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// Wait five seconds, then fail if (mrevWaitOne(5000, false) && rihrRapiInit == RapiS_OK) { // Notify caller that connection is established itReason = INVOKE_STARTUPSTARTUP_SUCCESS; m_ctlInvokeTargetInvoke(m_deleCallback); } else { // On failure, disconnect from RAPI RapiCeRapiUninit(); strBuffer = "Timeout - no device present"; itReason = INVOKE_STARTUPSTATUS_MESSAGE; m_ctlInvokeTargetInvoke(m_deleCallback); // Notify caller that connection failed itReason = INVOKE_STARTUPSTARTUP_FAILED; m_ctlInvokeTargetInvoke(m_deleCallback); } // Trigger that thread has ended m_thrd = null; } } // class StartupThread } // namespace FindPrograms
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The StartupThread class provides a wrapper around the startup thread We put this class into its own file to make it easy for you to reuse This class uses the Invoke function to communicate from the background thread to the user interface thread This class depends on having a control (or a form) and on that control having a delegate to receive the interthread calls Our startup wrapper has three methods: a constructor, Run, and ThreadMainStartup The ThreadMainStartup method is the thread entry point, and it does the actual work of calling the CeRapiInitEx function to initiate the RAPI connection and calling the CeRapiUninit function if the attempt to connect fails A background thread allows the user interface thread to be available to receive user input A downside to multithreaded programming is that the two threads need to be carefully crafted to avoid conflicts, which are usually very hard to reproduce because they are often related to timing and thus are hard to find Our approach to this problem is for each thread to run in a separate class and not interact with others, with two exceptions: thread startup code and thread-safe communication code In this code,
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The Remote API
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there is a one-way communication from the worker thread to the main thread The communication is handled by the only function in the Control class guaranteed to be thread-safe: ControlInvoke
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Why ControlInvoke Is Thread-Safe
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The ControlInvoke method is thread-safe (in both the desktop NET Framework and the NET Compact Framework) because it relies on an underlying Win32 feature that is guaranteed to be thread-safe That feature is the SendMessage function, a Win32 function that has been around since the first version of Windows This function sends a Win32 message to a window, waiting for the message to be delivered before returning If the recipient is busy with another message, the SendMessage function blocks until the previous message has been handled Over the years, this function has been the backbone for interthread and interprocess communication in a variety of Windows technologies, including Dynamic Data Exchange (DDE), Object Linking and Embedding (OLE), and Component Object Model (COM) It is not surprising, then, that this time-tested mechanism is also used by the NET Control class
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The desktop NET Framework provides two overloads for the Invoke method:
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public object Invoke(Delegate); public virtual object Invoke(Delegate, object[]);
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Each overload accepts a parameter of type Delegate (similar to a C function pointer) The second overload, which accepts an array of values to be used as parameters to the called function, is the version most often used in the desktop NET Framework The NET Compact Framework, however, supports only the first type of overload, and because this is a NET Compact Framework book we are going to resist the temptation to do things the desktop NET Framework way Here is a call to the Invoke method to notify the main thread that a successful RAPI connection has been established:
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// Notify caller that connection is established itReason = INVOKE_STARTUPSTARTUP_SUCCESS; ctlInvokeTargetInvoke(deleCallback);
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The recipient of this function call is a function named StartupCallback, a member of the MainForm class in our FindPrograms sample We cannot just pass a function pointer like we do in unmanaged C, however Instead, we pass the NET equivalent, a delegate of type EventHandler Here is the declaration for the delegate, along with the other declarations needed to support the startup thread:
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// Startup thread definitions StartupThread thrdStartup = null; private EventHandler deleStartup; private bool bRapiConnected = false;
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We initialize the delegate as follows:
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deleStartup = new EventHandler(thisStartupCallback);
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and then pass it to the constructor for our startup thread wrapper object:
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// Create thread to connect to RAPI thrdStartup = new StartupThread(this, deleStartup); if (!thrdStartupRun()) thrdStartup = null;
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After creating the StartupThread object, we call the Run member to start it This two-step approach create the object, then initialize the object may be familiar to C++ programmers from other object-oriented APIs A two-step approach works because a constructor cannot easily provide a return value, nor can it report a reason when it fails A stand-alone initialization function Run in this example can do both Our interthread delegate function, StartupCallback, takes two parameters: sender and e The parameters have names that suggest they might be useful, but what each name suggests does not quite match with what each parameter actually contains The first, sender, identifies the recipient of the function call the control that is receiving the Invoke call and not the sender as the name implies The second parameter, e, is of type EventArgs Because the first parameter gives us no useful information, you might suppose that the second parameter which seems to want to contain event arguments would yield a useful hint or two And while EventArgs is the base class for every other event argument class, this type contains no event data So, where does that leave us With two parameters that are essentially empty
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