如何在ViewModel层次结构中冒泡更改？

Question

我的MainView.xaml包含我的SmartForm视图:

<Grid Margin="10">
    <views:SmartForm/>
</Grid>

SmartForm视图加载ItemsControl

<Grid Margin="10">
    <ItemsControl
        ItemsSource="{Binding DataTypeViews}"/>
</Grid>

这是DataTypeViews的ObservableCollection:

List<FormField> formFields = new List<FormField>();
formFields.Add(new FormField { IdCode = "firstName", Label = "First Name", Value = "Jim" });
formFields.Add(new FormField { IdCode = "lastName", Label = "Last Name", Value = "Smith" });
formFields.Add(new FormField { IdCode = "address1", Label = "Address 1", Value = "123 North Ashton Rd." });
formFields.Add(new FormField { IdCode = "address2", Label = "Address 2", Value = "Box 23434" });
formFields.Add(new FormField { IdCode = "city", Label = "City", Value = "New Haven" });
formFields.Add(new FormField { IdCode = "state", Label = "State", Value = "NM" });
formFields.Add(new FormField { IdCode = "zip", Label = "Zip Code", Value = "34234" });

foreach (FormField formField in formFields)
{
    DataTypeView dtv = new DataTypeView();
    DataTypeViewModel dtvm = new DataTypeViewModel(formField);
    dtv.DataContext = dtvm;
    DataTypeViews.Add(dtv);
}

每个视图显示构建表单的标签和文本框:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="90"/>
        <ColumnDefinition Width="400"/>
    </Grid.ColumnDefinitions>
    <StackPanel Orientation="Horizontal" Grid.Column="0">
        <TextBlock Text="{Binding Label}" FontSize="14"/>
        <TextBlock Text=": " FontSize="14"/>
    </StackPanel>
    <TextBox Grid.Column="1" Text="{Binding Value}" FontSize="12"/>
</Grid>

如何将DataTypeViewModel中发生的文本框更改冒泡到SmartFormViewModel中？

或者换句话说:如果ViewModel A包含ViewModel B的集合,并且在ViewModel B中发生了更改,那么如何将该更改冒泡到ViewModel A？

Answer 1

我认为你应该使用你可以在这里阅读的中介模式.

基本上它是一个静态类,它允许ViewModels(或任何类)相互通信并来回传递参数.

基本上,ViewModel A开始监听某种消息类型(例如ViewModelBChanged),并且每当该事件发生时ViewModelB只是通知任何正在收听此消息类型的人,它也可以传递它想要的任何信息.

这是调解员的骨架.

public static class MyMediator
{
    public static void Register(Action<object> callback, string message);

    public static void NotifyColleagues(string message, object args);
}

ViewModel A会这样做(可能在构造函数中):

MyMediator.Register(ProcessMessage,"ViewModelBChanged")

然后必须声明一个这样的函数:

void ProcessMessage(object args)
{
    //Do some important stuff here
}

只要想要告诉ViewModel A,ViewModel B就会调用它

MyMediator.NotifyColleagues("ViewModelBChanged",this);

中介类将负责调用viewModel A的回调函数.然后每个人都很高兴.

就个人而言,我喜欢将这些字符串消息值放在这样的静态类中

static class MediatorMessages
{
    public static string ViewModelBChanged= "ViewModelBChanged";
}

这样你就可以做到以下(而不是上面的):

 MyMediator.Register(ProcessMessage,MediatorMessages.ViewModelBChanged)
 MyMediator.NotifyColleagues(MediatorMessages.ViewModelBChanged,this);

如果这还不清楚只是谷歌MVVM中介并点击你的心脏内容:)

Answer 2

您可以让父虚拟机连接到子虚拟机上的PropertyChanged事件.这是一种PITA,用于跟踪已添加/删除的子项等等,因此您可以考虑将您的子VM存储在我的ItemObservableCollection:

public sealed class ItemObservableCollection<T> : ObservableCollection<T>
    where T : INotifyPropertyChanged
{
    public event EventHandler<ItemPropertyChangedEventArgs<T>> ItemPropertyChanged;

    protected override void InsertItem(int index, T item)
    {
        base.InsertItem(index, item);
        item.PropertyChanged += item_PropertyChanged;
    }

    protected override void RemoveItem(int index)
    {
        var item= this[index];
        base.RemoveItem(index);
        item.PropertyChanged -= item_PropertyChanged;
    }

    protected override void ClearItems()
    {
        foreach (var item in this)
        {
            item.PropertyChanged -= item_PropertyChanged;
        }

        base.ClearItems();
    }

    protected override void SetItem(int index, T item)
    {
        var oldItem = this[index];
        oldItem.PropertyChanged -= item_PropertyChanged;
        base.SetItem(index, item);
        item.PropertyChanged -= item_PropertyChanged;
    }

    private void item_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        OnItemPropertyChanged((T)sender, e.PropertyName);
    }

    private void OnItemPropertyChanged(T item, string propertyName)
    {
        ItemPropertyChanged.Raise(this, new ItemPropertyChangedEventArgs<T>(item, propertyName));
    }
}

然后,您的父虚拟机可以通过以下方式监听子项的所有更改:

_formFields.ItemPropertyChanged += (s, e) => Foo();