[转]异步command的实现

在WPF中，有时有些重操作，需要异步完成，来避免Ui锁死。

MVVM模式下，可以通过异步Command来实现此过程。

首先定义一个IAsyncCommand ，实现ICommand：

    public interface IAsyncCommand : ICommand
    {
        Task ExecuteAsync(object parameter);
    }

然后实现一个实现该接口的抽象Base：

    public abstract class AsyncCommandBase : IAsyncCommand
    {
        public abstract bool CanExecute(object parameter);
        public abstract Task ExecuteAsync(object parameter);

        public async void Execute(object parameter)
        {
            await ExecuteAsync(parameter);
        }
        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }
        protected void RaiseCanExecuteChanged()
        {
            CommandManager.InvalidateRequerySuggested();
        }
    }

再实现上述抽象类：

    public class AsyncCommand<TResult> : AsyncCommandBase, INotifyPropertyChanged
    {
        private readonly Func<CancellationToken, Task<TResult>> _command;
        private readonly CancelAsyncCommand _cancelCommand;
        private NotifyTaskCompletion<TResult> _execution;

        public AsyncCommand(Func<CancellationToken, Task<TResult>> command)
        {
            _command = command;
            _cancelCommand = new CancelAsyncCommand();
        }

        public override bool CanExecute(object parameter)
        {
            return Execution == null || Execution.IsCompleted;
        }

        public override async Task ExecuteAsync(object parameter)
        {
            _cancelCommand.NotifyCommandStarting();
            Execution = new NotifyTaskCompletion<TResult>(_command(_cancelCommand.Token));
            RaiseCanExecuteChanged();
            await Execution.TaskCompletion;
            _cancelCommand.NotifyCommandFinished();
            RaiseCanExecuteChanged();
        }

        public ICommand CancelCommand
        {
            get { return _cancelCommand; }
        }

        public NotifyTaskCompletion<TResult> Execution
        {
            get { return _execution; }
            private set
            {
                _execution = value;
                OnPropertyChanged();
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }

        private sealed class CancelAsyncCommand : ICommand
        {
            private CancellationTokenSource _cts = new CancellationTokenSource();
            private bool _commandExecuting;

            public CancellationToken Token { get { return _cts.Token; } }

            public void NotifyCommandStarting()
            {
                _commandExecuting = true;
                if (!_cts.IsCancellationRequested)
                    return;
                _cts = new CancellationTokenSource();
                RaiseCanExecuteChanged();
            }

            public void NotifyCommandFinished()
            {
                _commandExecuting = false;
                RaiseCanExecuteChanged();
            }

            bool ICommand.CanExecute(object parameter)
            {
                return _commandExecuting && !_cts.IsCancellationRequested;
            }

            void ICommand.Execute(object parameter)
            {
                _cts.Cancel();
                RaiseCanExecuteChanged();
            }

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

            private void RaiseCanExecuteChanged()
            {
                CommandManager.InvalidateRequerySuggested();
            }
        }
    }

    public static class AsyncCommand
    {
        public static AsyncCommand<object> Create(Func<Task> command)
        {
            return new AsyncCommand<object>(async _ => { await command(); return null; });
        }

        public static AsyncCommand<TResult> Create<TResult>(Func<Task<TResult>> command)
        {
            return new AsyncCommand<TResult>(_ => command());
        }

        public static AsyncCommand<object> Create(Func<CancellationToken, Task> command)
        {
            return new AsyncCommand<object>(async token => { await command(token); return null; });
        }

        public static AsyncCommand<TResult> Create<TResult>(Func<CancellationToken, Task<TResult>> command)
        {
            return new AsyncCommand<TResult>(command);
        }
    }

其中的NotifyTaskCompletion实现了任务结束通知：

    public sealed class NotifyTaskCompletion<TResult> : INotifyPropertyChanged
    {
        public NotifyTaskCompletion(Task<TResult> task)
        {
            Task = task;
            TaskCompletion = WatchTaskAsync(task);
        }
        private async Task WatchTaskAsync(Task task)
        {
            try
            {
                await task;
            }
            catch
            {
            }
            var propertyChanged = PropertyChanged;
            if (propertyChanged == null)
                return;
            propertyChanged(this, new PropertyChangedEventArgs("Status"));
            propertyChanged(this, new PropertyChangedEventArgs("IsCompleted"));
            propertyChanged(this, new PropertyChangedEventArgs("IsNotCompleted"));
            if (task.IsCanceled)
            {
                propertyChanged(this, new PropertyChangedEventArgs("IsCanceled"));
            }
            else if (task.IsFaulted)
            {
                propertyChanged(this, new PropertyChangedEventArgs("IsFaulted"));
                propertyChanged(this, new PropertyChangedEventArgs("Exception"));
                propertyChanged(this, new PropertyChangedEventArgs("InnerException"));
                propertyChanged(this, new PropertyChangedEventArgs("ErrorMessage"));
            }
            else
            {
                propertyChanged(this, new PropertyChangedEventArgs("IsSuccessfullyCompleted"));
                propertyChanged(this, new PropertyChangedEventArgs("Result"));
            }
        }
        public Task<TResult> Task { get; private set; }
        public Task TaskCompletion { get; private set; }
        public TResult Result
        {
            get
            {
                return (Task.Status == TaskStatus.RanToCompletion) ?
                    Task.Result : default(TResult);
            }
        }
        public TaskStatus Status { get { return Task.Status; } }
        public bool IsCompleted { get { return Task.IsCompleted; } }
        public bool IsNotCompleted { get { return !Task.IsCompleted; } }
        public bool IsSuccessfullyCompleted
        {
            get
            {
                return Task.Status ==
                    TaskStatus.RanToCompletion;
            }
        }
        public bool IsCanceled { get { return Task.IsCanceled; } }
        public bool IsFaulted { get { return Task.IsFaulted; } }
        public AggregateException Exception { get { return Task.Exception; } }
        public Exception InnerException
        {
            get
            {
                return (Exception == null) ?
                    null : Exception.InnerException;
            }
        }
        public string ErrorMessage
        {
            get
            {
                return (InnerException == null) ?
                    null : InnerException.Message;
            }
        }
        public event PropertyChangedEventHandler PropertyChanged;
    }

假设为一个Button绑定一个IAsyncCommand，这个Button对应的命令就可以异步执行，而不锁死UI了。

※在原文章的代码中NotifyTaskCompletion实现有个bug，下载下来的代码是修复过的。千万别复制文章中的实现~