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One of the key features that the asynchronous results pattern offers is that the caller determines whether to call a method asynchronously The called object may choose to provide asynchronous methods explicitly, perhaps even using the asynchronous results pattern internally However, this is not a requirement for the caller to make an asynchronous call Called methods may choose to provide asynchronous APIs explicitly when the called class can implement the asynchronous functionality more efficiently than the caller can, or when it is determined that asynchronous method calls are likely, such as with methods that are relatively slow If it is determined that a method should explicitly provide an asynchronous calling pattern, it is a good practice for the API to follow the asynchronous results design pattern An explicit implementation, therefore, should include methods that correspond to BeginInvoke() , along with events that callers can subscribe to in order to be notified when a thread completes For example, in NET 20, the SystemNetWebClient class includes asynchronous method calls for downloading files To begin the download, it includes the DownloadFileAsync() method Additionally, the caller can register for notification when the download is complete using the DownloadFileCompleted event The NET 20 implementation of SystemNetWebClient also includes a DownloadProgressChanged event for publishing when a download operation successfully transfers some of its data, as well as a CancelAsync() method to discontinue the download These are not explicitly part of the asynchronous results pattern, and therefore, they are not available unless provided by the called class explicitly These methods are another reason why programmers may explicitly implement asynchronous APIs, instead of just relying on the caller to use the asynchronous results pattern To help with such methods, NET 20 includes the SystemComponentModelBackgroundWorker class Although not supplied by SystemNetWebClient, a method corresponding to EndInvoke() is also a nice addition for explicitly implemented asynchronous calls The asynchronous results pattern relies on the built-in thread pool that provides support for reusing threads rather than always creating new ones The pool has a number of threads, dependent on the number of processors, and the thread pool will wait for a free thread before servicing the request If a request for a new thread is needed but the creation of a new thread would undermine the value of multithreading (because the cost of an additional thread would outweigh its benefit), the thread pool won't allocate a thread until another thread is released back to the thread pool
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Frequently with multithreaded operations, you not only want to be notified when the thread completes, but you also want the method to provide an update on the status of the operation Often, users want to be able to cancel long-running tasks The NET Framework 20 includes a BackgroundWorker class for programming this type of pattern Listing 168 is an example of this pattern It calculates pi to the number of digits specified
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Listing 168 Using the Background Worker Pattern
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System; SystemThreading; SystemComponentModel; SystemText;
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public class PiCalculator { public static BackgroundWorker calculationWorker = new BackgroundWorker(); public static AutoResetEvent resetEvent = new AutoResetEvent(false); public static void Main() { int digitCount; ConsoleWrite( "Enter the number of digits to calculate:"); if (intTryParse(ConsoleReadLine(), out digitCount)) { ConsoleWriteLine("ENTER to cancel"); // C# 20 Syntax for registering delegates calculationWorkerDoWork += CalculatePi; // Register the ProgressChanged callback calculationWorkerProgressChanged+= UpdateDisplayWithMoreDigits; calculationWorkerWorkerReportsProgress = true; // Register a callback for when the // calculation completes calculationWorkerRunWorkerCompleted += new RunWorkerCompletedEventHandler(Complete); calculationWorkerWorkerSupportsCancellation = true; // Begin calculating pi for up to digitCount digits calculationWorkerRunWorkerAsync(digitCount); ConsoleReadLine(); // If cancel is called after the calculation // has completed it doesn't matter calculationWorkerCancelAsync(); // Wait for Complete() to run
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resetEventWaitOne(); } else { ConsoleWriteLine( "The value entered is an invalid integer"); } }
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private static void CalculatePi( object sender, DoWorkEventArgs eventArgs) { int digits = (int)eventArgsArgument; StringBuilder pi = new StringBuilder("3", digits + 2); calculationWorkerReportProgress(0, piToString()); // Calculate rest of pi, if required if (digits > 0) { for (int i = 0; i < digits; i += 9) {
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// Calculate next i decimal places int nextDigit = PiDigitCalculatorStartingAt( i + 1); int digitCount = MathMin(digits - i, 9); string ds = stringFormat("{0:D9}", nextDigit); piAppend(dsSubstring(0, digitCount)); // Show current progress calculationWorkerReportProgress( 0, dsSubstring(0, digitCount)); // Check for cancellation if (calculationWorkerCancellationPending) { // Need to set Cancel if you need to // distinguish how a worker thread completed // ie, by checking // RunWorkerCompletedEventArgsCancelled eventArgsCancel = true; break; } } } eventArgsResult = piToString(); } private static void UpdateDisplayWithMoreDigits( object sender, ProgressChangedEventArgs eventArgs) { string digits = (string)eventArgsUserState; ConsoleWrite(digits); }
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static void Complete( object sender, RunWorkerCompletedEventArgs eventArgs) { // } } --------------------------------------------------------------------------------------------------------------------------------------------------------public class PiDigitCalculator { // }
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