Async Programming

' ✗ WRONG — blocks the UI thread for 3 seconds
Private Sub btnLoad_Click(...) Handles btnLoad.Click
    lblStatus.Text = "Loading..."          ' never actually shows!
    Thread.Sleep(3000)                     ' UI frozen for 3s
    lblStatus.Text = "Done"
End Sub

' ✓ CORRECT — UI thread free during Await
Private Async Sub btnLoad_Click(...) Handles btnLoad.Click
    btnLoad.Enabled = False               ' prevent double-click
    lblStatus.Text  = "Loading..."          ' shows immediately
    Try
        Dim students = Await LoadStudentsAsync()
        dgv.DataSource = students
        lblStatus.Text = $"Loaded {students.Count} records"
    Catch ex As Exception
        lblStatus.Text = $"Error: {ex.Message}"
    Finally
        btnLoad.Enabled = True             ' always re-enable
    End Try
End Sub

' The async work function — runs off UI thread
Private Async Function LoadStudentsAsync() As Task(Of List(Of Student))
    Return Await Task.Run(Function()
        Using conn As New SqliteConnection(CONN_STR)
            conn.Open()
            Dim da As New SqliteDataAdapter("SELECT * FROM Students", conn)
            Dim dt As New DataTable()
            da.Fill(dt)
            Return dt.AsEnumerable() _
                .Select(Function(r) New Student With {
                    .Name  = r("Name").ToString(),
                    .Grade = CDbl(r("Grade"))
                }).ToList()
        End Using
    End Function)
End Function

' --- Event handler ---
Private Async Sub btnImport_Click(...) Handles btnImport.Click
    pbImport.Minimum = 0
    pbImport.Maximum = 100
    pbImport.Value   = 0
    btnImport.Enabled = False

    ' Progress(Of Integer) marshals to UI thread automatically
    Dim progress = New Progress(Of Integer)(Sub(pct)
        pbImport.Value   = pct
        lblProgress.Text = $"{pct}% complete"
    End Sub)

    Await ImportLargeFileAsync("C:\data\students.csv", progress)
    btnImport.Enabled = True
    lblProgress.Text  = "Import complete!"
End Sub

' --- Background work with progress ---
Private Async Function ImportLargeFileAsync(
        path As String,
        progress As IProgress(Of Integer)) As Task
    Await Task.Run(Sub()
        Dim lines = IO.File.ReadAllLines(path)
        Dim total  = lines.Length
        For i = 0 To total - 1
            ' process line i…
            Thread.Sleep(5)   ' simulate work
            progress.Report(CInt((i + 1) * 100 \ total))
        Next
    End Sub)
End Function

Private _cts As CancellationTokenSource = Nothing

Private Async Sub btnStart_Click(...) Handles btnStart.Click
    _cts = New CancellationTokenSource()
    btnStart.Enabled  = False
    btnCancel.Enabled = True
    Try
        Await ProcessDataAsync(_cts.Token)
        lblStatus.Text = "Completed!"
    Catch ex As OperationCanceledException
        lblStatus.Text = "Cancelled by user."
    Catch ex As Exception
        lblStatus.Text = $"Error: {ex.Message}"
    Finally
        btnStart.Enabled  = True
        btnCancel.Enabled = False
        _cts.Dispose()
    End Try
End Sub

Private Sub btnCancel_Click(...) Handles btnCancel.Click
    _cts?.Cancel()   ' null-safe cancel
End Sub

Private Async Function ProcessDataAsync(token As CancellationToken) As Task
    For i = 1 To 100
        token.ThrowIfCancellationRequested()   ' check on each iteration
        Await Task.Delay(50, token)            ' cancellable delay
    Next
End Function