通过MVVM Light RelayCommand绑定泛型类型

Question

我正在使用隐式数据模板，该模板应用于 ItemsControl 的项目：

<ItemsControl ItemsSource="{Binding Path=CategoryAttributeVMs}"/>

<DataTemplate DataType="{x:Type my:CategoryAttributeDateFieldVM}">
    <DockPanel>
        <.....>
        <Button Command="{Binding Path=MainViewModel.CopyToChildrenCommand,
                                  Source={StaticResource Locator}}"
                CommandParameter="{Binding Mode=OneWay}">
    </DockPanel>
</DataTemplate>

<DataTemplate DataType="{x:Type my:CategoryAttributeIntegerFieldVM}">
    <DockPanel>
        <.....>
        <Button Command="{Binding Path=MainViewModel.CopyToChildrenCommand,
                                  Source={StaticResource Locator}}"
                CommandParameter="{Binding Mode=OneWay}">
    </DockPanel>
</DataTemplate>

这是数据绑定到我的 ViewModel，如下所示：

public abstract class CategoryAttributeVM
{
    protected CategoryAttributeVM(ICategoryAttribute categoryAttribute)
    {
        _categoryAttribute = categoryAttribute;
    }

    private readonly ICategoryAttribute _categoryAttribute;

    public string Name { get { return _categoryAttribute.Name; } }
    public object Value { get { return _categoryAttribute.Value; } }
}

public abstract class CategoryAttributeVM<T> : CategoryAttributeVM
{
    protected CategoryAttributeVM(ICategoryAttribute<T> categoryAttribute)
        : base(categoryAttribute) { _categoryAttribute = categoryAttribute; }

    private readonly ICategoryAttribute<T> _categoryAttribute;

    public new T Value
    {
        get { return _categoryAttribute.Value; }
        set { _categoryAttribute.Value = value; }
    }
}

public class CategoryAttributeDateFieldVM : CategoryAttributeVM<DateTime?>
{
    public CategoryAttributeDateFieldVM(ICategoryAttributeDateField categoryAttributeDateField)
        : base(categoryAttributeDateField) { }
}

public class CategoryAttributeIntegerFieldVM: CategoryAttributeVM<Int32?>
{
    public CategoryAttributeIntegerFieldVM(ICategoryAttributeIntegerField categoryAttributeIntegerField)
       : base(categoryAttributeIntegerField) { }
}

public class MainViewModel : ViewModelBase
{
    public MainViewModel()
    {
        CategoryAttributeVMs = new List<CategoryAttributeVM>( new CategoryAttributeVM[]
            {
                new CategoryAttributeDateFieldVM(new CategoryAttributeDateField("DateField", DateTime.Now)),
                new CategoryAttributeIntegerFieldVM(new CategoryAttributeIntegerField("IntField", 123))
            });

        CategoryAttributeVM2s = new List<CategoryAttributeVM>(new CategoryAttributeVM[]
            {
                new CategoryAttributeDateFieldVM(new CategoryAttributeDateField("DateField", null)),
                new CategoryAttributeIntegerFieldVM(new CategoryAttributeIntegerField("IntField", null))
            });

        CopyToChildrenCommand = new RelayCommand<CategoryAttributeVM>(DoCopyToChildrenCommand);
    }

    public IEnumerable<CategoryAttributeVM> CategoryAttributeVMs { get; private set; }
    private IEnumerable<CategoryAttributeVM> CategoryAttributeVM2s { get; private set; }

    // This an MVVM Light RelayCommand
    public RelayCommand<CategoryAttributeVM> CopyToChildrenCommand { get; private set; }

    private void DoCopyToChildrenCommand(CategoryAttributeVM ca)
    {
        foreach (var item in CategoryAttributeVM2s)
        {
            // How to copy the ca.Value to item.Value where ca and item are of the same type?
            if (item.GetType() == ca.GetType())
                item.Value = ca.Value;  // !! No worky because item.Value refers to the CategoryAttributeVM.Value, which is a readonly Object property
        }
    }
}

如何从 CategoryAttributeVM2s 获取与“ca”类型匹配的对象？ 我想我错过了一些基本的东西，但看不到它。

泛型方法会很好，但据我所知，RelayCommand 不能以这种方式变得通用：

// This would be great
private void DoCopyToChildrenCommand<T, U>(T ca) where T : CategoryAttributeVM<U>
{
    foreach (var item in CategoryAttributeVM2s.OfType<T>())
        item.Value = ca.Value;  // item.Value is type U, which is perfect
}

// But this can't be, obviously
public RelayCommand<CategoryAttributeVM<U>> CopyToChildrenCommand<U> { get; private set; }

有什么想法吗？

Answer 1

我不是 100% 确定我理解您的问题，但为什么不在您的 RelayCommand 中使用 object 并使 CanExecute() 方法返回 true仅当对象可以转换为您的泛型类型时？

ICommand MyCommand { get; set; }

MyCommand = new RelayCommand<object>(
    param => DoCopyToChildrenCommand<T, U>(param as T),
    param => param is T);

Answer 2

最后，我在 CategoryAttributeVM 和 CategoryAttributeVM 上添加了抽象方法的重载：

public abstract class CategoryAttributeVM 
{ 
    <...>
    public abstract void UpdateValueFrom(CategoryAttributeVM sourceCategoryAttributeVM);
} 

public abstract class CategoryAttributeVM<T> : CategoryAttributeVM 
{
    <...>
    public override void UpdateValueFrom(CategoryAttributeVM sourceCategoryAttributeVM)
    {
        if (GetType() == sourceCategoryAttributeVM.GetType())
            Value = (T)sourceCategoryAttributeVM.Value;
    }
}

然后我可以在 DoCopyToChildrenCommand 中使用它：

private void DoCopyToChildrenCommand(CategoryAttributeVM ca)
{
    foreach (var attribute in CategoryAttributeVM2s)
        attribute.UpdateValueFrom(ca);
}