更改您的PointCollections属性为依赖属性：

Change your PointCollections Property to a dependency property:

public PointCollection Pts
        {
            get { return (PointCollection)GetValue(PtsProperty); }
            set { SetValue(PtsProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Pts.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty PtsProperty =
            DependencyProperty.Register("Pts", typeof(PointCollection), typeof(ViewModel), new UIPropertyMetadata(new PointCollection()));

BTW执行此操作，您不需要触发PropertyChanged事件。

BTW Doing this, you won't need to fire the PropertyChanged event.

哦，对不起，你的对象需要继承自DependencyObject

Oh sorry, and your object needs to inherit from DependencyObject

    public class ViewModel : DependencyObject 
{ 
//... 
}