Question

I have a number of classes that do stuff, typically step through a recordset and call a webservice or two for each record.

At the moment this all runs in the GUI thread and hangs painting. First thought was to use a BackgroundWorker and implement a nice progress bar, handle errors, completion etc. All the nice things a Background worker enables.

As soon as the code hit the screen it started to smell. I was writing a lot of the background worker into each class, repeating most of the ProcessRows method in a bw_DoWork method and thinking there should be a better way, and it’s probably already been done.

Before I go ahead and reinvent the wheel is there a pattern or implementation for a class that seperates out the background worker? It would take classes that implement an interface such as ibackgroundable, but the classes could still be run standalone, and would require minimal change to implement the interface.

Edit: A simplified example requested by @Henk:

I have:

    private void buttonUnlockCalls_Click(object sender, EventArgs e)
    {
        UnlockCalls unlockCalls = new UnlockCalls();
        unlockCalls.MaxRowsToProcess = 1000;
        int processedRows = unlockCalls.ProcessRows();
        this.textProcessedRows.text = processedRows.ToString();
    }

I think I want:

    private void buttonUnlockCalls_Click(object sender, EventArgs e)
    {
        UnlockCalls unlockCalls = new UnlockCalls();
        unlockCalls.MaxRowsToProcess = 1000;

        PushToBackground pushToBackground = new PushToBackground(unlockCalls)
        pushToBackground.GetReturnValue = pushToBackground_GetReturnValue;
        pushToBackground.DoWork();
    }

    private void pushToBackground_GetReturnValue(object sender, EventArgs e)
    {
        int processedRows = e.processedRows;
        this.textProcessedRows.text = processedRows.ToString();
    }

I could go ahead and do this, but don’t want to reinvent.

The answer I’m looking for would along the lines of “Yes, Joe did a good implementation of that (here)” or “That’s a Proxy Widget pattern, go read about it (here)”

Answer 1

Each operation needs to implement the following interface:

/// <summary>
/// Allows progress to be monitored on a multi step operation
/// </summary>
interface ISteppedOperation
{
    /// <summary>
    /// Move to the next item to be processed.
    /// </summary>
    /// <returns>False if no more items</returns>
    bool MoveNext();

    /// <summary>
    /// Processes the current item
    /// </summary>
    void ProcessCurrent();

    int StepCount { get; }
    int CurrentStep { get; }
}

This seperates the enumeration of the steps from the processing.

Here is a sample operation:

class SampleOperation : ISteppedOperation
{
    private int maxSteps = 100;

    //// The basic way of doing work that I want to monitor
    //public void DoSteppedWork()
    //{
    //    for (int currentStep = 0; currentStep < maxSteps; currentStep++)
    //    {
    //        System.Threading.Thread.Sleep(100);
    //    }
    //}

    // The same thing broken down to implement ISteppedOperation
    private int currentStep = 0; // before the first step
    public bool MoveNext()
    {
        if (currentStep == maxSteps)
            return false;
        else
        {
            currentStep++;
            return true;
        }
    }

    public void ProcessCurrent()
    {
        System.Threading.Thread.Sleep(100);
    }

    public int StepCount
    {
        get { return maxSteps; }
    }

    public int CurrentStep
    {
        get { return currentStep; }
    }

    // Re-implement the original method so it can still be run synchronously
    public void DoSteppedWork()
    {
        while (MoveNext())
            ProcessCurrent();
    }
}

This can be called from the form like this:

private void BackgroundWorkerButton_Click(object sender, EventArgs eventArgs)
{
    var operation = new SampleOperation();

    BackgroundWorkerButton.Enabled = false;

    BackgroundOperation(operation, (s, e) =>
        {
            BackgroundWorkerButton.Enabled = true;
        });
}

private void BackgroundOperation(ISteppedOperation operation, RunWorkerCompletedEventHandler runWorkerCompleted)
{
    var backgroundWorker = new BackgroundWorker();

    backgroundWorker.RunWorkerCompleted += runWorkerCompleted;
    backgroundWorker.WorkerSupportsCancellation = true;
    backgroundWorker.WorkerReportsProgress = true;

    backgroundWorker.DoWork += new DoWorkEventHandler((s, e) =>
    {
        while (operation.MoveNext())
        {
            operation.ProcessCurrent();

            int percentProgress = (100 * operation.CurrentStep) / operation.StepCount;
            backgroundWorker.ReportProgress(percentProgress);

            if (backgroundWorker.CancellationPending) break;
        }
    });

    backgroundWorker.ProgressChanged += new ProgressChangedEventHandler((s, e) =>
    {
        var progressChangedEventArgs = e as ProgressChangedEventArgs;
        this.progressBar1.Value = progressChangedEventArgs.ProgressPercentage;
    });

    backgroundWorker.RunWorkerAsync();
}

I haven’t done it yet but I’ll be moving BackgroundOperation() into a class of its own and implementing the method to cancel the operation.