c# ObservableCollection: 如何实现 CollectionChanged 事件

Question

我想知道如何使用 ObservableCollection 和 CollectionChanged 事件。我有一个画布类，可以在上面画笔画。我是将 CollectionChanged 事件处理程序放在 Canvas 类中并监听集合更改，还是放入一个托管笔画集合的模型。

public partial class CollectionBindingPage : ContentPage
{
    private ObservableCollection<Object> c;

    public CollectionBindingPage()
    {
        InitializeComponent();

        c.CollectionChanged += (sender, e) => { 
            //Update the display when strokes was added or removed.
        };
    }


    public class Object
    {
        public string A { get; set; }
        public string B { get; set; }
    }

    public class ViewModel
    {
        public ObservableCollection<Object> collection { get; set; }
    }
}

对不起，我是第一次使用这个。

Answer 1

e 属于 NotifyCollectionChangedEventArgs 类型，它包含 NewItems、OldItems 和 Action。

c.CollectionChanged += (sender, e) => { 
    Console.WriteLine($"{e.Action}");
    // check e.Action and draw your stuff       
};

https://msdn.microsoft.com/en-us/library/system.collections.specialized.notifycollectionchangedeventargs(v=vs.110).aspx

cmets中提到的解决方案的草图

class MyObject : INotifyPropertyChanged
{
    private string _a;
    public event PropertyChangedEventHandler PropertyChanged;

    public string A
    {
        get { return _a; }
        set
        {
            _a = value;
            OnPropertyChanged();
        }
    }

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

class Pagey : ContentPage
{
    private ObservableCollection<MyObject> c = new ObservableCollection<MyObject>();

    // somwhere in your code 
    c.CollectionChanged += OnCollectionChanged;

    private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs args)
    {
        if (args.NewItems != null)
        {
            foreach (var item in args.NewItems.Cast<MyObject>())
            {
                item.PropertyChanged += OnChanged;
            }
        }

        if(args.OldItems != null)
        {
            foreach (var item in args.OldItems.Cast<MyObject>())
            {
                item.PropertyChanged -= OnChanged;
            }
        }

        Redraw();
    }

    private void OnChanged(object sender, PropertyChangedEventArgs e)
    {
        Redraw();
    }

    private void Redraw()
    {

    }
}