c# - Show Transparent Loading Spinner above other Controls

Question

I am working in a spinner control. I want the control to support transparent backcolor. When the arc is drawn, there is a blank space in the middle, I want that space to be truly transparent, so that I could put another control behind it and it would not be covered by the spinner.

I tried overriding the CreateParams void.
Also I set the style to support TransparentColor.
Tried overriding OnPaintBackground void, but I cannot achieve the real transparent backcolor.

So, what can you suggest me to do?

Answer 1

To make a transparent layer, you should override painting of the control and draw the control in this order, First draw all controls in the same container which are under your control (based on z-index) on a bitmap. Then draw that bitmap on graphics of your control. At last draw content of your control. Also the BackColor of your control should be Color.Transparent.

Also as another option to make transparent layer, you can exclude some regions from your control when drawing.

In the following samples I used the first technique and created 2 controls. A spinning circles transparent control. and a transparent picturebox control.

In both samples I used a delay between loading rows to showing a spinner make sense.

Sample 1 - Using a SpinningCircles Control

SpinningCircles control draws circles and supports transparency. The control doesn't animate at design-time, but it animates at run-time. Also it doesn't consume resources when it is not visible.

Sample 2 - Using a TransparentPictureBox Control and a transparent animated gif TransparentPictureBox control supports transparency, so I used an animated gif as its image and as you can see, the gif is showing correctly.

Sample 1 Code - SpinningCircles

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Linq;
using System.Windows.Forms;
public class SpinningCircles : Control
{
    int increment = 1;
    int radius = 4;
    int n = 8;
    int next = 0;
    Timer timer;
    public SpinningCircles()
    {
        timer = new Timer();
        this.Size = new Size(100, 100);
        timer.Tick += (s, e) => this.Invalidate();
        if (!DesignMode)
            timer.Enabled = true;
        SetStyle(ControlStyles.AllPaintingInWmPaint |
                 ControlStyles.OptimizedDoubleBuffer |
                 ControlStyles.ResizeRedraw | ControlStyles.UserPaint |
                 ControlStyles.SupportsTransparentBackColor, true);
        BackColor = Color.Transparent;
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        if (Parent != null && this.BackColor == Color.Transparent)
        {
            using (var bmp = new Bitmap(Parent.Width, Parent.Height))
            {
                Parent.Controls.Cast<Control>()
                      .Where(c => Parent.Controls.GetChildIndex(c) > Parent.Controls.GetChildIndex(this))
                      .Where(c => c.Bounds.IntersectsWith(this.Bounds))
                      .OrderByDescending(c => Parent.Controls.GetChildIndex(c))
                      .ToList()
                      .ForEach(c => c.DrawToBitmap(bmp, c.Bounds));

                e.Graphics.DrawImage(bmp, -Left, -Top);
            }
        }
        e.Graphics.SmoothingMode = SmoothingMode.HighQuality;
        int length = Math.Min(Width, Height);
        PointF center = new PointF(length / 2, length / 2);
        int bigRadius = length / 2 - radius - (n - 1) * increment;
        float unitAngle = 360 / n;
        if (!DesignMode)
            next++;
        next = next >= n ? 0 : next;
        int a = 0;
        for (int i = next; i < next + n; i++)
        {
            int factor = i % n;
            float c1X = center.X + (float)(bigRadius * Math.Cos(unitAngle * factor * Math.PI / 180));
            float c1Y = center.Y + (float)(bigRadius * Math.Sin(unitAngle * factor * Math.PI / 180));
            int currRad = radius + a * increment;
            PointF c1 = new PointF(c1X - currRad, c1Y - currRad);
            e.Graphics.FillEllipse(Brushes.Black, c1.X, c1.Y, 2 * currRad, 2 * currRad);
            using (Pen pen = new Pen(Color.White, 2))
                e.Graphics.DrawEllipse(pen, c1.X, c1.Y, 2 * currRad, 2 * currRad);
            a++;
        }
    }
    protected override void OnVisibleChanged(EventArgs e)
    {
        timer.Enabled = Visible;
        base.OnVisibleChanged(e);
    }
}

Sample 2 Code - TransparentPictureBox Code

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Linq;
using System.Windows.Forms;
class TransparentPictureBox : PictureBox
{
    public TransparentPictureBox()
    {
        this.BackColor = Color.Transparent;
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        if (Parent != null && this.BackColor == Color.Transparent)
        {
            using (var bmp = new Bitmap(Parent.Width, Parent.Height))
            {
                Parent.Controls.Cast<Control>()
                      .Where(c => Parent.Controls.GetChildIndex(c) > Parent.Controls.GetChildIndex(this))
                      .Where(c => c.Bounds.IntersectsWith(this.Bounds))
                      .OrderByDescending(c => Parent.Controls.GetChildIndex(c))
                      .ToList()
                      .ForEach(c => c.DrawToBitmap(bmp, c.Bounds));

                e.Graphics.DrawImage(bmp, -Left, -Top);
            }
        }
        base.OnPaint(e);
    }
}