How to wait for onPaint to finish painting (C#)

Question

Hi based on the thread here: How to create a jpg image dynamically in memory with .NET?

I have this method:

        int maxVal = 50;
        int maxXCells = r.Next(maxVal);
        int maxYCells = r.Next(maxVal);
        int cellXPosition = r.Next(maxVal);
        int cellYPosition = r.Next(maxVal);
        int boxSize = 10;


        Graphics fg = this.CreateGraphics();


        using (var bmp = new System.Drawing.Bitmap(maxXCells * boxSize + 1, maxYCells * boxSize + 1))
        {


            using (Graphics g = Graphics.FromImage(bmp))
            {
                g.Clear(Color.Yellow);
                Pen pen = new Pen(Color.Black);
                pen.Width = 1;

                //Draw red rectangle to go behind cross
                Rectangle rect = new Rectangle(boxSize * (cellXPosition - 1), boxSize * (cellYPosition - 1), boxSize, boxSize);
                g.FillRectangle(new SolidBrush(Color.Red), rect);

                //Draw cross
                g.DrawLine(pen, boxSize * (cellXPosition - 1), boxSize * (cellYPosition - 1), boxSize * cellXPosition, boxSize * cellYPosition);
                g.DrawLine(pen, boxSize * (cellXPosition - 1), boxSize * cellYPosition, boxSize * cellXPosition, boxSize * (cellYPosition - 1));

                //Draw horizontal lines
                for (int i = 0; i <= maxXCells; i++)
                {
                    g.DrawLine(pen, (i * boxSize), 0, i * boxSize, boxSize * maxYCells);
                }

                //Draw vertical lines            
                for (int i = 0; i <= maxYCells; i++)
                {
                    g.DrawLine(pen, 0, (i * boxSize), boxSize * maxXCells, i * boxSize);
                }
            }
            fg.DrawImage(bmp, 0, 0);
            fg.Dispose();

        }

It fires based on the random event (can be many times in a second). What should I do, to draw a new image only after current drawing has finished? At the moment, I can seen, that if the event to run this method fires faster than drawing has finished, the screen flickers. What are the common solution to avoiding drawing before previous drawing has finished?

This what I included in OnPaint method:

   protected override void OnPaint(System.Windows.Forms.PaintEventArgs pe)
    {
        finishedInvalidating = false;
        fg = this.CreateGraphics();
        lock (bmp)
        {
            fg.DrawImage(bmp, 0, 0);
        }
        fg.Dispose();
        finishedInvalidating = true;

But this does not solve the problem

---------------------- update ------------------

 public partial class LadderFrm : Form
{
    Bitmap bmp;

    int numCols = 3;
    int colWidth = 100;
    int numRows = 30;
    int rowHeight = 20;
    bool finishedInvalidating = false;

    decimal lastprice;

    public LadderFrm()
    {
        bmp = new System.Drawing.Bitmap(numCols * colWidth + 1, numRows * rowHeight + 1);
        prepareLadderGraphics();
    }



    // prepare initial ladder background
    private void prepareLadderGraphics()
    {

        using (Graphics g = Graphics.FromImage(bmp))
        {
            g.Clear(Color.LightGray);
            Pen pen = new Pen(Color.Black);
            pen.Width = 1;

            // drawCells
            for (int i = 0; i < numCols; i++)
            {
                for (int j = 0; j < numRows; j++)
                {
                    Rectangle rect = new Rectangle(i * colWidth, j * rowHeight, colWidth, rowHeight);
                    g.DrawRectangle(pen, rect);
                }
            }
            g.Dispose();
        }

    }


    protected override void OnPaint(System.Windows.Forms.PaintEventArgs pe)
    {
        Graphics fg = this.CreateGraphics();
        lock (fg)
        {
            lock (bmp)
            {
                fg.DrawImage(bmp, 0, 0);
            }
        }
        fg.Dispose();
    }

    public void OrderBookUpdateFn(OrderBookEvent orderBookEvent)
    {



        if (lastprice != orderBookEvent.ValuationAskPrice)
        {
            lastprice = orderBookEvent.ValuationAskPrice;
            lock (bmp)
            {
                using (Graphics g = Graphics.FromImage(bmp))
                {
                    Pen pen = new Pen(Color.Black);
                    pen.Width = 1;
                    for (int i = 0; i < numRows; i++)
                    {
                        Rectangle rect = new Rectangle(colWidth + 1, i*rowHeight + 1, colWidth - 1, rowHeight - 1);
                        g.FillRectangle(new SolidBrush(Color.LightGray), rect);
                        g.DrawString(lastPrice.ToString(), new Font(FontFamily.GenericSansSerif, 10), new SolidBrush(Color.Black), new Point(colWidth + 1, i*rowHeight + 1));
                    }
                    g.Dispose();
                }
                this.Invalidate(new Rectangle(0, 0, 1, 1));
            }
    }
}

Above code works, pretty well, however I am not sure if its correct. This is just for test purpose. Method OrderBookUpdateFn(OrderBookEvent orderBookEvent) gets fired very often (sometimes tens of times a second).

Answer 1

The event handlers all run in the main thread of the application, so it's not possible that a method to handle an event is started before the method that handles the previous event is finished.

If you experience flickering, it's not because of overlapping event handlers.

Answer 2

You could try overriding the OnPaint method, call base.OnPaint(); and then do your manual painting after.

protected override void OnPaint(PaintEventArgs e)
{
   // call base
   base.OnPaint(e);

   // Do your stuff after the rest has been painted
   if(this.picture != null && this.pictureLocation != Point.Empty)
   {
      e.Graphics.DrawImage(this.picture, this.pictureLocation);
   }
}

Answer 3

to avoid flickering of selfpainted controls you can DoubleBuffer the drawing of the form / control. Add this to the constructor:

this.SetStyle(
  ControlStyles.AllPaintingInWmPaint |
  ControlStyles.UserPaint |
  ControlStyles.DoubleBuffer,true);

some anti-flicker techniques