, EventArgs.Empty, nameof(CanExecuteChanged));

This fixed the problem for me.

Answer 4

Here is an implementation of AsyncCommand that I created for this NuGet Package: AsyncAwaitBestPractices.MVVM.

This implementation was inspired by @John Thiriet's blog post, "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();
    }
}