xamarin.android - 为什么向 MvxListView 派生对象添加 Header View 会破坏绑定(bind)过程？

Question

我目前正在将 MvvmCross 用于新的应用程序项目，并且我正在尝试使用 MvxListView 显示标题 View 。阅读诸如 this 之类的讨论让我明白在设置适配器后无法添加 headerview，因此必须覆盖构造函数，以便在设置 MvxAdapter 之前添加 header 。这导致我实现了以下类:

public class HeaderListView : MvxListView
{
    public FrameLayout HeaderFrame { get; set; }

    public HeaderListView(Context context, IAttributeSet attrs)
        : this(context, attrs, new MvxAdapter(context))
    {
    }

    public HeaderListView(Context context, IAttributeSet attrs, IMvxAdapter adapter) 
        : base(context, attrs, null)
    {
        InitializeHeader(context);
        // Note: Any calling derived class passing a null adapter is responsible for setting
        // it's own itemTemplateId
        if (adapter == null)
            return;

        var itemTemplateId = MvxAttributeHelpers.ReadListItemTemplateId(context, attrs);
        adapter.ItemTemplateId = itemTemplateId;
        Adapter = adapter;
    }

    private void InitializeHeader(Context context)
    {
        HeaderFrame = new FrameLayout(context);
        AddHeaderView(HeaderFrame);
    }
}

请特别注意第二个构造函数中的 InitializeHeader 调用。当我注释掉这一行时，测试应用程序启动正常，并且此 HeaderListView 的行为与标准 MvxListView 没有区别。但是，取消对该行的注释，你会看到一个空白的 ListView 和一些绑定(bind)错误:

MvxBind:Error: 10.86 Problem seen during binding execution for binding ItemsSource for Names - problem TargetInvocationException: Exception has been thrown by the target of an invocation.
02-02 23:33:01.948 I/mono-stdout( 2323): MvxBind:Error: 10.86 Problem seen during binding execution for binding ItemsSource for Names - 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) [0x00000] in <filename unknown>:0 
  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Binding.Bindings.Target.MvxPropertyInfoTargetBinding.SetValueImpl (System.Object target, System.Object value) [0x00000] in <filename unknown>:0 
02-02 23:33:01.948 I/mono-stdout( 2323):      at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
02-02 23:33:01.948 I/mono-stdout( 2323):   at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <filename unknown>:0 
02-02 23:33:01.958 I/mono-stdout( 2323):   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 Cirrious.MvvmCross.Binding.Droid.Views.MvxListView.set_ItemsSource (IEnumerable value) [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
02-02 23:33:01.958 I/mono-stdout( 2323):   at Cirrious.MvvmCross.Binding.Bindings.Target.MvxConvertingTargetBinding.SetValue (System.Object value) [0x00000] in <filename unknown>:0 
02-02 23:33:01.968 I/mono-stdout( 2323):   at Cirrious.MvvmCross.Binding.Bindings.MvxFullBinding.UpdateTargetFromSource (System.Object value) [0x00000] in <filename unknown>:0 
02-02 23:33:01.968 I/mono-stdout( 2323): InnerException was NullReferenceException: Object reference not set to an instance of an object
02-02 23:33:01.968 I/mono-stdout( 2323):      at Cirrious.MvvmCross.Binding.Droid.Views.MvxListView.set_ItemsSource (IEnumerable value) [0x00000] in <filename unknown>:0 
02-02 23:33:01.968 I/mono-stdout( 2323):   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) [0x00000] in <filename unknown>:0 

我完全无法理解这个错误，因为导致它出现的唯一区别是我在分配适配器之前分配了一个标题 View ，但这与适配器没有任何关系。此错误的来源是什么，我该如何解决？

演示此问题的示例项目的完整源代码可用 on github .

Answer 1

原因与将页眉和页脚添加到 ListView 的怪异方式有关。当您将页眉或页脚添加到 ListView 时，当您为 ListView 设置适配器时，它会将您的适配器包装在一个返回页眉和页脚的 HeaderViewListAdapter 中，就好像它们只是您的适配器返回的实际行之前和之后的行一样。

MvxListView 失败的原因是它的 Adpater 属性，如下所示:

    public new IMvxAdapter Adapter
    {
        get { return base.Adapter as IMvxAdapter; }
        set
        {
            // Code to copy ItemsSource and template ID

            base.Adapter = value;
        }
    }

它不存储您提供给它的 IMvxAdapter 实例。它只是将其设置在 base.Adapter 中，并期望稍后获得 base.Adapter 时它将与之前设置的值相同。但当页眉和页脚添加到列表时，情况就不是这样了。您从 base.Adapter 返回的类将是一个 HeaderViewListAdapter，它包装了之前设置的 IMvxAdapter 实例。

作为 MvxList 的用户，没有好的方法来解决这个问题(除了复制和更改 MvxList)。 Stuart 的解决方法是不依赖 base.Adapter 作为 getter，而是将值存储到 setter 的字段中并从 getter 返回它。