MvvmCross 可绑定 MonoTouch.Dialog RadioElement

Question

我正在尝试创建一个可绑定的 RadioElement 以在 MvvmCross 中的 MonoTouch.Dialog 实现中使用，并按照https://github.com/MvvmCross/MvvmCross-Tutorials/tree/master/DialogExamples/DialogExamples.Touch/BindableElements 中的模式（如MvvmCross Monotouch.Dialog binding data to a table 中的建议），我创建了以下类：

public class MvxBindableRadioElement : RadioElement, IBindableElement
{
    public IMvxBindingContext BindingContext { get; set; }

    public MvxBindableRadioElement ()
    {
        this.CreateBindingContext();

        this.DelayBind(() => {
            var set = this.CreateBindingSet<MvxBindableRadioElement, PropertyCategory>();
            set.Bind().For(me => me.Caption).To(p => p.Id);
            set.Bind().For(me => me.Value).To(p => p.Value);
            set.Apply();
        });
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing) {
            BindingContext.ClearAllBindings();
        }

        base.Dispose(disposing);
    }

    public virtual object DataContext
    {
        get { return BindingContext.DataContext; }
        set { BindingContext.DataContext = value; }
    }
}

PropertyCategory 是一个基本模型：

public class PropertyCategory
{
    public int Id { get; set; }
    public string Value { get; set; }
}

这个用法如下：

new Section {
    new RootElement ("Category", new RadioGroup ()) {
        new BindableSection<MvxBindableRadioElement> ().Bind(bindings, e => e.ItemsSource, vm => vm.PropertyCategories)
    }.Bind (bindings, e => e.RadioSelected, vm => vm.PropertyCategory) as Element
}

其中BindableSection 取自上述 MvvmCross 存储库。

调试我能够验证MvxBindableSection的newElements变量是否正确填充了MvxBindableRadioElements，但是在执行TableView.ReloadData()行时会出现以下错误：

MvxBind:Error: 12.12 Problem seen during binding execution for binding ItemsSource for PropertyCategories - 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 NullReferenceException: Object reference not set to an instance of an object
at <redacted>.iOS.Views.MvxBindableSection`1[<redacted>.Views.MvxBindableRadioElement].NotifyDataSetChanged () [0x000b9] in <redacted>/MonoTouch.Dialog/MvxBindableSection.cs:87 
at <redacted>.iOS.Views.MvxBindableSection`1[<redacted>.Views.MvxBindableRadioElement].SetItemsSource (IEnumerable value) [0x00094] in <redacted>/MonoTouch.Dialog/MvxBindableSection.cs:56 
at <redacted>.iOS.Views.MvxBindableSection`1[<redacted>.Views.MvxBindableRadioElement].set_ItemsSource (IEnumerable value) [0x00003] in <redacted>/MonoTouch.Dialog/MvxBindableSection.cs:30 
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

为了解决这个问题，我将上面提到的 MvvmCross 存储库中的 MvxBindableRadioElement 替换为 CustomStringElement：

new MvxBindableSection<CustomStringElement>().Bind(bindings, element => element.ItemsSource, vm => vm.PropertyCategories)

这就像一个魅力。为什么CustomStringElement 有效，而MvxBindableRadioElement 无效？我是否必须创建一个可绑定的RadioGroup 来包装MvxBindableRadioElements？

编辑：

这里是内部异常（NullReferenceException）：

at CrossUI.Touch.Dialog.Elements.RadioElement.SubscribeToRoot () [0x00000] in <filename unknown>:0 
at CrossUI.Touch.Dialog.Elements.RadioElement.GetCellImpl (MonoTouch.UIKit.UITableView tv) [0x00000] in <filename unknown>:0 
at CrossUI.Touch.Dialog.Elements.Element.GetCell (MonoTouch.UIKit.UITableView tv) [0x00000] in <filename unknown>:0 
at CrossUI.Touch.Dialog.DialogViewController+Source.GetCell (MonoTouch.UIKit.UITableView tableView, MonoTouch.Foundation.NSIndexPath indexPath) [0x00000] in <filename unknown>:0 
at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
at MonoTouch.UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0004c] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:38 
at PGPCapture.iOS.Application.Main (System.String[] args) [0x00008] in <redacted>/Main.cs:21 

Answer 1

好的，所以问题在于BindableSection 的实现（取自MvvmCross-Tutorials / DialogExamples）： Section 元素的 Parent 未设置（即为 null），除非您使用 AddAll() 添加元素。这个问题实际上发生在 MT.Dialog 本身的内部。

更新后的MvxBindableSection 可用here。