Using the ThreadPool to Run Animated Gifs in C# and .NET

ImageAnimator Static Methods	Description
Animate	Sets up the image and event handler for updating the frame of the animated gif
UpdateFrames	Move to the next GIF

Table 2 - ImageAnimator Class Methods

The RunAnimation thread delegate is shown below. From the code, you can see the initial call to Animate. The image is created from the object passed into the thread which is unboxed to get the name of the image. This string is also used to locate the array position of the image because the file names are conveniently named gif1, gif2, and gif3 in this example:

Listing 2 - Initial code for starting the Animated Gif file

void RunAnimation(object filename)
{
// Image is created from the filename
Image theImage = Image.FromFile((string)filename);
// the image is set up for animation using the
// ImageAnimator class and an event handler
ImageAnimator.Animate(theImage, newEventHandler(this.OnFrameChanged));
// we also need to parse the image name to
// determine the position of the gif in the array
string name = (string)filename;
int periodPosition = name.LastIndexOf(".");
periodPosition--; // get number
int index =Convert.ToInt32(name[periodPosition].ToString());
Gifs[index-1] = theImage;
}

OnFrameChanged is called each time the UpdateFrame is called for the image. Inside OnFrameChanged we call invalidate to force a repaint of the gif and a subsequent call to UpdateFrame:

Listing 3 - Event Handler for UpdateFrame event of the Animated Gif

private void OnFrameChanged(object o, EventArgs e)
{
this.Invalidate();// force a repaint of the gif
}

All of the painting of the Gifs are handled in the OnPaint event. Each thread will trigger the OnPaint event through the OnFrameChanged Event which is also contained in the individual threads. The Paint Event Handler, shown below, does the painting of the images for all three threads:

Listing 4 - Painting the Gif Images on the Form

private void Form1_Paint(object sender,System.Windows.Forms.PaintEventArgs e)
{
//Get the next frame ready for rendering for the image in this thread.
ImageAnimator.UpdateFrames();
Graphics g = e.Graphics;
//Draw the current frame in the animation for all of the images
for (int i = 0; i < 3; i++)
{
if (Gifs[i] != null)
g.DrawImage(Gifs[i], (i%2)*150, (i/2)*150, Gifs[i].Width, Gifs[i].Height);
}
// if AutoReset event was triggered, paint the red circle with the HI
if (AllowHi)
{
g.FillEllipse(Brushes.Red, 200, 175, 50, 50);
g.DrawString(CircleString, TheFont, Brushes.Black,215, 190, new StringFormat());
}
}

Youll also note in the listing above the AllowHi boolean flag. This flag is set in the callback after WaitForSingleObject callback is triggered (either by setting the signal of the AutoResetEvent or by timing out after 20 seconds). If you click on the Trigger Hi button on the form, it signals the AutoResetEvent (by calling Set) which in turn triggers the WaitForSingleObject callback.

The callback delegate passed by the WaitForSingleObject method of the ThreadPool is shown below:

void WaitThreadFunc(object test, bool b)
{
AllowHi = true;
CircleString = (string)test;
}

Notice that you can also pass an object into the callback from the WaitForSingleObject function of the ThreadPool. In this case a string with the message displayed in the red circle is passed into the method.

The behavior the combined code exhibits when running the example is the following.

1. If the button is pressed, the red circle with the Hi pops up immediately.
2. If the button is not pressed, the red circle with the Hi will show up anyway after 20 seconds (the timeout period).

Conclusion

Thread pools are a no-brainer way to implement thread management and the ThreadPool class gives us the tools we need to create and manage threads. The gifs were brought to you courtesy of a cool artist site I found while searching for animated gifs to use to create this article. The next step, you may have guessed, is to thread the animated gifs in ASP.NET.

References

• The Complete Visual C# Programmer's Guide Authors of C# Corner
• Animated Gif Artists Guild