[This program is really a precursor to some point geometry programs that do such things as find a minimal spanning tree, find the convex hull, etc.]

Subroutine SaveData writes the X and Y coordinates of the points in the m_Points array into a file. Subroutine LoadData reads those values back into the array.

Subroutine DrawPoints loops through the points drawing them.

Private m_Points() As POINTAPI

' Save data into the file.
Private Sub SaveData(ftitle As String, fname As String)
Dim fnum As Integer
Dim i As Integer

    ' Open the file.
    fnum = FreeFile
    Open fname For Output As fnum

    ' Save the number of points.
    Write #fnum, m_NumPoints

    ' Save the points.
    For i = 1 To m_NumPoints
        With m_Points(i)
            Write #fnum, .X, .Y
        End With
    Next i

    ' Close the file.
    Close fnum

    ' Save the file name and title.
    FileTitle = ftitle
    FileName = fname

    ' Make sure the caption gets updated.
    DataModified = True
    SetDataChanged False
End Sub

' Load data from the file.
Private Sub LoadData(ftitle As String, fname As String)
Dim fnum As Integer
Dim i As Integer

    ' Open the file.
    fnum = FreeFile
    Open fname For Input As fnum

    ' Read the number of points.
    Input #fnum, m_NumPoints
    ReDim m_Points(1 To m_NumPoints)

    ' Read the points.
    For i = 1 To m_NumPoints
        With m_Points(i)
            Input #fnum, .X, .Y
        End With
    Next i

    ' Close the file.
    Close fnum

    ' Save the file name and title.
    FileTitle = ftitle
    FileName = fname

    ' Make sure the caption gets updated.
    DataModified = True
    SetDataChanged False

    ' Draw the new polygon.
    DrawPoints
End Sub

' Draw the points.
Private Sub DrawPoints()
Dim i As Integer

    Cls
    For i = 1 To m_NumPoints
        Circle (m_Points(i).X, m_Points(i).Y), RADIUS
    Next i
End Sub

When the user clicks with the mouse, the program creates another point and lets the user start moving it.

Private Sub Form_MouseMove(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    If Not m_Drawing Then Exit Sub

    Circle (m_LastX, m_LastY), RADIUS
    m_LastX = X
    m_LastY = Y
    Circle (m_LastX, m_LastY), RADIUS
End Sub

Private Sub Form_MouseDown(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    If m_Drawing Then
        ' Already drawing.
        ' Stop or place a point here and continue
        If Button = vbRightButton Then
            ' Stop drawing.
            m_Drawing = False
            DrawMode = vbCopyPen
            DrawPoints
        Else
            ' Add another point to the list.
            Circle (m_LastX, m_LastY), RADIUS
            AddPoint X, Y
            DrawMode = vbCopyPen
            Circle (m_Points(m_NumPoints).X, _
                m_Points(m_NumPoints).Y), RADIUS
            DrawMode = vbInvert
            Circle (m_Points(m_NumPoints).X, _
                m_Points(m_NumPoints).Y), RADIUS
        End If
    Else
        ' Start drawing.
        m_Drawing = True
        Cls
        m_NumPoints = 1
        ReDim m_Points(1 To m_NumPoints)
        With m_Points(m_NumPoints)
            .X = X
            .Y = Y
        End With
        m_LastX = X
        m_LastY = Y
        Circle (m_LastX, m_LastY), RADIUS

        DrawMode = vbInvert
        Circle (m_LastX, m_LastY), RADIUS

        ' Mark the data as modified.
        SetDataChanged True
    End If
End Sub

Private Sub AddPoint(ByVal X As Long, ByVal Y As Long)
    m_NumPoints = m_NumPoints + 1
    ReDim Preserve m_Points(1 To m_NumPoints)
    With m_Points(m_NumPoints)
        .X = X
        .Y = Y
    End With
End Sub

Function DataSafe returns True if it is safe for the program to discard the current data. If there are no changes to the data, the function simply returns True. If the data has been modified, the program asks the user if it is okay to discard or save the changes and takes appropriate action.

The form's QueryUnload event handler uses DataSafe to decide whether it should close the program's form.

Private DataModified As Boolean

' Set DataModified. Display an asterisk in the
' form's Caption next to the file name if
' appropriate.
Private Sub SetDataChanged(changed As Boolean)
    ' Don't bother if it's already been done.
    If DataModified = changed Then Exit Sub
    
    DataModified = changed
    If changed Then
        Caption = "Editor*[" & FileTitle & "]"
    Else
        Caption = "Editor [" & FileTitle & "]"
    End If
End Sub

' Return True if the data is safe.
Private Function DataSafe() As Boolean
    ' No problem if the data is unmodified.
    If Not DataModified Then
        DataSafe = True
        Exit Function
    End If
    
    ' See if the user wants to save changes.
    Select Case MsgBox("The data has been modified. Do you " & _
        "want to save the changes?", _
            vbYesNoCancel)
        Case vbYes
            ' Save the data. Procedure SaveData
            ' will reset DataModified.
            mnuFileSave_Click
            DataSafe = Not DataModified
            
        Case vbNo
            ' Discard the changes to the data.
            DataSafe = True
            
        Case vbNo
            ' Cancel.
            DataSafe = False
    End Select
End Function

' Make sure the data is safe to unload.
Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode _
    As Integer)
    Cancel = Not DataSafe
End Sub