Polyline using DataBinding and PointCollection for continuous update

Question

first of all I describe my objective I want to achive. I want to visualise a continuous data stream (maximum 1000 values per second but could be reduced). This data stream should be visualised as a chart - being more precise it's a visualisation of an ECG among other things. My first idea was using polyline and bind it to a point collection. The problem here is that nothing is shown on the UI. Perhaps it's a wrong aproach for this task. Better ideas are welcomed. Here ist my code so far. First the View:

 
<Canvas>
  <Polyline Points="{Binding Points}" Stroke="Red" StrokeThickness="2" />
</Canvas>

For the sake of simplicity I use the code-behind even though I use the MVVM-pattern. That's also the reason why I want to use the binding and not just the name of the polyline and add the values.

public partial class MainWindow : Window
{
   private short[] data = new short[]{ 10,30,50,70,90,110,130,150,170,190,210 };
   private short[] data1 = new short[] { 15,14,16,13,17,12,18,11,19,10,24 };

    public MainWindow()
    {
        InitializeComponent();
        for (int i = 0; i < data.Length; i++)
        {
            Points.Add(new Point(data[i], data1[i]));
        }
    }

    private PointCollection _points = new PointCollection();
    public PointCollection Points
    {
        get { return _points; }
    }


}

I know that is no good coding style but for first tests its enough for me. I use array data for x-values and data1 for y-values. Can anyone tell me whats wrong with that binding? What's to be done for a continuous update of the view, whenever new values occur?
Thanks for your help in advance.

[Updated new version] The view:

<Window.Resources>
        <my:PointCollectionConverter x:Key="myPointsConverter"/>
</Window.Resources>
    <Grid Name="grid">
        <Polyline x:Name="ekglineI" Points="{Binding Points, Converter={StaticResource myPointsConverter}}" Stroke="Red" StrokeThickness="2"  />
        <Button Content="Button" Click="button1_Click" />
</Grid>

The code-behind which draws a polyline on startup and later on when a button is clicked.

public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private short[] data = new short[] { 10, 30, 50, 70, 90, 110, 130, 150, 170, 190, 210 };
        private short[] data2 = new short[] { 230, 250, 270, 290, 300, 310, 330, 350, 370, 390, 410 };
        private short[] data1 = new short[] { 15, 14, 16, 13, 17, 12, 18, 11, 19, 10, 24 };

public MainWindow()
        {
            InitializeComponent();
            grid.DataContext = this;
            for (int i = 0; i < data.Length; i++)
            {
                Points.Add(new Point(data[i], data1[i]));
            }
        }
        public event PropertyChangedEventHandler PropertyChanged;
        private ObservableCollection _points = new ObservableCollection();
        public ObservableCollection Points
        {
            get { return _points; }
        }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        for (int i = 0; i < data2.Length; i++)
        {
            Points.Add(new Point(data2[i], data1[i]));
        }
        PropertyChanged(this, new PropertyChangedEventArgs("Points"));
    }

Now what I want to do is getting rid of this line: grid.DataContext = this; so that I can use my MVVM or is there another possibility?

Answer 1

In order to bind the Polyline Points attribute to your viewmodel successfully (i.e. to have it update when the bound PointCollection changes), you should avoid changing the PointCollection as a collection (Clear, Add, etc). The Polyline will not notice that, even binding to an ObservableCollection of Points with a custom converter will not help.

Instead, you should consider your PointCollection as a property: set it with a newly created PointCollection, and fire a NotifyPropertyChanged event:

    private PointCollection points = new PointCollection();
    public PointCollection Points 
    {
        get { return points; }
        set
        {
            points = value;
            NotifyPropertyChanged("Points");
        }
    }

    public void SomeUpdateFunc() 
    {
        PointCollection pc = new PointCollection();

        // Do some adding: pc.Add(new Point(x, y)); etc

        this.Points = pc; // set via the setter, so the notification will fire
    }

Now the Polyline should be updated properly, good luck!

Answer 2

Kai to make the change notification propagate to your bindings you should be making use of a collection which implements change notificaiton, PointCollection does not do this. You could create your own collection however I'd recommend making use of ObservableCollection<T>.

In addition here is a similar SO post which also touches on a few other options for making the UI aware of your changes.

Answer 3

There is at least one possible way to remove grid.DataContext = this;

Add Binding to RelativeSource to the grid itself. In this case the xaml file will looks like

<Window x:Class="WpfApplication2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525" xmlns:my="clr-namespace:WpfApplication2">
<Grid Name="grid" DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=my:MainWindow, AncestorLevel=1}}">
    <Canvas>
        <Polyline Points="{Binding Points}" Stroke="Red" StrokeThickness="2" />
    </Canvas>
</Grid>

And the code behind will be like this

 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
 using System.Text;
 using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Data;
 using System.Windows.Documents;
 using System.Windows.Input;
 using System.Windows.Media;
 using System.Windows.Media.Imaging;
 using System.Windows.Navigation;
 using System.Windows.Shapes;
 using System.ComponentModel;

 namespace WpfApplication2
 {
    public partial class MainWindow : Window , INotifyPropertyChanged
    {
        public MainWindow()
        {
            InitializeComponent();

            for (int i = 0; i < data.Length; i++)
            {
                 Points.Add(new Point(data[i], data1[i]));
            }
            NotifyPropertyChanged("Points");                
        }

        private void NotifyPropertyChanged(String info)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(info));
            }
        }

        public PointCollection Points { get { return _points; } }

        public event PropertyChangedEventHandler PropertyChanged;
        private PointCollection _points = new PointCollection();
        private short[] data = new short[] { 10, 30, 50, 70, 90, 110, 130, 150, 170, 190, 210 };
        private short[] data1 = new short[] { 15, 14, 16, 13, 17, 12, 18, 11, 19, 10, 24 };


    }
}