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Because neither parameter is useful, we structure our code so that this function has only one possible caller, the thread wrapper object, thrdStartup The recipient of the call then accesses that caller s public data to receive any parameters needed for the interthread function call and responds accordingly Because of the way the Invoke method works, our background thread is blocked while the main thread runs the Invoke target function In this way, our code is thread-safe Listing 93 shows the code for the StartupCallback function, which receives an interthread call courtesy of the Invoke method in the Control class
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Listing 93: StartupCallback Accepting Invoke Calls from the Startup Thread
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private void StartupCallback(object sender, SystemEventArgs e) { INVOKE_STARTUP it = thism_thrdStartupitReason; switch(it) { case INVOKE_STARTUPSTARTUP_SUCCESS: m_bRapiConnected = true; EnableUI(); break; case INVOKE_STARTUPSTARTUP_FAILED: ResetUI(); break; case INVOKE_STARTUPSTATUS_MESSAGE: sbarMainText = m_thrdStartupstrBuffer; break; } }
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A key element connecting the main thread to the startup thread is the connection status flag, m_bRapiConnected When the startup thread establishes the RAPI connection, it sends the main thread a STARTUP_SUCCESS code, and the main thread sets the connected flag to true:
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m_bRapiConnected = true;
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This flag reminds us to shut down our RAPI connection at the appropriate time For example, as shown in Listing 94, when the main form closes, we also shut down any running worker threads
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91 RAPI Fundamentals
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Listing 94: Cleanup of Background Threads and RAPI Connection
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private void MainForm_Closed(object sender, SystemEventArgs e) { // If threads are running, trigger shutdown if (thism_thrdStartup != null) thism_thrdStartupbThreadContinue = false; if (thism_thrdFindFiles != null) thism_thrdFindFilesbThreadContinue = false; if (m_bRapiConnected) { RapiCeRapiUninit(); m_bRapiConnected = false; } }
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9134 Defensive Coding
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RAPI is a communications protocol, which means you must code defensively When a connection has been established, your program must be able to handle lengthy operations reading or writing data that may take a long time because either the connection is slow (USB or serial), or the volume of data is large, or both Also, when communicating between two machines, the connection may get interrupted For example, ActiveSync is interrupted when a user removes a device from its docking cradle In addition to interrupted connections, your code must handle the possibility that no connection meaning no device is available Lengthy operations can be caused by blocking functions, by a slow connection, or simply by the need to move a lot of data Blocking functions return only when the requested action is finished For example, the CeRapiInit function returns only after a RAPI connection is established, and the function hangs when no device is present Most RAPI functions, in fact, are blocking (also called synchronous) and finish their work before returning Functions that return a large amount of data might take several seconds or even several minutes to finish While these issues require some effort to address, none are entirely daunting To avoid hanging a program s user interface, use background threads Without threads, an application can become unresponsive to a user s input and perhaps make a user worry that the program has hung Threads and feedback to the user with progress information help ease such user concerns
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The Remote API
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Thefinalchallenge,adisconnectedconnection,canbealittlemoredifficult to address For individual function calls, you must use vigilance in checking return values In addition, you will want to use exception handling (the try and catch keywords) so that when something fails deep within your code, you can escape the problem by raising an exception and recovering from the failure higher up the call stack Exception handling simplifies the handling of unexpected failures
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92 Accessing the Object Store
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The majority of RAPI functions involve reading from or writing to the object store As detailed in 5, Storage, the object store is the RAMbased storage area with three parts: a file system, the registry, and property databases The Windows CE file system is very similar to the desktop file system Both are hierarchical, supporting nested subdirectories (sometimes called folders) and long file paths that can be up to 260 characters long The file system in the object store provides the primary storage area for a Windows CE device Additional file systems known as installable file systems can be added to extend available storage space Two common installable file systems are Compact Flash memory cards and Secure Digital (SD) storage cards Both are sometimes referred to as memory cards because they contain nonvolatile flash memory, but both are used for file storage and not as program memory You can access both object store files as well as installable file system files by using the RAPI functions The registry provides a hierarchy for storing application and system settings The registry is as important to Windows CE as it is to desktop Windows The registry contains settings that the operating system uses for a wide range of tasks; examples include device drivers to load and applications to launch at system boot time Using RAPI functions, you can access any part of the registry of a connected Windows CE device The third part of the object store, property databases, provides a memory-efficient way to store and access sparsely populated record information These databases are used by the built-in PDA applications the calendar, inbox, task list, and contact list for their respective storage needs In general, a property database provides an easy way to store small, relatively static data As the number of records increases beyond 1,000 records or as the number of changes increases, the performance of CE property databases drops The SQL Server CE database meets the needs of
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