MVVM和VM集合

Question

常见的情形:具有项目模型集合的模型.
例如一个有人的集合的房子.

如何正确地为MVVM构造这一点 - 特别是在使用添加和删除更新Model和ViewModel集合方面？

模型House包含模型的集合People(通常为a List<People>).
视图模型HouseVM包含它包装的House对象和视图模型的ObservableCollection PeopleVM(ObservableCollection<PeopleVM>).请注意,我们最终会在HouseVM中保存两个集合(需要同步):
1.2 HouseVM.House.List<People>
.HouseVM.ObservableCollection<PeopleVM>

当House更新为新人(添加)或人员离开(删除)时,现在必须在两个集合中处理该事件: Model House People集合和 VM HouseVM PeopleVM ObservableCollection.

这个结构是否正确MVVM？
反正是否有必要对添加和删除进行双重更新？

Answer 1

你的一般方法是完美的MVVM,有一个ViewModel公开其他ViewModel的集合是一个非常常见的场景,我在这里使用它.我不建议直接在ViewModel中公开项目,比如nicodemus13说,因为你最终将视图绑定到没有ViewModel的模型中,用于你的集合的项目.所以,你的第一个问题的答案是:是的,这是有效的MVVM.

您在第二个问题中解决的问题是您的房屋模型中的人员模型列表与您的房屋ViewModel中的ViewModel人员列表之间的同步.您必须手动执行此操作.所以,没有办法避免这种情况.

您可以做什么:实现自定义ObservableCollection<T>,ViewModelCollection<T>将其更改推送到基础集合.要获得双向同步,请将模型的集合也设置为ObservableCollection <>并注册到CollectionChangedViewModelCollection中的事件.

这是我的实施.它使用ViewModelFactory服务等,但只需查看一般主体.我希望它有所帮助......

/// <summary>
/// Observable collection of ViewModels that pushes changes to a related collection of models
/// </summary>
/// <typeparam name="TViewModel">Type of ViewModels in collection</typeparam>
/// <typeparam name="TModel">Type of models in underlying collection</typeparam>
public class VmCollection<TViewModel, TModel> : ObservableCollection<TViewModel>
    where TViewModel : class, IViewModel
    where TModel : class

{
    private readonly object _context;
    private readonly ICollection<TModel> _models;
    private bool _synchDisabled;
    private readonly IViewModelProvider _viewModelProvider;

    /// <summary>
    /// Constructor
    /// </summary>
    /// <param name="models">List of models to synch with</param>
    /// <param name="viewModelProvider"></param>
    /// <param name="context"></param>
    /// <param name="autoFetch">
    /// Determines whether the collection of ViewModels should be
    /// fetched from the model collection on construction
    /// </param>
    public VmCollection(ICollection<TModel> models, IViewModelProvider viewModelProvider, object context = null, bool autoFetch = true)
    {
        _models = models;
        _context = context;

        _viewModelProvider = viewModelProvider;

        // Register change handling for synchronization
        // from ViewModels to Models
        CollectionChanged += ViewModelCollectionChanged;

        // If model collection is observable register change
        // handling for synchronization from Models to ViewModels
        if (models is ObservableCollection<TModel>)
        {
            var observableModels = models as ObservableCollection<TModel>;
            observableModels.CollectionChanged += ModelCollectionChanged;
        }


        // Fecth ViewModels
        if (autoFetch) FetchFromModels();
    }

    /// <summary>
    /// CollectionChanged event of the ViewModelCollection
    /// </summary>
    public override sealed event NotifyCollectionChangedEventHandler CollectionChanged
    {
        add { base.CollectionChanged += value; }
        remove { base.CollectionChanged -= value; }
    }

    /// <summary>
    /// Load VM collection from model collection
    /// </summary>
    public void FetchFromModels()
    {
        // Deactivate change pushing
        _synchDisabled = true;

        // Clear collection
        Clear();

        // Create and add new VM for each model
        foreach (var model in _models)
            AddForModel(model);

        // Reactivate change pushing
        _synchDisabled = false;
    }

    private void ViewModelCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        // Return if synchronization is internally disabled
        if (_synchDisabled) return;

        // Disable synchronization
        _synchDisabled = true;

        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Add:
                foreach (var m in e.NewItems.OfType<IViewModel>().Select(v => v.Model).OfType<TModel>())
                    _models.Add(m);
                break;

            case NotifyCollectionChangedAction.Remove:
                foreach (var m in e.OldItems.OfType<IViewModel>().Select(v => v.Model).OfType<TModel>())
                    _models.Remove(m);
                break;

            case NotifyCollectionChangedAction.Reset:
                _models.Clear();
                foreach (var m in e.NewItems.OfType<IViewModel>().Select(v => v.Model).OfType<TModel>())
                    _models.Add(m);
                break;
        }

        //Enable synchronization
        _synchDisabled = false;
    }

    private void ModelCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (_synchDisabled) return;
        _synchDisabled = true;

        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Add:
                foreach (var m in e.NewItems.OfType<TModel>()) 
                    this.AddIfNotNull(CreateViewModel(m));
                break;

            case NotifyCollectionChangedAction.Remove:
                    foreach (var m in e.OldItems.OfType<TModel>()) 
                        this.RemoveIfContains(GetViewModelOfModel(m));
                break;

            case NotifyCollectionChangedAction.Reset:
                Clear();
                FetchFromModels();
                break;
        }

        _synchDisabled = false;
    }

    private TViewModel CreateViewModel(TModel model)
    {
        return _viewModelProvider.GetFor<TViewModel>(model, _context);
    }

    private TViewModel GetViewModelOfModel(TModel model)
    {
        return Items.OfType<IViewModel<TModel>>().FirstOrDefault(v => v.IsViewModelOf(model)) as TViewModel;
    }

    /// <summary>
    /// Adds a new ViewModel for the specified Model instance
    /// </summary>
    /// <param name="model">Model to create ViewModel for</param>
    public void AddForModel(TModel model)
    {
        Add(CreateViewModel(model));
    }

    /// <summary>
    /// Adds a new ViewModel with a new model instance of the specified type,
    /// which is the ModelType or derived from the Model type
    /// </summary>
    /// <typeparam name="TSpecificModel">Type of Model to add ViewModel for</typeparam>
    public void AddNew<TSpecificModel>() where TSpecificModel : TModel, new()
    {
        var m = new TSpecificModel();
        Add(CreateViewModel(m));
    }
}