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The DCOMDRAW, DCDSERVE, and DCDMARSH samples together form a distributed drawing application. Using Distributed COM (DCOM) technology, they allow users on different client machines in a network to interactively draw on a shared drawing. All DCOMDRAW clients see the same drawing and any client can take ownership of the pen to draw. The user of DCOMDRAW can use a mouse or tablet device to draw in the client window and can choose the color and width of the electronic ink. As clients draw, the application echoes the drawing activity from DCDSERVE to all connected clients using DCOM over the network. Although these samples do not build an optimized and complete application, the functionality is suggestive of the shared whiteboards offered by some workgroup applications.
Although DCOMDRAW, DCDSERVE, and DCDMARSH work on the same machine across process boundaries, the main goal is to provide a distributed application that enables multiple DCOMDRAW clients on different machines to access a single shared drawing object housed in a DCDSERVE server running on a common server machine. A typical scenario would have numerous client machines hooked to an intranet consisting of a Windows NT network domain or peer-to-peer workgroup. DCOMDRAW can run on any of these client machines. DCDSERVE is installed and runs on a common server machine in the domain. The user of the DCOMDRAW client can interactively choose the remote server machine to determine which DCDSERVE COM server to use for the shared drawing.
DCDSERVE manages a shared, single COPaper COM object that models a sheet of white drawing paper. Clients can use the methods of the COPaper object to draw on the paper surface using "ink" of specified color and width. This functionality is outwardly similar to the "scribble" tutorial samples in many versions of the Microsoft Visual C++® product. The drawing paper features of COPaper objects are exposed to DCOMDRAW clients by a custom ISharePaper interface. COPaper implements the ISharePaper interface. A clear architectural distinction is kept between client and server. No graphical user interface (GUI) is provided by the COPaper object. It relies on the DCOMDRAW client for all GUI behavior. COPaper encapsulates only the server-based capture and storage of the drawn ink data.
The ink data that is drawn on the COPaper surface can be stored in and loaded from compound files. The ISharePaper::Save and ISharePaper::Load method implementations in COPaper use structured storage to store the current data of the shared drawing in a compound file kept with DCDSERVE.EXE on the machine acting as the common server.
The DCOMDRAW sample creates and uses the connectable COPaper COM object that is provided as the CLSID_SharePaper component in the DCDSERVE server. The DCOMDRAW client creates a COPaper object and controls it using the methods of the ISharePaper interface. DCOMDRAW obtains drawing data from the user and displays that data in a window that it manages. DCOMDRAW uses COPaper's ISharePaper interface to save the drawing data in COPaper and to direct file-storage operations on this data.
COPaper only manages the drawing data; it performs no GUI actions. DCOMDRAW provides the GUI for the drawing application by encapsulating this functionality in a central CGuiPaper C++ object.
DCOMDRAW also implements the custom IPaperSink interface on a COPaperSink COM object and connects this interface to an appropriate connection point on the COPaper object in DCDSERVE. COPaper uses the connected IPaperSink interface to send notifications back to the COPaperSink in DCOMDRAW. The normal GUI repainting of COPaper's drawing data is done in DCOMDRAW using COPaper's connectable object technology. Thus in this sample, COM's connectable-object technology uses DCOM to function transparently across the network. In earlier samples such as STOCLIEN and STOSERVE, COM's connectable-object technology was shown operating only in-process.
The two ISharePaper and IPaperSink custom interfaces are declared in PAPINT.H, which is located in the common INC directory. PAPINT.H is automatically generated in the DCDMARSH sample. The GUIDs for the interfaces and objects are defined in PAPGUIDS.H located in that same directory.
This lesson focuses primarily on how to take care of COM process security on the client side and how to load a remote object using DCOM. Using CoInitializeSecurity to set process security is covered. CoCreateInstanceEx using the MULTI_QI structure is covered.
Because client and server run in separate processes--usually on different computers--both DCDSERVE and DCOMDRAW rely on standard marshaling for the ISharePaper and IPaperSink custom interfaces. The DCDMARSH sample provides this support, so you must build (or otherwise register) DCDMARSH.DLL on both machines prior to building and running DCDSERVE and DCOMDRAW across machines.
To set a DCOMDRAW client on one machine to control the common DCDSERVE server on another, both machines must have DCOM (Distributed COM) installed. DCOM is included in Windows NT 4.0 or above and in Windows 98. If a computer is running Windows 95, you must install the DCOM95 add on. DCOM95 can currently be obtained by download from Microsoft's world wide Web site at: http://www.microsoft.com/com/.
