Chain CollectionChanged 事件

Question

我想看看如何将 x 个 ObservableCollections.CollectionChanged 事件链接在一起，作为 N 级深度对象树暴露给消费者可以收听的单个父级 CollectionChanged 事件？本质上，我想将所有子 CollectionChanged 事件汇集或冒泡到最顶层的父级。我注意到许多解决类似问题的解决方案都假设了固定数量的级别，比如 2 深。我的想法是支持任何级别的深度。

最初我希望我可以将 FieldInfos 的实例传递给子构造函数并直接附加到处理程序。但是我收到一条错误消息，指出“事件 'CollectionChanged' 只能出现在 += 或 -= 的左侧。

谢谢，

public class FieldInfos
    {
        public event NotifyCollectionChangedEventHandler CollectionChanged;

        private ObservableCollection<Field> _fields;

        public ObservableCollection<Field> Fields => _fields ?? (_fields = new ObservableCollection<Field>());
    }



    public class Field
    {
        public string Name;

        private ObservableCollection<FieldInstance> _instances;
        public ObservableCollection<FieldInstance> Instances => _instances ?? (_instances = new ObservableCollection<FieldInstance>());
    }

    public class FieldInstance
    {
        public string Id { get; set; }
    }

Answer 1

最简单的方法是继承原始 ObservableCollection<T>。

您至少需要一个接口来避免协变问题。您也可以拥有自己的类来实现 INotifyDescendantsChanged 接口。

public interface INotifyDescendantsChanged
{
    event NotifyCollectionChangedEventHandler DescendantsChanged;
}

public class ObservableBubbleCollection<T> : ObservableCollection<T>, INotifyDescendantsChanged
{
    public event NotifyCollectionChangedEventHandler DescendantsChanged;
    protected virtual void OnDescendantsChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        NotifyCollectionChangedEventHandler handler = DescendantsChanged;
        if (handler != null)
            handler(sender, e);
    }

    private readonly Func<T, INotifyDescendantsChanged> childSelector;
    public ObservableBubbleCollection() { }
    public ObservableBubbleCollection(Func<T, INotifyDescendantsChanged> childSelector)
    {
        this.childSelector = childSelector;
    }

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        base.OnCollectionChanged(e);
        OnDescendantsChanged(this, e);

        if (childSelector == null)
            return;

        if (e.NewItems != null)
            foreach (var item in e.NewItems.Cast<T>())
                childSelector(item).DescendantsChanged += OnDescendantsChanged;

        if (e.OldItems != null)
            foreach (var item in e.OldItems.Cast<T>())
                childSelector(item).DescendantsChanged -= OnDescendantsChanged;
    }
}

要使用它，请替换 ObservableCollection 的实例并将选择器传递给集合。

public class FieldInfos
{
    private ObservableBubbleCollection<Field> _fields;

    public ObservableBubbleCollection<Field> Fields => _fields ?? (_fields = new ObservableBubbleCollection<Field>(fi => fi.Instances));
}

public class Field
{
    public string Name;

    private ObservableBubbleCollection<FieldInstance> _instances;
    public ObservableBubbleCollection<FieldInstance> Instances => _instances ?? (_instances = new ObservableBubbleCollection<FieldInstance>());
}

public class FieldInstance
{
    public string Id { get; set; }
}

static class Program
{
    static void Main(string[] args)
    {
        var fi = new FieldInfos();
        fi.Fields.DescendantsChanged += (sender, e) =>
        {
            Console.WriteLine("Change from {0}", sender.GetType());
        };

        var field = new Field();
        fi.Fields.Add(field);

        field.Instances.Add(new FieldInstance());
        Console.ReadLine();
    }
}

Chain CollectionChanged 事件

Chain CollectionChanged Events

events

event-handling

observablecollection

reactive-programming

system.reactive