Threading and ArcGIS

Question

Hi!

I have just stumbled across the Backgroundworker object, and it seems like the tool I'm looking for to make my GUI responding while performing calculations. I am writing IO plugins for ArcGIS.

I am doing some data processing outside ArcGIS, which works fine using the backgroundworker. But when I'm inserting the data into ArcGIS, the backgroundworker seems to increase the duration time by a factor 9 or so. Placing the processing code outside the DoWork method, increases the performance by a factor 9.

I have read about this several places on the net, but I have no experience in multithreaded programming and the terms like STA and MTA means nothing to me. link text I have also tried to use a simple implementation of threading, but with similar results.

Does anyone know what I can do to be able to use ArcGIS processing and maintaining a responsive GUI?

EDIT: I have included a sample of my interaction with the background worker. If I put the code located in the StartImporting method in the cmdStart_Click method, it executes much faster.

private void StartImporting(object sender, DoWorkEventArgs e)
{
    DateTime BeginTime = DateTime.Now;
    // Create a new report object.
    SKLoggingObject loggingObject = new SKLoggingObject("log.txt");
    loggingObject.Start("Testing.");

    SKImport skImporter = new SKImport(loggingObject);
    try
    {
        // Read from a text box - no writing.
    skImporter.Open(txtInputFile.Text);
    }
    catch
    {
    }
    SKGeometryCollection convertedCollection = null;

    // Create a converter object.
    GEN_SK2ArcGIS converter = new GEN_SK2ArcGIS(loggingObject);

    // Convert the data.
    convertedCollection = converter.Convert(skImporter.GetGeometry());

    // Create a new exporter.
    ArcGISExport arcgisExporter = new ArcGISExport(loggingObject);

    // Open the file.            
    // Read from a text box - no writing.
    arcgisExporter.Open(txtOutputFile.Text);

    // Insert the geometry collection.
    try
    {
    arcgisExporter.Insert(convertedCollection);
    }
    catch
    {
    }
    TimeSpan totalTime = DateTime.Now - BeginTime;
    lblStatus.Text = "Done...";

}

private void ChangeProgress(object sender, ProgressChangedEventArgs e) 
{
    // If any message was passed, display it.
    if (e.UserState != null && !((string)e.UserState).Equals(""))
    {
    lblStatus.Text = (string)e.UserState;
    }
    // Update the progress bar.
    pgStatus.Value = e.ProgressPercentage;
}

private void ImportDone(object sender, RunWorkerCompletedEventArgs e)
{
    // If the process was cancelled, note this.
    if (e.Cancelled)
    {
    pgStatus.Value = 0;
    lblStatus.Text = "Operation was aborted by user...";
    }
    else
    {
    }

}

private void cmdStart_Click(object sender, EventArgs e)
{
    // Begin importing the sk file to the geometry collection.

    // Initialise worker.
    bgWorker = new BackgroundWorker();
    bgWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(ImportDone);
    bgWorker.ProgressChanged += new ProgressChangedEventHandler(ChangeProgress);
    bgWorker.DoWork += new DoWorkEventHandler(StartImporting);
    bgWorker.WorkerReportsProgress = true;
    bgWorker.WorkerSupportsCancellation = true;

    // Start worker.
    bgWorker.RunWorkerAsync();

}

private void cmdCancel_Click(object sender, EventArgs e)
{
    bgWorker.CancelAsync();
}

Kind Regards, Casper

Answer 1

Typically in order to maintain a responsive GUI you will want to execute your code that does the work in a different thread. This is made very easy with .net using the BeginInvoke method: http://msdn.microsoft.com/en-us/library/aa334867(VS.71).aspx

In a nutshell, include all the non GUI code in a separate class (or classes) and rather than calling each metho directly you create a delegate and call the BeginInvoke method on that. The method will then go off and do it's thing with no further interaction with the GUI. If you wish to have it update the GUI (e.g. a progress bar) then you can raise events from the class and catch them from the GUI however you will need to ensure that controls are updated in a thread safe fashion. If you want the GUI to update when the method completes then you can use the EndInvoke method to handle that

Answer 2

I have continued trying to find a solution, and the following is what I ended up doing. The code is cut and paste from various files and presented to give an idea of what I did. It demonstrates how I can call methods which are communicating with ArcGIS using a thread. The code allows me to update the GUI in the main thread, abort the operation, and do post-operation stuff. I ended up using the first threading part from the link I posted initially.

