如何在 ViewModel 属性中包装模型属性

Question

而不是直接将整个 Model 暴露给 View，我想要 ViewModel属性只是每个 Model 属性的代理。例如;

private Product _product;

public string ProductName
{
     get { return _product.ProductName; }
     set
     {
          SetProperty(ref _product.ProductName, value);
     }
}

但是上面的例子导致了A property, indexer or dynamic member access may not be passed as an out or ref parameter的错误。

我该如何解决这个问题？

P.S。我的模型不是由 INPC 接口实现的。它们只是简单的 POCO 类.

Answer 1

这里不需要设置属性，你可以这样做：

private Product _product;

public string ProductName
{
     get { return _product.ProductName; }
     set
     {
          _product.ProductName = value;
     }
}

Answer 2

您想要的是一个 façade 或 decorator 对象，它将在您的 VM 中充当您的模型，而不是用 ViewModel 属性包装每个模型属性。这使您不仅可以重用您的模型 (facades/decorators)，而且还可以将关注点保留在它们所属的位置。您可以像提供的 chipples 一样定义您的属性，但在 setter 中调用 OnPropertyChanged()。包装其他属性时不能使用 SetProperty 方法。

与此类似的内容：

class Person
{
    public string Name { get; set; }
}

class PersonFacade : BindableBase
{
    Person _person;

    public string Name
    {
        get { return _person.Name; }
        set
        {
            _person.Name = value;
            OnPropertyChanged();
        }
    }
}

class ViewModel : BindableBase
{
    private PersonFacade _person;
    public PersonFacade Person
    {
        get { return _person; }
        set { SetProperty(ref _person, value); }
    }
}

Answer 3

使用像 ViewModelBase 这样的助手 class 很简单，它简化了引发 PropertyChanged 事件的过程：

public class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    protected void OnPropertyChanged<T>(Expression<Func<T>> propertyExpression)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(ExtractPropertyName(propertyExpression)));
    }

    private static string ExtractPropertyName<T>(Expression<Func<T>> propertyExpression)
    {
        if (propertyExpression == null)
            throw new ArgumentNullException("propertyExpression");

        var memberExpression = propertyExpression.Body as MemberExpression;
        if (memberExpression == null)
            throw new ArgumentException("memberExpression");

        var property = memberExpression.Member as PropertyInfo;
        if (property == null)
            throw new ArgumentException("property");

        var getMethod = property.GetGetMethod(true);
        if (getMethod.IsStatic)
            throw new ArgumentException("static method");

        return memberExpression.Member.Name;
    }
}

然后，简单的 POCO class 人：

class Person
{
    public string Name { get; set; }

    public double Age { get; set; }
}

我们可以像这样包装到 ViewModel：

public class PersonViewModel : ViewModelBase
{
    private readonly Person person = new Person();

    public string Name
    {
        get { return person.Name; }
        set
        {
            person.Name = value;
            OnPropertyChanged(() => Name);
        }
    }

    public double Age
    {
        get { return person.Age; }
        set
        {
            person.Age = value;
            OnPropertyChanged(() => Age);
        }
    }
}

Answer 4

也许这是一个旧线程，但 C# MVVM 属性让我感到压力。假设您每天需要编写 200 个属性。

我有另一种方法来创建 BaseClass。

public abstract class NotifyPropertiesBase : INotifyPropertyChanged, INotifyPropertyChanging
{
    public event PropertyChangingEventHandler PropertyChanging;
    public event PropertyChangedEventHandler PropertyChanged;

    readonly Dictionary<string, object> _propertyStore = new Dictionary<string, object>();

    public PropertyChangingEventArgs NotifyChanging([CallerMemberName] string propertyName = null)
    {
        var arg = new PropertyChangingEventArgs(propertyName);
        PropertyChanging?.Invoke(this, arg);
        return arg;
    }
    public PropertyChangedEventArgs NotifyChanged([CallerMemberName] string propertyName = null)
    {
        var arg = new PropertyChangedEventArgs(propertyName);
        PropertyChanged?.Invoke(this, arg);
        return arg;
    }
    public void SetPropValue(object newValue, [CallerMemberName] string propertyName = null)
    {
        if (GetType().GetMember(propertyName).Count() != 1)
            throw new NotSupportedException($"\"{propertyName}\" Not Supported or maybe its not a Property");
        var member = GetType().GetMember(propertyName).FirstOrDefault();
        if (member.MemberType != System.Reflection.MemberTypes.Property)
            throw new NotSupportedException($"Not Support Member Type {member.MemberType}");
        var pInfo = member.DeclaringType.GetProperties().First();

        NotifyChanging(propertyName);

        if (!_propertyStore.ContainsKey(propertyName))
            _propertyStore.Add(propertyName, newValue);
        else
            _propertyStore[propertyName] = newValue;

        NotifyChanged(propertyName);
    }
    public T GetPropertyValue<T>([CallerMemberName] string propertyName = null)
    {
        return (T)GetPropertyValue(propertyName);
    }
    public object GetPropertyValue([CallerMemberName] string propertyName = null)
    {
        if (GetType().GetMember(propertyName).Count() != 1)
            throw new NotSupportedException($"\"{propertyName}\" Not Supported or maybe its not a Property");
        var member = GetType().GetMember(propertyName).FirstOrDefault();
        if (member.MemberType != System.Reflection.MemberTypes.Property)
            throw new NotSupportedException($"Not Support Member Type {member.MemberType}");
        var pInfo = member.DeclaringType.GetProperties().First();

        if (!_propertyStore.ContainsKey(propertyName))
        {
            _propertyStore.Add(propertyName, GetDefault(pInfo.PropertyType));
        }

        return _propertyStore[propertyName];
    }
    object GetDefault(Type t)
    {
        if (t.IsValueType)
        {
            return Activator.CreateInstance(t);
        }
        return null;
    }
}

Class 用法：

class Program
{
    static void Main(string[] args)
    {
        var t = new Test();
        t.PropertyChanged += T_PropertyChanged;
        t.ValueTest = "Hello World!";

        var data = t.GetPropertyValue(nameof(t.ValueTest));
        Console.Write(data);
    }

    private static void T_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        Console.WriteLine(e.PropertyName);
    }
}

public class Test : NotifyPropertiesBase
{
    public string ValueTest
    {
        get => GetPropertyValue<string>();
        set => SetPropValue(value);
    }
}

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