Imports System.ComponentModel
Imports System.Timers
Imports System.Collections.ObjectModel
Imports System.Windows.Threading

Partial Class MainWindow
    Implements INotifyPropertyChanged

    Private CurrentTime As TimeSpan
    Private LapTime As TimeSpan
    Private LapCount As Integer
    Private tmr As Timer

    Private ReadOnly OneSecond As TimeSpan =
        TimeSpan.FromSeconds(1)

    Public ReadOnly Property Laps As New ObservableCollection(Of Lap)

    Public Event PropertyChanged As PropertyChangedEventHandler _
        Implements INotifyPropertyChanged.PropertyChanged

    ' Reproduction context:
    ' Delay DataContext binding until the first lap exists.
    Private ReadOnly reproductionTimer As New DispatcherTimer With {
        .Interval = TimeSpan.FromMilliseconds(50)
    }

    Public Sub New()
        InitializeComponent()

        AddHandler reproductionTimer.Tick,
            AddressOf DetectFirstLap

        reproductionTimer.Start()
    End Sub

    Private Sub DetectFirstLap(
        sender As Object,
        e As EventArgs
    )
        If Laps.Count = 0 Then
            Return
        End If

        reproductionTimer.Stop()

        ' Simulates the missing application context:
        ' binding is established after the first item exists.
        DataContext = Me
    End Sub

    Private Sub StartTimer(
        sender As Object,
        e As RoutedEventArgs
    ) Handles btnStart.Click

        CurrentTime = TimeSpan.Zero
        LapTime = TimeSpan.Zero
        LapCount = 0

        tmr = New Timer(1000) With {
            .Enabled = True
        }

        AddHandler tmr.Elapsed, AddressOf UpdateClock
    End Sub

    Private Sub UpdateClock(
        source As Object,
        e As ElapsedEventArgs
    )
        Dispatcher.Invoke(Sub() PopulateTimer())
    End Sub

    Private Sub PopulateTimer()
        CurrentTime = CurrentTime.Add(OneSecond)
        txtDuration.Text = CurrentTime.ToString()
    End Sub

    Private Sub CreateLap(
        sender As Object,
        e As RoutedEventArgs
    ) Handles btnLap.Click

        SaveLap()
    End Sub

    Private Sub SaveLap()
        LapTime = CurrentTime - LapTime
        LapCount += 1

        Laps.Add(
            New Lap With {
                .Count = LapCount,
                .Duration = LapTime
            })

        LapTime = CurrentTime
    End Sub
End Class

Public Class Lap
    Public Property Count As Integer
    Public Property Duration As TimeSpan
End Class