data-binding - MvvmCross AttributedText Converter 数据绑定(bind)生成空指针警告，但标签呈现正常——有什么问题吗？

Question

我有一个包含 UILabel 的 MvxTableViewCell，我想在其中绑定(bind)/设置 attributedText。我所用的似乎有效(在屏幕上)，但同时我也遇到了这些错误:

2014-12-16 17:35:59.626 clientTouch[51481:1311271] MvxBind:Error: 13.25 Problem seen during binding execution for binding AttributedText for NotificationAttributedText - problem TargetInvocationException: Exception has been thrown by the target of an invocation.
      at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0005c] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:238 
  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Reflection/MethodBase.cs:114 
  at Cirrious.MvvmCross.Binding.Bindings.Target.MvxPropertyInfoTargetBinding.SetValueImpl (System.Object target, System.Object value) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Binding.Bindings.Target.MvxConvertingTargetBinding.SetValue (System.Object value) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Binding.Bindings.MvxFullBinding.UpdateTargetFromSource (System.Object value) [0x00000] in <filename unknown>:0 
InnerException was ArgumentNullException: Argument cannot be null.
Parameter name: value
      at MonoTouch.UIKit.UILabel.set_AttributedText (MonoTouch.Foundation.NSAttributedString value) [0x0000b] in /Developer/MonoTouch/Source/monotouch/src/build/compat/UIKit/UILabel.g.cs:272 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00044] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:230

产生上述问题的特定绑定(bind)是这样的:

this.DelayBind (() => {
       var set = this.CreateBindingSet<NotificationsCellView, NotificationActivitySubViewModel>();

       set.Bind(NameUiLabel)
          .For(v => v.AttributedText)
          .To(vm => vm.NotificationAttributedText)
          .WithConversion("StringToAttributedTextTest");

       ... more bindings here ...

   }

真正的 StringToAttributedText 代码有点长，所以我做了一个非常简单的代码 (StringToAttributedTextTest)，它产生了同样的问题:

public class StringToAttributedTextTestConverter: MvxValueConverter<string, NSMutableAttributedString>
{
    protected override NSMutableAttributedString Convert(
             string value, 
             Type targetType, 
             object parameter, 
             System.Globalization.CultureInfo culture)
    {
        return new NSMutableAttributedString("hi honey", UIFont.FromName("Courier", 13f));
    }
}

就像我说的——它呈现得很好，但它确实会产生那些警告，这对我来说似乎很糟糕。

鉴于上述值“转换器”完全忽略了数据源，它似乎指向一个不正确的值转换器实现，但此时它只是一行代码!

进一步隔离问题，当我将绑定(bind)替换为仅使用常规文本，并将其与 View 模型中的普通旧字符串绑定(bind)时，我根本没有收到任何警告(当然我以这种方式丢失了属性文本)。

set.Bind(NameUiLabel).For(v => v.Text).To(vm => vm.NotificationAttributedText);

非常感谢任何帮助!

Answer 1

将生成警告，因为当重用 MvxTableViewCell 时，其绑定(bind)上下文将设置为 null。这将导致绑定(bind)使用它们的回退值 - 默认情况下也是 null。

要解决此问题(删除警告)，您可以尝试为单元格内的绑定(bind)设置 string.Empty 回退值。

有关回退行为的更多信息，请搜索 UnsetValue - Mvx 试图在该区域模仿 WPF。