The multiple computers must be connected in a properly configured network. For details on setting up a network, see your Windows NT product documentation or the Windows NT Resource Kit. For more details on computer and network setup for running with DCOM, see the "Network and Setup Issues" section at the end of the REMCLIEN lesson. Note that the less restrictive security in Windows 95 and Windows 98 prevents the SCM under DCOM from automatically launching DCDSERVE on behalf of a remote DCOMDRAW client. You must manually pre-launch DCDSERVE on these operating systems if you attempt a remote load of DCDSERVE from a DCOMDRAW running on another machine. The DCDSERVE and DCOMDRAW lessons assume that you install DCDSERVE on a machine running Windows NT Server or Workstation.
For functional descriptions and a tutorial code tour of DCOMDRAW, see the Code Tour section in DCOMDRAW.HTM. For details on the external user operation of DCOMDRAW, see both the Usage and Operation sections in DCOMDRAW.HTM. To read DCOMDRAW.HTM, run TUTORIAL.EXE in the main tutorial directory and click the DCOMDRAW lesson in the table of lessons. You can do the same thing by locating the main tutorial directory in the Windows Explorer and double-clicking the DCOMDRAW.HTM file. For more details on how DCDSERVE works and exposes its services to DCOMDRAW, see DCDSERVE.HTM in the main tutorial directory. The makefile for DCDSERVE automatically registers that server in the registry of the host machine, so you must build DCDSERVE on the remote machine before attempting to run DCOMDRAW.
For details on setting up your system to build and test the code samples in this COM Tutorial series, see Building the Code Samples. The supplied makefile (MAKEFILE) is Microsoft NMAKE-compatible. To create a debug build, issue the NMAKE command in the Command Prompt window.
For convenient use in Microsoft's Visual Studio, a project file is provided for each sample. To load the project for the DCOMDRAW sample, you can run Visual Studio at the Command Prompt in the sample's directory as follows:
MSDEV DCOMDRAW.DSP
You can also simply double-click the DCOMDRAW.DSP file in the Windows Explorer to load a sample's project into Visual Studio. From within Visual Studio you can then browse the C++ classes of the sample source and generally perform the other edit-compile-debug operations. Note that, as part of the Platform SDK, the compilation of these samples from within Visual Studio requires the proper setting of directory paths in Visual Studio. For more details, see Building the Code Samples.
DCOMDRAW is an application that you can execute directly from Windows or from the command-prompt window. No command-line parameters are recognized by DCOMDRAW. DCOMDRAW currently runs on the Windows 95 operating system with the DCOM95 update for Windows 95 installed. It will run on Windows 98 which includes DCOM. It will also run under version 4 or later of Windows NT Workstation or Windows NT Server.
The DCDSERVE, DCDMARSH, and DCOMDRAW samples must be compiled before you can run the client. For more details on building the samples, see Building the Code Samples.
To access and control the remote DCDSERVE server, the DCOMDRAW client requires that you have your network and machines set up properly for DCOM operation. See the "Network Setup and Issues" section of the REMCLIEN lesson for more details. If you have already built the appropriate samples and set up your machines for DCOM operation, DCOMDRAW.EXE is the client executable to run for this sample.
The DCOMDRAW.EXE application provides the user interface for this sample application. It uses the functionality of the associated, but independent, DCDSERVE.EXE out-of-process server. Here is a summary of operation from the standpoint of DCOMDRAW.EXE as a client of DCDSERVE.EXE.
Since two machines are involved in this lesson, this lesson refers to the machine where DCDSERVE runs as Machine-S (server) and to the machine where DCOMDRAW runs as Machine-C (client).
The are many ways to set up to build and run DCOMDRAW for this tutorial. Here are two alternate ways to get started:
Once registered, DCDSERVE server requires manual security configuration using the DCOMCNFG utility. For more details, see "Installing DCDSERVE" in the DCDSERVE.HTM file.
With the executables present and the servers properly registered on both machines, you can run the DCOMDRAW application on Machine-C, relying on COM to locate and load the remote DCDSERVE server on Machine-S. DCOMDRAW provides a Load from Remote Server dialog box in which you can enter the name of the remote machine on which the DCDSERVE server runs.
DCOMDRAW.EXE provides menus for loading and saving the shared drawing and for controlling the drawing pen color and width. The following menus are provided:
Menu Selection: File/Load Remote...
