ReactiveUI:具有非集合属性的CanExecute

Question

我已经看到了ReactiveUI: Using CanExecute with a ReactiveCommand的问题，但是我的问题是我有一个字符串属性UniqueID，并且我希望它只在长度等于7的时候执行。我似乎想不出一个不会使程序崩溃的观察者。做这件事的正确而简单的方法是什么？

public class MainViewModel : ReactiveValidatedObject
{
    public MainViewModel()
    {
        RetrieveRecord = new ReactiveAsyncCommand(/* what goes here for CanExecute */);
        RetrieveRecord.Subscriber(x => Record = new Record(UniqueId));

        // or do we use the method RetrieveRecord.CanExecute()?
        // the next line crashes the app

        RetrieveRecord.CanExecute(UniqueId.Length == 7);
    }

    public ReactiveAsyncCommand RetrieveRecord { get; private set; }

    string _uniqueId;
    public string UniqueId
    {
        get { return _uniqueId; }
        set
        {
            _clientId = value;
            this.RaisePropertyChanged(x => x.UniqueId);
        }
    }
}

Answer 1

这样如何：

RetrieveRecord = new ReactiveAsyncCommand(
    this.WhenAny(x => x.UniqueId, x => x.Value.Length == 7));