Have BackgroundWorker wait for UI thread to show image

Question

Ok so I have a WPF application which on button press runs through a list of tests like this:

1. RunTest button pressed in GUI
2. BackgroundWorker is created and starts running DoWork (see below):

3. When t.Name == "TestSpecial" I want to show an image in the UI before I proceed. It's crucial that the image is showing before I continue.

   DoWork(...)
   {
       List<Test> tests = ....
   
       for (Test t in tests)
       {
           switch(t.Name)
           {
               case "Test1":
                   //Do something
                   break;
               case "Test2":
                   //Do something else 
                   break;
               ...
               case "TestSpecial":
                   // Here I want to:
                   // 1. Tell UI to show an image on the screen
                   // 2. Wait for the image to show up
                   // 3. Continue with my test
                   break;
               case "TestN+1":
                   //Do something completly different
                   break;
           }
       }
   }
   

I could probably use the Dispatcher.InvokeAsync or similar to get the image to show but is there any way to wait for it to finish or in some other way garantuee that the image is showing before proceeding?

Answer 1

I think that if you use BackgroundWorker as its shown here http://msdn.microsoft.com/en-us/library/cc221403%28v=vs.95%29.aspx with ProgressChanged event it will do the trick.

EDIT: Here is code from that msdn article aplied on your case

//create worker
BackgroundWorker bw = new BackgroundWorker();

//set progress to true
bw.WorkerReportsProgress = true; 

// handle events
bw.DoWork += new DoWorkEventHandler(DoWork);
bw.ProgressChanged += new ProgressChangedEventHandler(bw_ProgressChanged);

//method for work
private DoWork(object sender, DoWorkEventArgs e)
{

BackgroundWorker worker = sender as BackgroundWorker;
List<Test> tests = ....

for (Test t in tests)
{
    switch(t.Name)
    {
        case "Test1":
            //Do something
            break;
        case "Test2":
            //Do something else 
            break;
        ...
        case "TestSpecial":
            // Here I want to:
            // 1. Tell UI to show an image on the screen
            // 2. Wait for the image to show up
            // 3. Continue with my test
            worker.ReportProgress(1);
            break;
        case "TestN+1":
            //Do something completly different
            break;
    }
}
} 

//method for progress
private void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    //set your image here
}

Answer 2

I think that poor old @TomKos was onto the answer... if only he'd have explained himself better.

First, please take a look at my example implementation of the BackgroundWorker class in the Progress Bar update from Background worker stalling post here on StackOverflow.

Now, in the DoWork method, please notice the call to backgroundWorker.ReportProgress(i);. This passes a number back to the UI thread that represents the percentage of the work that is complete.

You could implement something like this and pass a particular value back to the UI when you want to display your image. At some stage in your DoWork method, you could pass the value back like this:

private void DoWork(object sender, DoWorkEventArgs e)
{
    // Do some long running process here
    backgroundWorker.ReportProgress(1); // Send signal to show the image
    // Maybe do some more long running process here
}

You would handle it in the UI something like this:

private void ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    if (e.ProgressPercentage == 1)
    {
        // Show your image
    }
}

Answer 3

100% of the time there will be a delay from when you tell the UI to show the image and when the image is on the screen. Gamers spends lots of money on graphics cards for a reason.

Loaded is the last event.
The Loaded event is raised before the final rendering, but after the layout system has calculated all necessary values for rendering.

There is no Rendered event (that I am aware of).

I would try a delay in the Loaded event.
100,000 is not that many.
Even 1/10 of a second delay would only be few hours.