Displays the Load from Remote Server dialog box, in which you specify the
network machine name of the computer where the remote DCDSERVE server is
located and registered. DCOM recognizes machine names that are in a valid
UNC format (for example, Machine-S or \\Machine-S), a DNS format (for
example, www.mymachine.com), or an IP address format (for example,
123.45.6.7).
Menu Selection: File/Load Local
Loads (or reloads) the drawing from a DCDSERVE registered on the local
machine (that is, the machine where this client is running).
Menu Selection: File/Save
Saves the current drawing. Beeps if this DCOMDRAW does not own the pen.
Menu Selection: File/Exit
Exits DCOMDRAW.
Menu Selection: Draw/Take Pen
Takes ownership of the pen to permit this DCOMDRAW to draw on the shared
drawing. Beeps if another client already owns the pen.
Menu Selection: Draw/Give Pen
Relinquishes ownership of the pen to permit any other DCOMDRAW client to
take the pen draw on the shared drawing.
Menu Selection: Draw/Redraw
Forces a redraw of the current DCOMDRAW client's display of the shared
drawing.
Menu Selection: Draw/Erase
Erases all content of the currently shared drawing. Beeps if this DCOMDRAW
client does not own the pen.
Menu Selection: Color
Shows the Pick Color dialog allowing you to choose the current color of
the drawing pen. Beeps if this DCOMDRAW client does not own the pen.
Menu Selection: Thin
Sets current pen width to Thin. Beeps if this DCOMDRAW client does not own
the pen.
Menu Selection: Medium
Sets current pen width to Medium. Beeps if this DCOMDRAW client does not own
the pen.
Menu Selection: Fat
Sets current pen width to Fat. Beeps if this DCOMDRAW client does not own
the pen.
Menu Selection: Help/DCOMDRAW Tutorial
Opens the DCOMDRAW.HTM tutorial file in the Web browser.
Menu Selection: Help/DCDSERVE Tutorial
Opens the DCDSERVE.HTM tutorial file in the Web browser.
Menu Selection: Help/DCDMARSH Tutorial
Opens the DCDMARSH.HTM tutorial file in the Web browser.
Menu Selection: Help/Read Source File
Displays the Open common dialog box so you can open a source file from this
lesson or another one in the Windows Notepad.
Menu Selection: Help/About DCOMDRAW
Displays the About dialog box for this application, a standard part of
this series of code samples. The code illustrates how to use the CAboutBox
class provided by APPUTIL.LIB.
When first run on a machine, DCOMDRAW loads a shared drawing from the DCDSERVE registered on the local machine. That first instance of DCOMDRAW is given "master" ownership of the drawing pen. This means that it can immediately make drawing changes to the drawing. Any subsequent instances of DCOMDRAW on that same machine will access the same locally shared drawing, but will not own the pen. This means that they cannot draw and may merely see drawing activity as it is done in the DCOMDRAW client that owns the pen. The pen cursor changes to an 'X' indicating that the pen is not owned and that drawing is off. When drawing is on, the pen cursor size changes (that is, a thin, medium, or fat pen) to indicate the chosen pen width. The DCOMDRAW title bar also has an indicator of pen ownership. If DCOMDRAW owns the pen, "***TALK***" appears in the title bar. If the client does not own the pen, "LISTEN" appears in the title bar. The usage model here assumes that while users are sharing the drawing they are also talking over some audio channel (possibly telephone). Typically, when a client user is drawing they are talking to explain it; otherwise the client user con only watch another client's drawing activity.
When you own the pen in DCOMDRAW, you press and hold the left mouse button as you move the mouse to draw ink.
The File/Load Remote menu choice allows you to load or connect to a shared drawing maintained on a remote machine. If multiple clients on different client machines load from that same remote machine then they will all share the same drawing maintained by the DCDSERVE registered on that server machine.
Files Description
DCOMDRAW.TXT Short description of the sample.
MAKEFILE The generic Win32 makefile for building the code sample
application of this tutorial lesson.
DCOMDRAW.H The include file for the DCOMDRAW application. Contains
class declarations, function prototypes, and resource
identifiers.
DCOMDRAW.CPP The main implementation file for DCOMDRAW.EXE. Has WinMain
and CMainWindow implementation, as well as the main menu
dispatching.
DCOMDRAW.RC The application resource definition file.
DCOMDRAW.ICO The application icon resource.
PENCURT.CUR Pen cursor for the Thin pen width.
PENCURM.CUR Pen cursor for the Medium pen width.
PENCURF.CUR Pen cursor for the Fat pen width.
