异步 MVVM 命令

Question

我一直在关注 Stephen Cleary 在 MSDN 杂志 (Patterns for Asynchronous MVVM Applications) 上相当出色的系列文章，并一直在“hello world”风格的应用程序中使用他的 IAsyncCommand 模式。

但是，他没有解决的一个领域是需要传入命令参数（使用此模式）。举个简单的例子，在密码控件出于安全原因可能没有数据绑定的情况下进行身份验证。

我想知道是否有人设法让他的AsyncCommand 使用参数，如果是，他们会分享他们的发现吗？

Answer 1

让 Stephen Cleary 的 IAsyncCommand 模式与在生成要执行的任务时接受参数的函数一起工作，只需要对他的 AsyncCommand 类和静态辅助方法进行一些调整。

从上面链接中的 AsyncCommand4 示例中找到的他的类开始，让我们修改构造函数以获取一个函数，该函数带有一个参数（对象类型 - 这将是命令参数）以及一个 CancellationToken 并返回一个任务。我们还需要在 ExecuteAsync 方法中进行一次更改，以便我们可以在执行命令时将参数传递给此函数。我创建了一个名为 AsyncCommandEx 的类（如下所示）来演示这些更改。

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

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

    public ICommand CancelCommand
    {
        get { return _cancelCommand; }
    }

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

    public event PropertyChangedEventHandler PropertyChanged;

    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(parameter, _cancelCommand.Token));
        RaiseCanExecuteChanged();
        await Execution.TaskCompletion;
        _cancelCommand.NotifyCommandFinished();
        RaiseCanExecuteChanged();
    }

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

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

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

        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; }
        }

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

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

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

更新静态 AsyncCommand 帮助器类也有助于更轻松地创建命令参数感知 IAsyncCommand。为了处理接受或不接受命令参数的函数的可能组合，我们将方法数量加倍，但结果还不错：

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

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

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

    public static AsyncCommandEx<TResult> Create<TResult>(Func<object, Task<TResult>> command)
    {
        return new AsyncCommandEx<TResult>((param, _) => command(param));
    }

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

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

    public static AsyncCommandEx<TResult> Create<TResult>(Func<CancellationToken, Task<TResult>> command)
    {
        return new AsyncCommandEx<TResult>(async (param, token) => await command(token));
    }

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

要继续 Stephen Cleary 的示例，您现在可以构建一个 AsyncCommand，它接受从命令参数（可以绑定到 UI）传入的对象参数：

CountUrlBytesCommand = AsyncCommandEx.Create((url,token) => MyService.DownloadAndCountBytesAsync(url as string, token));