The dispatcher will execute this delegate only when there are no important events to process. Ui responsiveness is more important than number checking. We also pass a new delegate representing the number-checking routine. Public void CheckNextNumber / Reset flag. Text String num 2; if (continuecalculating) ginInvoke( stemIdle, new private bool NotAPrime false; Public Sub CheckNextNumber ' reset flag. Text String End If num 2 If continuecalculating Then stemIdle, new NextPrimeDelegate(AddressOf eckNextNumber) End If End Sub Private notAPrime As boolean false This method checks if the next odd number is prime. If it is prime, the method directly updates the bigPrime textBlock to reflect its discovery.
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Windows Imports ntrols Imports reading Imports reading Namespace sdksamples Partial Public Class mainWindow Inherits Window Public Delegate sub NextPrimeDelegate 'current number to check Private num small As Long 3 Private continuecalculating As boolean false public Sub New w Initializecomponent End Sub Private sub StartOrStop(byval sender. NotAPrime false for i as Long 3 to math. Sqrt(num) If num Mod i 0 Then ' set not a prime flag to true. NotAPrime true exit For End If Next ' if a prime number. If Not NotAPrime Then bigPrime. Text String End If num 2 If continuecalculating Then stemIdle, new NextPrimeDelegate(AddressOf eckNextNumber) End If End Sub Private notAPrime As boolean false End Class End Namespace The following example shows the homework event handler for the button. Private void StartOrStop(object sender, eventArgs e) if (continuecalculating) continuecalculating false; ntent "Resume else continuecalculating true; ntent "Stop ginInvoke( rmal, new private sub StartOrStop(byval sender As Object, byval e as eventArgs) If continuecalculating Then continuecalculating false ntent "Resume" Else continuecalculating True ntent "Stop" rmal, new NextPrimeDelegate(AddressOf. Sometime after this event handler has completed its work, the dispatcher will select this delegate for execution. As we mentioned earlier, beginInvoke is the dispatcher member used to schedule a delegate for execution. In this case, we choose the systemIdle priority.
Let's take a look at the code. The following example shows the xaml that creates the user interface. Window1" xmlns:x"m/winfx/2006/xaml" Title"Prime numbers" Width"260" height"75" stackPanel Orientation"Horizontal" VerticalAlignment"Center" button Content"Start" Click"StartOrStop" Name"startStopButton" Margin"5,0,5,0" / textBlock margin"10,5,0,0" Biggest Prime found: /TextBlock textBlock name"bigPrime" /StackPanel /Window The following example shows the code-behind. Using System; using System. Windows; using ntrols; using reading; using reading; namespace sdksamples public partial class Window1 : Window public delegate void NextPrimeDelegate /Current number to check private long num 3; private bool continuecalculating false; public Window1 : base initializecomponent private void StartOrStop(object sender, eventArgs e) if (continuecalculating) summary continuecalculating. NotAPrime false; for (long i 3; i math. Sqrt(num i) if (num i 0) / Set not a prime flag to true. NotAPrime true; break; / If a prime number. Text String num 2; if (continuecalculating) ginInvoke( stemIdle, new private bool NotAPrime false; imports System Imports System.
The best way to split processing time between calculation and event handling is to manage calculation from the dispatcher. By using the beginInvoke method, we can schedule prime number checks in the same queue that ui events are drawn from. In our example, we schedule only a single prime number check at a time. After the prime number check is complete, we schedule the next check immediately. This check proceeds only after pending ui events have been handled. Microsoft Word accomplishes spell checking using this mechanism. Spell checking is done in the background using the idle time of the ui thread.
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The wpf threading model doesnt allow input to interrupt an operation happening in the ui thread. This means you must be sure to return to the dispatcher periodically to process pending input events before they get stale. Consider the following example: This simple application counts upwards from three, searching for prime numbers. When the user clicks the Start button, the search begins. When the program finds a prime, it updates the user interface with its discovery. At any point, the user can stop the search. Although simple enough, the prime number search could go on forever, which presents some difficulties.
If we handled the entire search inside of the click event handler of the button, we would never give the ui thread a chance to handle other starting events. The ui would be unable to respond to input or process messages. It would never repaint and never respond to button clicks. We could conduct the prime number search in a separate thread, but then we would need to deal with synchronization issues. With a single-threaded approach, we can directly update the label that lists the largest prime found. If we break up the task of calculation into manageable chunks, we can periodically return to the dispatcher and process events. We can give wpf an opportunity to repaint and process input.
Dispatcher of the ui thread. The dispatcher class provides two methods for registering work items: Invoke and BeginInvoke. Both methods schedule a delegate for execution. Invoke is a synchronous call that is, it doesnt return until the ui thread actually finishes executing the delegate. BeginInvoke is asynchronous and returns immediately.
The dispatcher orders the elements in its queue by priority. There are ten levels that may be specified when adding an element to the dispatcher queue. These priorities are maintained in the dispatcherPriority enumeration. Detailed information about DispatcherPriority levels can be found in the windows sdk documentation. Threads in Action: The samples a single-Threaded Application with a long-Running Calculation Most graphical user interfaces (GUIs) spend a large portion of their time idle while waiting for events that are generated in response to user interactions. With careful programming this idle time can be used constructively, without affecting the responsiveness of the.
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DispatcherObject can call its public, verifyAccess method. VerifyAccess examines the, dispatcher associated with the current thread and compares it to the. Dispatcher brief reference stored during construction. If they dont match, verifyAccess throws an exception. VerifyAccess is intended to be called at the beginning of every method belonging. If only one thread can modify the ui, how do background lab threads interact with the user? A background thread can ask the ui thread to perform an operation on its behalf. It does this by registering a work item with the.
Windows does this book to ensure the integrity of ui components. A list box could look strange if its contents were updated by a background thread during painting. Wpf has a built-in mutual exclusion mechanism that enforces this coordination. Most classes in wpf derive from. At construction, a, dispatcherObject stores a reference to the, dispatcher linked to the currently running thread. DispatcherObject associates with the thread that creates. During program execution,.
a large calculation or needs to query a database on some remote server? Usually, the answer is to handle the big operation in a separate thread, leaving the ui thread free to tend to items in the. When the big operation is complete, it can report its result back to the ui thread for display. Historically, windows allows ui elements to be accessed only by the thread that created them. This means that a background thread in charge of some long-running task cannot update a text box when it is finished.
Most applications use a single ui thread, although in some situations it is best to use several. Well discuss this with an example later. The ui thread queues work healthy items inside an object called. The, dispatcher selects work items on a priority basis and runs each one to completion. Every ui thread must have at least one. Dispatcher, and each, dispatcher can execute work items in exactly one thread. The trick to building responsive, user-friendly applications is to maximize the. Dispatcher throughput by keeping the work items small.
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Contributors, windows Presentation foundation (WPF) is designed to save developers from the difficulties of threading. As a result, the majority of wpf developers won't have to write an interface that uses more than one thread. Because multithreaded programs are complex and difficult to debug, they should be avoided when single-threaded solutions exist. No matter how well architected, however, no ui framework will ever be able to provide a single-threaded solution for every sort of problem. Wpf comes close, but there are still situations where multiple threads improve user interface (UI) responsiveness or application performance. After discussing some background material, this paper explores some of these situations and then concludes with a discussion of some lower-level details. Overview and the dispatcher, typically, literature wpf applications start with two threads: one for handling rendering and another for managing the. The rendering thread effectively runs hidden in the background while the ui thread receives input, handles events, paints the screen, and runs application code.