PENCURN.CUR Pen cursor for when drawing is off (pen is not owned).
GUIPAPER.H The class declaration for the CGuiPaper C++ class.
GUIPAPER.CPP Implementation file for the CGuiPaper C++ class.
SINK.H The class declaration for the COPaperSink COM object class.
SINK.CPP Implementation file for the COPaperSink COM object class.
DCOMDRAW.DSP Microsoft Visual Studio Project file.
Like all code samples in the series, DCOMDRAW uses many of the utility classes and services provided by APPUTIL. For more details on APPUTIL, study the APPUTIL library source code in the APPUTIL directory and the lesson in the APPUTIL.HTM file in the main tutorial directory.
DCOMDRAW requires the DCOM facility in COM. Early in its WinMain function, DCOMDRAW calls APPUTIL's DComOk function to confirm that DCOM is installed on the system. DComOk does this by ensuring that the CoInitializeEx function is exported from the OLE32.DLL system library. CoInitializeEx was initially supplied with the DCOM functionality in Windows NT 4.0 and its presence is thus a simple way to confirm that DCOM is present. Similarly, DComOk will return FALSE on Windows 95 if the DCOM95 add-on is not installed.
DCOMDRAW and DCDSERVE borrow much from the STOCLIEN and STOSERVE samples. As a result, this lesson does not discuss in detail how CGuiPaper encapsulates the GUI behavior of drawing paper, how the interactive mouse activity is captured and displayed, how COPaper records the drawing data, how IPaperSink and the connectable-object technology works, and how the drawing data is actually stored in the structured storage of a compound file. For more details on these topics, see the STOCLIEN/STOSERVE lessons.
The major topics covered in this code tour are:
The DCOMDRAW sample relies heavily on COM's connectable-object technology for communication from COPaper back to the client. Interactive drawing as well as on-demand re-painting use this technology. In particular, COPaper calls the methods of an IPaperSink interface on a COPaperSink object in DCOMDRAW. For COM to carry these calls across the network you must take care of some security issues. We saw in the DCDSERVE lesson that its process is acting under the identity of a DCOMDraw user. We explicitly created that user account when installing DCDSERVE. Unless we create an account for this specific user on every client machine with the proper permissions, this DCOMDraw identity in the server will not have permissions to call back into (or otherwise access) DCOMDRAW on the client side. So a technique is needed to permit this access into the client from an outside identity.
A call to CoInitializeSecurity grants this access for the entire DCOMDRAW client process. The call establishes a security context in the client that permits COPaper on the server side to call back into DCOMDRAW on the client side. Here is such a CoInitializeSecurity call in a code fragment from the WinMain function in DCOMDRAW.CPP:
...
...
// If we were compiled for UNICODE and the platform seems OK with this
// then proceed. Else we error and exit the app.
if (UnicodeOk())
{
// Call to initialize the COM Library. Use the SUCCEEDED macro
// to detect success. If fail then exit app with error message.
// Tell COM that this client process will use the Single Threaded
// Apartment model.
if (SUCCEEDED(CoInitialize(NULL)))
{
// Ensure that DCOM (Distributed COM) is installed.
if (DComOk())
{
// Initialize for Client process security. Allow COPaper to
// call back into the client (ie, the client can impersonate
// the server identity).
hr = CoInitializeSecurity(
NULL, //Points to security descriptor
-1, //Count of entries in asAuthSvc
NULL, //Array of names to register
NULL, //Reserved for future use
RPC_C_AUTHN_LEVEL_NONE, //Default authentication level
// for proxies
RPC_C_IMP_LEVEL_IMPERSONATE, //Default impersonation level
// for proxies
NULL, //Reserved; must be set to NULL
EOAC_NONE, //Additional client or
// server-side capabilities
NULL); //Reserved for future use
if (SUCCEEDED(hr))
{
...
...
Initialize the client app and run the main message loop.
...
...
}
else
Error: Can't initialize security.
}
else
Error: DCOM is not installed.
// We're exiting this app (either normally or by init failure) so
// shut down the COM Library.
CoUninitialize();
}
else
Error: Can't initialize COM.
}
else
Error: Platform Can't handle UNICODE.
...
...
