MSDN Magazine - October 2007 - (Page 69)

Figure 7 Using a BackgroundWorker in WPF BackgroundWorker _backgroundWorker = new BackgroundWorker(); // Set up the Background Worker Events _backgroundWorker.DoWork += _backgroundWorker_DoWork; backgroundWorker.RunWorkerCompleted += _backgroundWorker_RunWorkerCompleted; // Run the Background Worker _backgroundWorker.RunWorkerAsync(5000); // Worker Method void _backgroundWorker_DoWork(object sender, DoWorkEventArgs e) { // Do something } // Completed Method void _backgroundWorker_RunWorkerCompleted( object sender, RunWorkerCompletedEventArgs e) { if (e.Cancelled) { statusText.Text = “Cancelled”; } else if (e.Error != null) { statusText.Text = “Exception Thrown”; } else { statusText.Text = “Completed”; } } Windows Forms 2.0, Microsoft introduced a class for non-UI thread handling to simplify the development model for user interface developers. This class is called the BackgroundWorker. Figure 7 shows typical usage of the BackgroundWorker class. The BackgroundWorker component works well with WPF because underneath the covers it uses the AsyncOperationManager class, which in turn uses the SynchronizationContext class to deal with synchronization. In Windows Forms, the AsyncOperationManager hands off a WindowsFormsSynchronizationContext class that derives from the SynchronizationContext class. Likewise, in ASP.NET it works with a different derivation of SynchronizationContext called AspNetSynchronizationContext. These SynchronizationContextderived classes know how to handle the cross-thread synchronization of method invocation. In WPF, this model is extended with a DispatcherSynchronizationContext class. By using BackgroundWorker, the Dispatcher is being employed automatically to invoke cross-thread method calls. The good news is that since you are probably already familiar with this common pattern, you can continue using BackgroundWorker in your new WPF projects. Figure 8 DispatcherTimer Class in Action // Create a Timer with a Normal Priority _timer = new DispatcherTimer(); // Set the Interval to 2 seconds _timer.Interval = TimeSpan.FromMilliseconds(2000); // Set the callback to just show the time ticking away // NOTE: We are using a control so this has to run on // the UI thread _timer.Tick += new EventHandler(delegate(object s, EventArgs a) { statusText.Text = string.Format( “Timer Ticked: {0}ms”, Environment.TickCount); }); // Start the timer _timer.Start(); .NET Framework Base Class Library (BCL) you will find no fewer than three Timer classes: System.Threading.Timer, System.Timers. Timer, and System.Windows.Forms.Timer. Each of these timers is different. Alex Calvo’s article in MSDN Magazine explains when to use each of these Timer classes (see msdn.microsoft.com/msdnmag/issues/04/02/TimersinNET). DispatcherTimer Periodic execution of code is a common task in development within the Microsoft® .NET Framework, but using timers in .NET has been, at best, confusing. If you look for a Timer class in the WPF Threading References Windows Presentation Foundation Virtual Labs microsoft.com/india/virtuallabs/workflow_full.aspx WPF in the .NET Framework Developer Center For WPF applications, there is a new type of timer that utilizes the Dispatcher—the DispatcherTimer class. Like other timers, the DispatcherTimer class supports specifying an interval between ticks as well as the code to run when the timer’s event fires. You can see a fairly pedestrian use of the DispatcherTimer in Figure 8. Because the DispatcherTimer class is tied to Dispatcher you can also specify the DispatcherPriority as well as which Dispatcher to use. The DispatcherTimer class uses the Normal priority as the current Dispatcher by default, but you can override these values: _timer = new DispatcherTimer( DispatcherPriority.SystemIdle, form1.Dispatcher); msdn2.microsoft.com/en-us/netframework/aa663321.aspx WPF Fundamentals: Threading Model, MSDN Library msdn2.microsoft.com/ms741870 Programming the Windows Presentation Foundation by Chris Sells and Ian Griffiths (O’Reilly, 2005) oreilly.com/catalog/avalon It is well worth all the effort involved in planning your worker processes to enable more responsive applications. Doing some initial research can make that planning more succesful. I suggest exploring some of the sites mentioned in the “WPF Threading References” sidebar before you begin—along with this article, they should give you a good foundation for developing responsive apps. ■ WPF Multithreading october2007 69 http://msdn.microsoft.com/msdnmag/issues/04/02/TimersinNET http://msdn.microsoft.com/msdnmag/issues/04/02/TimersinNET http://microsoft.com/india/virtuallabs/workflow_full.aspx http://msdn2.microsoft.com/en-us/netframework/aa663321.aspx http://msdn2.microsoft.com/ms741870 http://oreilly.com/catalog/avalon

Table of Contents Feed for the Digital Edition of MSDN Magazine - October 2007

Cover Contents Toolbox CLR Inside Out Basic Instincts Data Points Cutting Edge Pooled Threads WPF Threads Parallel Linq Parallel Performance Mobile Apps Test Run Foundations Windows with C++ Netting C++ .NET Matters { End Bracket } Net Nuptials

MSDN Magazine - October 2007

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