wpf processing data before binding - wpf

Question

I have a canvas that shows an audio waveform, which is made using a lot of Lines. Each line is Tagged with it's time code so I can identify where in the audio it is.
I want to put a rectangle on the canvas based on data stored in an observable collection.
Basically, Timespan start and end points so I can show a block of audio.
The problem I'm having is that to show the rectangle, I have to know the Canvas left value and width.
I can get these by scanning through the canvas children until I find the correct line and getting its X1 value, but I don't know how to do it in a binding.
I want to bind the observable collection ItemsControl so I can show the rectangles on the canvas.
Is it possible to bind to a function or something that would do the calculations based on the data in the observable collection?
XAML:
<ScrollViewer Grid.Row="2" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Hidden">
<Canvas Name="waveform" >
<ItemsControl Name="RectArea"> <!-- Where I hope to have the rectangles appear on top of the waveform canvas -->
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Canvas />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Rectangle Stroke="Yellow" Fill="Yellow" Opacity="0.2" Height="200" Width="{Binding Width}" Canvas.Left="{Binding Left}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</Canvas>
</ScrollViewer>
Creating the waveform:
Dim seconds As Integer = 0
lines = New Dictionary(Of String, Line)
Using Reader As New AudioFileReader(openfile.FileName)
Dim samples = Reader.Length / (Reader.WaveFormat.Channels * Reader.WaveFormat.BitsPerSample / 8)
Dim f = 0.0F
Dim max = 0.0F
Dim batch As Integer = Math.Max(10, samples / samplecount)
Dim mid = 100
Dim yScale = 100
Dim buffer(batch) As Single
Dim read As Integer
Dim xPos = 0
read = Reader.Read(buffer, 0, batch)
While read = batch
For n As Integer = 0 To read
max = Math.Max(Math.Abs(buffer(n)), max)
Next
Dim line As New Line
line.X1 = xPos
line.X2 = xPos
line.Y1 = mid + (max * yScale)
line.Y2 = mid - (max * yScale)
line.Tag = Reader.CurrentTime
line.StrokeThickness = 1
line.Stroke = Brushes.DarkGray
AddHandler line.MouseDown, AddressOf Line_MouseDown
waveform.Children.Add(line)
' lines is a dictionary that holds all of the line information. nothing is bound to it, it just allows me to search against time code so I can highlight the line as the audio is playing'
If Not lines.ContainsKey(Reader.CurrentTime.Hours.ToString().PadLeft(2, "0") & Reader.CurrentTime.Minutes.ToString().PadLeft(2, "0") & Reader.CurrentTime.Seconds.ToString().PadLeft(2, "0") & Reader.CurrentTime.Milliseconds.ToString().PadLeft(3, "0").Substring(0, 1)) Then
lines.Add(Reader.CurrentTime.Hours.ToString().PadLeft(2, "0") & Reader.CurrentTime.Minutes.ToString().PadLeft(2, "0") & Reader.CurrentTime.Seconds.ToString().PadLeft(2, "0") & Reader.CurrentTime.Milliseconds.ToString().PadLeft(3, "0").Substring(0, 1), line)
End If
' Draw a tall black line and show timecode every 10 seconds to make it easier to see where you are on the code '
If Reader.CurrentTime.TotalSeconds > (seconds + 10) Then
seconds = Reader.CurrentTime.TotalSeconds
line = New Line
line.X1 = xPos
line.X2 = xPos
line.Y1 = mid + yScale
line.Y2 = mid - yScale
line.StrokeThickness = 1
line.Stroke = Brushes.Black
waveform.Children.Add(line)
Dim textblock As New TextBlock
textblock.Text = Reader.CurrentTime.Hours.ToString().PadLeft(2, "0") & ":" & Reader.CurrentTime.Minutes.ToString().PadLeft(2, "0") & ":" & Reader.CurrentTime.Seconds.ToString().PadLeft(2, "0") & "," & Reader.CurrentTime.Milliseconds.ToString().PadLeft(3, "0")
textblock.Foreground = Brushes.Black
Canvas.SetLeft(textblock, xPos)
Canvas.SetTop(textblock, yScale)
waveform.Children.Add(textblock)
End If
max = 0
xPos += 1
read = Reader.Read(buffer, 0, batch)
End While
waveform.Width = xPos
End Using
ObservableCollection:
Imports System.ComponentModel
Imports System.Collections.ObjectModel
Public Class ocAudioSelection
Implements INotifyPropertyChanged
Private _Changed As Boolean
Public Event PropertyChanged(sender As Object, e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged
Protected Overridable Sub OnPropertyChanged(ByVal Propertyname As String)
On Error GoTo sError
If Not Propertyname.Contains("Changed") Then
Changed = True
End If
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(Propertyname))
Exit Sub
sError:
MsgBox(ErrorToString)
End Sub
Public Property Changed() As Boolean
Get
Return _Changed
End Get
Set(ByVal value As Boolean)
If _Changed <> value Then
_Changed = value
OnPropertyChanged("Changed")
End If
End Set
End Property
Private _startTime As String
Private _endTime As String
Public Sub New()
End Sub
Public Sub New(startTime As String)
_startTime = startTime
End Sub
Public Sub New(startTime As String, endTime As String)
_startTime = startTime
_endTime = endTime
End Sub
Public Property StartTime As String
Get
Return _startTime
End Get
Set(value As String)
If value <> _startTime Then
_startTime = value
OnPropertyChanged("StartTime")
End If
End Set
End Property
Public Property EndTime As String
Get
Return _endTime
End Get
Set(value As String)
If value <> _endTime Then
_endTime = value 'TimeSpan.Parse()
OnPropertyChanged("EndTime")
End If
End Set
End Property
End Class