The reason for the initial loss of performance is probably due to the single-threaded apartment (STA) which is required by ArcGIS. The Backgroundworker seems to be MTA, thus not appropriate for working with ArcGIS

Well here it goes, I hope I haven't forgotten anything, and feel very free to edit my solution. It will both help me and probably also other people developing stuff for ArcGIS.

public class Program
{
    private volatile bool AbortOperation;
    Func<bool> AbortOperationDelegate;
    FinishProcessDelegate finishDelegate;
    UpdateGUIDelegate updateGUIDelegate;

    private delegate void UpdateGUIDelegate(int progress, object message);
    private delegate void FinishProcessDelegate();

    private void cmdBegin_Click(...)
    {
        // Create finish delegate, for determining when the thread is done.
        finishDelegate = new FinishProcessDelegate(ProcessFinished);
        // A delegate for updating the GUI.
        updateGUIDelegate = new UpdateGUIDelegate(UpdateGUI);
        // Create a delegate function for abortion.
        AbortOperationDelegate = () => AbortOperation;

        Thread BackgroundThread = new Thread(new ThreadStart(StartProcess));            
        // Force single apartment state. Required by ArcGIS.
        BackgroundThread.SetApartmentState(ApartmentState.STA);
        BackgroundThread.Start();
    }

    private void StartProcess()
    {    
        // Update GUI.
        updateGUIDelegate(0, "Beginning process...");

        // Create object.
        Converter converter = new Converter(AbortOperationDelegate);
        // Parse the GUI update method to the converter, so it can update the GUI from within the converter. 
        converter.Progress += new ProcessEventHandler(UpdateGUI);
        // Begin converting.
        converter.Execute();

        // Tell the main thread, that the process has finished.
        FinishProcessDelegate finishDelegate = new FinishProcessDelegate(ProcessFinished);
        Invoke(finishDelegate);

        // Update GUI.
        updateGUIDelegate(100, "Process has finished.");
    }

    private void cmdAbort_Click(...)
    {
        AbortOperation = true;
    }

    private void ProcessFinished()
    {
        // Post processing.
    }

    private void UpdateGUI(int progress, object message)
    {
        // If the call has been placed at the local thread, call it on the main thread.
        if (this.pgStatus.InvokeRequired)
        {
            UpdateGUIDelegate guidelegate = new UpdateGUIDelegate(UpdateGUI);
            this.Invoke(guidelegate, new object[] { progress, message });
        }
        else
        {
            // The call was made on the main thread, update the GUI.
            pgStatus.Value = progress;
            lblStatus.Text = (string)message;   
        }
    }
}

public class Converter
{
    private Func<bool> AbortOperation { get; set;}

    public Converter(Func<bool> abortOperation)
    {
        AbortOperation = abortOperation;
    }

    public void Execute()
    {
        // Calculations using ArcGIS are done here.
        while(...) // Insert your own criteria here.
        {
            // Update GUI, and replace the '...' with the progress.
            OnProgressChange(new ProgressEventArgs(..., "Still working..."));

            // Check for abortion at anytime here...
            if(AbortOperation)
            {
                return;
            }
        }
    }

    public event ProgressEventHandler Progress;
    private virtual void OnProgressChange(ProgressEventArgs e)
    {
        var p = Progress;
        if (p != null)
        {
            // Invoke the delegate. 
        p(e.Progress, e.Message);
        }
    }    
}

public class ProgressEventArgs : EventArgs
{
    public int Progress { get; set; }
    public string Message { get; set; }
    public ProgressEventArgs(int _progress, string _message)
    {
        Progress = _progress;
        Message = _message;
    }
}

public delegate void ProgressEventHandler(int percentProgress, object userState);

Answer 3

It is correct that you should use STA threads when working with the COM objects in ArcGIS. Still, you can get the convenience of the BackgroundWorker, which always is an MTA thread from the system's thread pool.

private static void OnBackgroundWorkerDoWork(object sender, DoWorkEventArgs e)
{
    BackgroundWorker worker = (BackgroundWorker)sender;
    ToolToStart tool = e.Argument as ToolToStart;

    if (tool != null)
    {
        tool.BackgroundWorker = worker;

        // The background worker thread is an MTA thread, 
        // and should not operate on ArcObjects/COM types.
        // Instead we create an STA thread to run the tool in.
        // When the the tool finishes the infomation from the STA thread 
        // is transferred to the background worker's event arguments.
        Thread toolThread = new Thread(STAThreadStart);
        toolThread.SetApartmentState(ApartmentState.STA);
        toolThread.Start(tool);

        toolThread.Join();
        e.Cancel = m_ToolCanceled;
        e.Result = m_ToolResult;
    }
}

The STA thread can now use the BackgroundWorker's methods, such as reporting progress, checking for cancellation and reporting results.

protected virtual void StatusUpdateNotify(ProgressState progressState)
{
    if (BackgroundWorker.CancellationPending)
    {
        throw new OperationCanceledException();
    }

    BackgroundWorker.ReportProgress(progressState.Progress, progressState);
}

In addition to only using STA threads when operating on ArcGIS objects, you should not share objects between two threds. From your code it seems like you access the GUI from the background worker: lblStatus.Text = "Done...";, which could be done in e.g. the delegate for RunWorkerComplete.