Question

In my WPF app i need to load some images. I only need to display one image at a time. If i load the image when it’s needed, there is a slightly delay. So i thought to myself: “Hey, why not do some preloading in a background thread? Can’t be that hard.” I have some experience with threads, but not enough to know that this thought was wrong. I started programming and run into some problems. I fixed some of the problems and i probably could fix the other problems too, but that would result in spaghetti code. So, I think starting from the scratch would be the best. What initial planing is needed to build a nice and little preloading thread? Is there a pattern or something like that?

Here’s my current setup:

• LinkedList<string> to stores pathes to the pictures and navigate to the next picture
• Dictionary<string, BitmapImage> to store the preloaded images

Answer 1

I’d use something like this:

class ImageManager
{
  private Dictionary<string, Image> images=
    new Dictionary<string,Image>();

  public Image get(string s) {  // blocking call, returns the image
    return load(s);
  }

  private Image load(string s) {  // internal, thread-safe helper
    lock(images) {
      if(!images.ContainsKey(s)) {
        Image img=// load the image s
        images.Add(s,img);
        return img; 
      }
      return images[s];
    }
  }

  public void preload(params string[] imgs) {  // non-blocking preloading call
    foreach(string img in imgs) { 
      BackgroundWorker bw=new BackgroundWorker();
      bw.DoWork+=(s,e)=>{ load(img); }  // discard the actual image return
      bw.RunWorkerAsync();
    }
  }
}

// in your main function
{
   ImageManager im=new ImageManager();
   im.preload("path1", "path2", "path3", "path4"); // non-blocking call

   // then you just request images based on their path
   // they'll become available as they are loaded
   // or if you request an image before it's queued to be loaded asynchronously 
   // it will get loaded synchronously instead, thus with priority because it's needed
}