DialogViewController中的CommandParameter绑定

Question

考虑下面这个简单的ViewModel

public class AboutViewModel
    : MvxViewModel
{
    private readonly IMvxWebBrowserTask _webBrowserTask;
    public AboutViewModel(IMvxWebBrowserTask webBrowserTask) { _webBrowserTask = webBrowserTask; }

    private MvxCommand<string> _showInStoreCommand;
    public ICommand ShowInStoreCommand
    {
        get
        {
            _showInStoreCommand = _showInStoreCommand ?? new MvxCommand<string>(DoShowInStoreCommand);
            return _showInStoreCommand;
        }
    }

    private void DoShowInStoreCommand(string url)
    {
        _webBrowserTask.ShowWebPage(url);
    }

    public string Wp8StoreUrl
    {
        get { return "http://windowsphone.com/s?appId=myappid"; }
    }

    public string AndroidStoreUrl
    {
        get { return "https://play.google.com/store/apps/details?id=mypackagename"; }
    }

    public string TouchStoreUrl
    {
        get { return "http://itunes.com"; }
    }
}

我想把它绑定到一个MvxDialogViewController中，然后我写成这样：

Root = new RootElement
{
    new Section()
    {
        new StringElement("Show in App Store").Bind(bindings, e => e.SelectedCommand, vm => vm.ShowInStoreCommand, new MvxCommandParameterValueConverter(), ViewModel.TouchStoreUrl),
    }
}

为什么这总是在DoShowInStoreCommand的url参数中给我null？当绑定Elements时，我如何使用命令参数？

编辑：

我尝试了以下方法，因为Slodge在他的答案中写的东西与任何扩展方法签名都不匹配，但是当它试图绑定时，它给了我错误：

 new StringElement("Show in App Store")
                .Bind(bindings, "SelectedCommand CommandParameter(ShowInStoreCommand, TouchStoreUrl)")

错误：

MvxBind: Error:   2.37 Problem parsing Lang binding MvxException: Must follow binding option CommandParameter with an '=' in SelectedCommand CommandParameter(ShowInStoreCommand, TouchStoreUrl)
      at Cirrious.MvvmCross.Binding.Parse.Binding.MvxBindingParser.ParseEquals (System.String block) [0x0003c] in c:\Projects\Misc\MVVMCROSS\Cirrious\Cirrious.MvvmCross.Binding\Parse\Binding\MvxBindingParser.cs:83 
  at Cirrious.MvvmCross.Binding.Parse.Binding.Swiss.MvxSwissBindingParser.ParseNextBindingDescriptionOptionInto (Cirrious.MvvmCross.Binding.Parse.Binding.MvxSerializableBindingDescription description) [0x0019a] in c:\Projects\Misc\MVVMCROSS\Cirrious\Cirrious.MvvmCross.Binding\Parse\Binding\Swiss\MvxSwissBindingParser.cs:56 
  at Cirrious.MvvmCross.Binding.Parse.Binding.Tibet.MvxTibetBindingParser.ParseNextBindingDescriptionOptionInto (Cirriou
s.MvvmCross.Binding.Parse.Binding.MvxSerializableBindingDescription description) [0x00033] in c:\Projects\Misc\MVVMCROSS\Cirrious\Cirrious.MvvmCross.Binding\Parse\Binding\Tibet\MvxTibetBindingParser.cs:49 
  at Cirrious.MvvmCross.Binding.Parse.Binding.Swiss.MvxSwissBindingParser.ParseBindingDescription (ParentIsLookingForComma parentIsLookingForComma) [0x00014] in c:\Projects\Misc\MVVMCROSS\Cirrious\Cirrious.MvvmCross.Binding\Parse\Binding\Swiss\MvxSwissBindingParser.cs:176 
  at Cirrious.MvvmCross.Binding.Parse.Binding.Swiss.MvxSwissBindingParser.ParseBindingDescription () [0x00001] in c:\Projects\Misc\MVVMCROSS\Cirrious\Cirrious.MvvmCross.Binding\Parse\Binding\Swiss\MvxSwissBindingParser.cs:159 
  at Cirrious.MvvmCross.Binding.Parse.Binding.MvxBindingParser.ParseTargetPropertyNameAndDescription () [0x0000f] in c:\Projects\Misc\MVVMCROSS\Cirrious\Cirrious.MvvmCross.Binding\Parse\Binding\MvxBindingParser.cs:72 
  at Cirrious.MvvmCross.Binding.Parse.Binding.MvxBindingParser.TryParseBindingSpecification (System
.String text, Cirrious.MvvmCross.Binding.Parse.Binding.MvxSerializableBindingSpecification& requestedBindings) [0x0001a] in c:\Projects\Misc\MVVMCROSS\Cirrious\Cirrious.MvvmCross.Binding\Parse\Binding\MvxBindingParser.cs:51 

Answer 1

您在以下位置创建的绑定：

    new StringElement("Show in App Store").Bind(
                                  bindings, 
                                  e => e.SelectedCommand, 
                                  vm => vm.ShowInStoreCommand, 
                                 new MvxCommandParameterValueConverter(), ViewModel.TouchStoreUrl),

是到ViewModel上的表达式vm => vm.ShowInStoreCommand的绑定，但它捕获 ViewModel.TouchStoreUrl的当前值

如果您想在MvvmCross中使用MultiBinding，那么您可以使用Tibet扩展(请参阅https://github.com/MvvmCross/MvvmCross/wiki/Databinding)来实现-但是这些扩展在流畅的表达式语法中不容易访问-相反，您必须使用基于字符串的语法。

因此，这种绑定可以使用如下命令来实现：

    new StringElement("Show in App Store").Bind(
                                  bindings, 
                                  e => e.SelectedCommand, 
                                  "CommandParameter(ShowInStoreCommand, TouchStoreUrl)")