为什么这个数据绑定不起作用？ [英] Why doesn't this data binding work?

问题描述

我有一个ViewModel类，它包含一个点列表，我试图将它绑定到一个Polyline。 Polyline接收点的初始列表，但是即使我实现了INotifyPropertyChanged，即使添加了附加点也没有注意到。怎么了？

I have a ViewModel class that contains a list of points, and I am trying to bind it to a Polyline. The Polyline picks up the initial list of points, but does not notice when additional points are added even though I implement INotifyPropertyChanged. What's wrong?

<StackPanel>
    <Button Click="Button_Click">Add!</Button>
    <Polyline x:Name="_line" Points="{Binding Pts}" Stroke="Black" StrokeThickness="5"/>
</StackPanel>

C＃边：

// code-behind
_line.DataContext = new ViewModel();
private void Button_Click(object sender, RoutedEventArgs e)
{
    // The problem is here: NOTHING HAPPENS ON-SCREEN!
    ((ViewModel)_line.DataContext).AddPoint();
}

// ViewModel class
public class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public PointCollection Pts { get; set; }

    public ViewModel()
    {
        Pts = new PointCollection();
        Pts.Add(new Point(1, 1));
        Pts.Add(new Point(11, 11));
    }

    public void AddPoint()
    {
        Pts.Add(new Point(25, 13));
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Pts"));
    }
}

推荐答案

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 
{ 
//... 
}

这篇关于为什么这个数据绑定不起作用？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！