RAPHICAL USER INTERFACES (CUIs) are of special interest to devel in Visual Studio .NET

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In this chapter, we'll review the GUI threading architecture broadly and then look at specifically how it is surfaced in Windows Forms and the Win dows Presentation Foundation (WPF) We'll also look at the mechanisms available for building responsive GUIs, including the NET Syn c h ron i z a t ionCont ext, which unifies GUI threading models on NET, and the Asyn c Ope rationMa n a g e r, which builds atop the Syn c h ro n i zat ionContext feature to simplify building higher-level services Asynchronous patterns like the event driven asynchronous programming model reviewed in 8, Asynchronous Programming Models, commonly use these features in their implementation
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GUI architectures on Windows have remained fairly consistent for the past two decades Although there are differences in the details-and in the capabilities and style of programming-USER32, Windows Forms, and WPF all use the same general architecture for reacting to user input and repainting the screen That architecture can be summed up as "single threaded and message based " Just a few lines of Petzold style code can be used to succinctly illustrate it
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This is called the message loop or, alternatively, the message pump We already had some exposure to this concept during the course of discussing functions like MsgWa it F o rMul t i p leObj e ct s E x in 5, Windows Kernel Synchronization Notice, however, that this loop is sequential One GetMe s s age call happens after the other To understand the message loop, you need to first understand how GUIs on Windows work Figure 1 6 1 illustrates the basic architecture Each thread that creates at least one window has a message queue, and it is this thread's job to process messages from the queue The thread is silently
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There is another aspect to the GUI architecture that is interesting, relevant, and somewhat unfortunate Any code that directly manipulates GUI elements must execute on the GUI thread Given that we already estab lished the GUI thread's sole purpose is to process messages, repaint the screen, and the like, you might wonder how this is even possible The answer is that such code runs inside of event handlers that are invoked in response to GUI events By invoking event handlers on the GUI thread, some complex issues are avoided-such as requiring developers to acquire locks while updating the GUI-in an attempt to provide a more convenient programming model Additionally, if events could be processed entirely asynchronously, strange glitches could occur due to interleaving multiple handlers and / or the framework deciding to repaint while a handler was in progress Given a window handle (HWND), you can easily find out the identity of its special thread
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Why is this design choice an issue Anything the GUI thread does in addition to dequeuing and dispatching messages from its queue prevents it from processing additional messages If the thread is running a user sup plied event handler and that event handler does some lengthy operation (such as a network I / O), subsequent messages will get clogged in the queue waiting for the GUI thread to return to its message loop The fact that code inside of event handlers automatically runs on the GUI thread leads developers down this path by default, often without knowing it Let's take a simple example In response to a button click, let's say that your application fires off a network request to download a file It does this on the GUI thread Now imagine that this could take some time, maybe 1 second The application is frozen and cannot repaint for 1 second If the user of your application tries to resize, close, or maximize the window, for exam ple, they will see visual artifacts (such as a blank white screen) because the GUI thread can't retrieve those messages and properly repaint But 1 second is a fairly brief delay; it will be slightly annoying, but not terrible But now imagine that the network connection drops out, and instead of 1 second, we must wait for a 30 second network timeout to occur What an awful user
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