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,只需要对其 AsyncCommand 类和静态帮助器方法进行一些调整。
从上面链接中的 AsyncCommand4 示例中找到的类开始,让我们修改构造函数以获取一个带有参数输入(对象类型 - 这将是命令参数)以及 CancellationToken 的函数,并返回一个任务。我们还需要对 ExecuteAsync 方法进行一次更改,以便在执行命令时将参数传递到此函数中。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 帮助程序类也将很有帮助,以便更轻松地创建命令参数感知 IAsyncCommands。为了处理使用或不使用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 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));