The above CoInitializeSecurity call should look familiar because it is identical to the one used in the DCDSERVE sample. See DCDSERVE.HTM where this call is covered in detail. In DCOMDRAW the call to CoInitializeSecurity shown above establishes a process security context in which call backs are not authenticated (that is, they are allowed to pass) and the calling identity (the DCOMDraw user) is impersonated in the client. This essentially opens the security door in this sample client. You can also use CoInitializeSecurity to create a more restrictive arrangement. For example, a non-NULL security descriptor could be passed as the first parameter. It could include a DACL (Discretionary Access Control List) that specifically grants permission to only the DCOMDraw user. Although this sample does not use them, there are also techniques whereby restricted security contexts can be established for the duration of individual interface method calls.
When you run DCOMDRAW it initially loads the drawing from a COPaper that is available from a DCDSERVE server that is registered on the local machine. At this point DCOMDRAW is a simple local drawing application. The drawing you make is stored in the DCDSERVE.PAP compound file located with the DCDSERVE on the local machine. If you run multiple instances of DCOMDRAW on this machine, they will all share that local drawing.
By choosing the File/Load Remote command you can enter the machine name for the remote drawing to be shared. The existing local drawing is then closed and a drawing is loaded into a COPaper running on the remote server machine. (Of course, this assumes that the remote machine is set up with DCDSERVE installed as described in the DCDSERVE lesson.) The REMCLIEN lesson also covered using CoCreateInstanceEx with a COSERVERINFO structure. DCOMDRAW uses the same technique. Here is the CGuiPaper::LoadRemote method from GUIPAPER.CPP:
HRESULT CGuiPaper::LoadRemote(void)
{
HRESULT hr = NOERROR;
BOOL bFirst;
COPaperSink* pCobSink = NULL;
HCURSOR hCurPrev;
HCURSOR hCurWait = LoadCursor(NULL, IDC_WAIT);
COSERVERINFO* pServerInfo = &g_ServerInfo;
MULTI_QI qiRes;
int iAns;
CDlgLoadRemote dlgLoadRemote;
// Ask user for the remote machine name. Cancel this whole load if
// he cancels the dialog.
iAns = dlgLoadRemote.ShowDialog(
m_hInst,
MAKEINTRESOURCE(IDD_LOAD_REMOTE),
m_hWnd);
// Ensure a remote machine name was specified by user.
if (IDOK == iAns && pServerInfo->pwszName[0])
{
hr = E_FAIL;
// Unload the existing COPaper object.
if (m_pISharePaper)
{
// Ask User if save of current drawing is desired.
AskSave();
// Change cursor to the hour glass.
hCurPrev = SetCursor(hCurWait);
// Clear the window of the previous drawing.
ClearWin();
// Make sure we unlock the paper object.
Lock(FALSE);
// Disconnect all Sinks--currently only one: PaperSink. This
// officially stops all PaperSink notifications.
hr = DisconnectPaperSink();
// Release the main interface pointer copy held in CGuiPaper.
// This causes destruction (in the server) of the current
// COPaper object.
RELEASE_INTERFACE(m_pISharePaper);
}
// Load a new COPaper from the server on a user-specifed remote machine.
// Call COM service to create an instance of the remote COPaper
// COM object. We are not aggregating this new object (viz, the
// NULL parameter), so we ask for its IShareDraw interface directly.
qiRes.pIID = &IID_ISharePaper;
qiRes.pItf = NULL;
qiRes.hr = 0;
hr = CoCreateInstanceEx(
CLSID_SharePaper,
NULL,
CLSCTX_REMOTE_SERVER,
pServerInfo,
1,
&qiRes);
if (SUCCEEDED(hr))
{
hr = qiRes.hr;
if (SUCCEEDED(hr))
{
// Grab our copy of the returned interface pointer. An AddRef was
// done by CoCreateInstanceEx on this interface pointer.
m_pISharePaper = (ISharePaper*)qiRes.pItf;
// Init the newly created COPaper object.
GetClientRect(m_hWnd, &m_WinRect);
hr = m_pISharePaper->InitPaper(&m_WinRect, &bFirst);
if (SUCCEEDED(hr))
{
// Reconnect all Sinks--currently only one: PaperSink.
// This restores all PaperSink notifications.
hr = ConnectPaperSink();
if (SUCCEEDED(hr))
{
if (bFirst)
{
// Lock and load.
Lock(TRUE);
hr = Load();
}
else
{
Lock(FALSE);
// If this is not first init then resize this client's
// window to match what the remote COPaper object is
// using for its window size.
hr = ResizeWin(m_WinRect.right, m_WinRect.bottom);
}
}
}
}
}
if (FAILED(hr))
{
HrMsg(m_hWnd, TEXT(REMOTE_CREATE_ERR_STR), hr);
// If error with remote load then restore local drawing.
hr = LoadLocal();
}
// Set Cursor back to what it was.
