如何防止面板内容在滚动时被擦除

Question

I have a question regarding how I can prevent some content drawn on a panel control from being erased when a scroll action brings it out of view.

What I am trying to do is create a 2D tile-map editor. Whenever a mouse click event happens on the panel, a tile should get drawn onto the panel. I have this working fine. But if I place the object on the panel and scroll to one side, and scroll back, the object I had placed is gone.

I have done some research and I have seen suggestions on implementing the paint event. The problem is that I do not understand what to implement here. I think most of my struggles is coming from not fully understanding the Graphics object.

Here is some of my code:

     private void canvas_MouseClick(object sender, MouseEventArgs e)
     {
        Graphics g = canvas.CreateGraphics();

        float x1 = CommonUtils.GetClosestXTile(e.X);
        float y1 = CommonUtils.GetClosestYTile(e.Y);

        if (currentTile != null)
        {
            g.DrawImage(currentTile, x1, y1);
            me.AddTile((int)currX, (int)currY, (int)x1, (int)y1, "C:\\DemoAssets\\tileb.png");
        }
        else
        {
            // dont do anything
        }
        g.Dispose();
    }

    private void canvas_Paint(object sender, PaintEventArgs e)
    {
        // update here?
    }

Answer 1

To hold multiple Tiles, you'd need a List to hold each clicked location along with its associated tile:

    List<Tuple<Image, PointF>> Tiles = new List<Tuple<Image, PointF>>();

    private void canvas_MouseClick(object sender, MouseEventArgs e)
    {
        if (currentTile != null)
        {
            float x1 = CommonUtils.GetClosestXTile(e.X);
            float y1 = CommonUtils.GetClosestYTile(e.Y);
            Tiles.Add(new Tuple<Image, PointF>(currentTile, new PointF(x1, y1)));
            canvas.Refresh();

            me.AddTile((int)currX, (int)currY, (int)x1, (int)y1, "C:\\DemoAssets\\tileb.png");
        }
    }

    private void canvas_Paint(object sender, PaintEventArgs e)
    {
        foreach (Tuple<Image, PointF> tile in Tiles)
        {
            e.Graphics.DrawImage(tile.Item1, tile.Item2);
        }
    }