异步 MVVM 命令

Question

我一直在关注 MSDN 杂志 (Patterns for Asynchronous MVVM Applications) 中 Stephen Cleary 的相当出色的系列文章，并且一直在 "hello world" 风格的应用程序中使用他的 IAsyncCommand 模式。

然而，他没有解决的一个领域是需要传入命令参数（使用此模式）。举一个简单的例子，以身份验证为例，出于安全原因，密码控件可能没有数据绑定。

我想知道是否有人设法让他的 AsyncCommand 使用参数，如果是这样，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采用参数的函数一起工作只需要对他的 AsyncCommand class 和静态帮助程序方法进行一些调整。

从他在上面 link 的 AsyncCommand4 示例中找到的 classes 开始，让我们修改构造函数以获取带有参数输入的函数（对象类型 - 这将是命令参数）以及 CancellationToken 并返回任务。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（如下所示）的 class 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 帮助程序 class 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，我们将方法的数量加倍，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));
    }
}

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

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

异步 MVVM 命令

Asynchronous MVVM commands

wpf

asynchronous

mvvm

task-parallel-library

async-await