SetCursor(hCurPrev);
}
return hr;
}
The CoCreateInstanceEx call uses the MULTI_QI structure even though only one interface is requested. This facility is especially useful in the context of distributed COM where numerous interfaces on a remote object are needed by a client. Rather than suffer the overhead of separate QueryInterface round-trips across the network, this facility allows more than one interface to be obtained in one CoCreateInstanceEx call.
After obtaining the m_pISharePaper interface pointer, it is used to call the InitPaper method. This call returns the window rectangle of the remote drawing and also a flag that indicates whether this is the first client to gain access to the newly created COPaper.
If this is the first client then an initial lock on the shared COPaper drawing is requested. This lock means that this first client owns the pen. COPaper is then asked to load the drawing content from its associated compound file. This is the DCDSERVE.PAP file located on the remote machine with DCDSERVE.
If this client is not the first client, then it does not request a lock on COPaper because it is assumed that an earlier client owns the pen. Since a COPaper object already exists and is loaded, this client resizes its window to match the existing drawing used by the other clients. In addition to drawing something, one of these earlier clients may have resized the drawing window. Finally, the drawing is painted in the client's window whether this client is the first or not.
Since the drawing data for the shared drawing is managed by a single instance of COPaper on the server side, DCOMDRAW must obtain this data from COPaper to display the drawing. Here is the CGuiPaper::PaintWin method from GUIPAPER.CPP.
HRESULT CGuiPaper::PaintWin(void)
{
HRESULT hr = E_FAIL;
COLORREF crInkColorTmp;
SHORT nInkWidthTmp;
BOOL bInkSavingTmp;
LONG i;
SHORT nInkType;
SHORT nX;
SHORT nY;
SHORT nInkWidth;
COLORREF crInkColor;
if (m_pISharePaper && !m_bPainting && !m_bInking)
{
hr = NOERROR;
m_bPainting = TRUE;
// Save and restore ink color and width since redraw otherwise
// ends up changing these values in CGuiPaper.
crInkColorTmp = m_crInkColor;
nInkWidthTmp = m_nInkWidth;
bInkSavingTmp = m_bInkSaving;
m_bInkSaving = FALSE;
for (i = 0; SUCCEEDED(hr); i++)
{
hr = m_pISharePaper->GetInk(
i,
&nInkType,
&nX,
&nY,
&nInkWidth,
&crInkColor);
if (SUCCEEDED(hr))
{
switch (nInkType)
{
case INKTYPE_START:
m_nInkWidth = nInkWidth;
m_crInkColor = crInkColor;
InkStart(nX, nY);
break;
case INKTYPE_DRAW:
InkDraw(nX, nY);
break;
case INKTYPE_STOP:
InkStop(nX, nY);
break;
default:
break;
}
}
}
m_nInkWidth = nInkWidthTmp;
m_crInkColor = crInkColorTmp;
m_bInkSaving = bInkSavingTmp;
m_bPainting = FALSE;
hr = NOERROR;
}
return hr;
}
The DCOMDRAW client plays an active role in getting the ink data for painting. Compare this to the repainting in the STOCLIEN sample. In that sample the client would request a redraw of the COPaper object. COPaper would then loop to send the data back to a sink in the client. But in this DCOMDRAW distributed drawing sample, the clients can all concurrently loop to obtain the data for their own separate repaints. Because both the DCDSERVE server and the DCOMDRAW client initialize COM in a Single Threaded Apartment (STA), COM ensures that multiple client calls to the shared COPaper are serialized on the same thread in DCDSERVE. This arrangement is convenient to program but can tax performance. For better performance, COPaper can be coded as a free-threaded object residing in the Multi-Threaded Apartment (MTA) of DCDSERVE.
Note that performance of the drawing and repainting in DCOMDRAW can vary depending on the complexity of your network. For example, during repaints COPaper in DCDSERVE may be calling COPaperSink in DCOMDRAW through intermediary servers or from trusted domains to trusting domains. DCOMDRAW and DCDSERVE are not optimized for maximum performance in these cases. One worthy optimization would cache individual ink points into collections called strokes. Each stroke would be a variant blob beginning with an InkStart and ending with an InkStop. Rather than each individual ink point being sent separately, strokes would be sent. This would replace the InkDraw calls that make up the bulk of the repaint traffic with a greatly reduced number of InkStroke calls. The drawing behavior would look less smooth but a repaint would be faster.
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