您提出的建议很合理,正是
Stephen Cleary创建的AsyncCommand
实现所做的(他是我认为在异步代码方面最重要的
专家之一)。
这里是文章中代码的完整实现(加上我为使用案例所做的一些调整)。
AsyncCommand.cs
using System;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Input;
namespace Infrastructure
{
public class AsyncCommand : AsyncCommand<object>
{
public AsyncCommand(Func<object, Task> command)
: base(async (parmater, token) => { await command(parmater); return null; }, null)
{
}
public AsyncCommand(Func<object, Task> command, Func<object, bool> canExecute)
: base(async (parmater, token) => { await command(parmater); return null; }, canExecute)
{
}
public AsyncCommand(Func<object, CancellationToken, Task> command)
: base(async (parmater, token) => { await command(parmater, token); return null; }, null)
{
}
public AsyncCommand(Func<object, CancellationToken, Task> command, Func<object, bool> canExecute)
: base(async (parmater, token) => { await command(parmater, token); return null; }, canExecute)
{
}
}
public class AsyncCommand<TResult> : AsyncCommandBase, INotifyPropertyChanged
{
private readonly Func<object, CancellationToken, Task<TResult>> _command;
private readonly CancelAsyncCommand _cancelCommand;
private readonly Func<object, bool> _canExecute;
private NotifyTaskCompletion<TResult> _execution;
private bool _allowMultipleInvocations;
public AsyncCommand(Func<object, Task<TResult>> command)
: this((parmater, token) => command(parmater), null)
{
}
public AsyncCommand(Func<object, Task<TResult>> command, Func<object, bool> canExecute)
: this((parmater, token) => command(parmater), canExecute)
{
}
public AsyncCommand(Func<object, CancellationToken, Task<TResult>> command)
: this(command, null)
{
}
public AsyncCommand(Func<object, CancellationToken, Task<TResult>> command, Func<object, bool> canExecute)
{
_command = command;
_canExecute = canExecute;
_cancelCommand = new CancelAsyncCommand();
}
public override bool CanExecute(object parameter)
{
var canExecute = _canExecute == null || _canExecute(parameter);
var executionComplete = (Execution == null || Execution.IsCompleted);
return canExecute && (AllowMultipleInvocations || executionComplete);
}
public override async Task ExecuteAsync(object parameter)
{
_cancelCommand.NotifyCommandStarting();
Execution = new NotifyTaskCompletion<TResult>(_command(parameter, _cancelCommand.Token));
RaiseCanExecuteChanged();
await Execution.TaskCompletion;
_cancelCommand.NotifyCommandFinished();
RaiseCanExecuteChanged();
}
public bool AllowMultipleInvocations
{
get { return _allowMultipleInvocations; }
set
{
if (_allowMultipleInvocations == value)
return;
_allowMultipleInvocations = value;
OnPropertyChanged();
}
}
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();
}
}
}
}
AsyncCommandBase.cs
using System;
using System.Threading.Tasks;
using System.Windows.Input;
namespace Infrastructure
{
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();
}
}
}
NotifyTaskCompletion.cs
using System;
using System.ComponentModel;
using System.Threading.Tasks;
namespace Infrastructure
{
public sealed class NotifyTaskCompletion<TResult> : NotifyTaskCompletion
{
public NotifyTaskCompletion(Task<TResult> task)
: base(task)
{
}
public TResult Result
{
get
{
return (Task.Status == TaskStatus.RanToCompletion) ?
((Task<TResult>)Task).Result : default(TResult);
}
}
}
public class NotifyTaskCompletion : INotifyPropertyChanged
{
public NotifyTaskCompletion(Task task)
{
Task = task;
if (!task.IsCompleted)
TaskCompletion = WatchTaskAsync(task);
else
TaskCompletion = 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 Task { get; private set; }
public Task TaskCompletion { get; private set; }
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;
}
}