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TN071: MFC IOleCommandTarget Implementation

NoteNote:

The following technical note has not been updated since it was first included in the online documentation. As a result, some procedures and topics might be out of date or incorrect. For the latest information, it is recommended that you search for the topic of interest in the online documentation index.

The IOleCommandTarget interface enables objects and their containers to dispatch commands to each other. For example, an object's toolbars may contain buttons for commands such as Print, Print Preview, Save, New, and Zoom. If such an object were embedded in a container that supports IOleCommandTarget, the object could enable its buttons and forward the commands to the container for processing when the user clicked them. If a container wanted the embedded object to print itself, it could make this request by sending a command through the IOleCommandTarget interface of the embedded object.

IOleCommandTarget is an Automation-like interface in that it is used by a client to invoke methods on a server. However, using IOleCommandTarget saves the overhead of making calls via Automation interfaces because programmers don't have to use the typically expensive Invoke method of IDispatch.

In MFC, the IOleCommandTarget interface is used by Active document servers to allow Active document containers to dispatch commands to the server. The Active document server class, CDocObjectServerItem, uses MFC interface maps (see TN038: MFC/OLE IUnknown Implementation) to implement the IOleCommandTarget interface.

IOleCommandTarget is also implemented in the COleFrameHook class. COleFrameHook is an undocumented MFC class that implements the frame window functionality of in-place editing containers. COleFrameHook also uses MFC interface maps to implement the IOleCommandTarget interface. COleFrameHook's implementation of IOleCommandTarget forwards OLE commands to COleDocObjectItem-derived Active document containers. This allows any MFC Active document container to receive messages from contained Active document servers.

MFC developers can take advantage of IOleCommandTarget by using MFC OLE command maps. OLE command maps are like message maps because they can be used to map OLE commands to member functions of the class that contains the command map. To make this work, place macros in the command map to specify the OLE command group of the command you want to handle, the OLE command, and the command ID of the WM_COMMAND message that will be sent when the OLE command is received. MFC also provides a number of predefined macros for standard OLE commands. For a list of the standard OLE commands that were originally designed for use with Microsoft Office applications, see the OLECMDID enumeration, which is defined in docobj.h.

When an OLE command is received by an MFC application that contains an OLE command map, MFC tries to find the command ID and command group for the requested command in the OLE command map of the application. If a match is found, a WM_COMMAND message is dispatched to the application containing the command map with the ID of the requested command. (See the description of ON_OLECMD below.) In this way, OLE commands dispatched to an application are turned into WM_COMMAND messages by MFC. The WM_COMMAND messages are then routed through the application's message maps using the MFC standard command routing architecture.

Unlike message maps, MFC OLE command maps are not supported by ClassWizard. MFC developers must add OLE command map support and OLE command map entries by hand. OLE command maps can be added to MFC Active document servers in any class that is in the WM_COMMAND message-routing chain at the time the Active document is in-place active in a container. These classes include the application's classes derived from CWinApp, CView, CDocument, and COleIPFrameWnd. In Active document containers, OLE command maps can only be added to the COleDocObjectItem-derived class. Also, in Active document containers, the WM_COMMAND messages will only be dispatched to the message map in the COleDocObjectItem-derived class.

Use the following macros to add command map functionality to your class:

DECLARE_OLECMD_MAP ()

This macro goes in the class declaration (typically in the header file) of the class that contains the command map.

BEGIN_OLECMD_MAP(theClass, baseClass)
theClass

Name of the class that contains the command map.

baseClass

Name of the base class of the class that contains the command map.

This macro marks the beginning of the command map. Use this macro in the implementation file for the class that contains the command map.

END_OLECMD_MAP()

This macro marks the end of the command map. Use this macro in the implementation file for the class that contains the command map. This macro must always follow the BEGIN_OLECMD_MAP macro.

ON_OLECMD(pguid, olecmdid, id)
pguid

Pointer to the GUID of the OLE command's command group. This parameter is NULL for the standard OLE command group.

olecmdid

OLE command ID of the command to be invoked.

id

ID of the WM_COMMAND message to be sent to the application containing the command map when this OLE command is invoked.

Use the ON_OLECMD macro in the command map to add entries for the OLE commands you want to handle. When the OLE commands are received, they will be converted to the specified WM_COMMAND message and routed through the application's message map using the standard MFC command-routing architecture.

The following example shows how to add OLE command-handling capability to an MFC Active document server to handle the OLECMDID_PRINT OLE command. This example assumes that you used AppWizard to generate an MFC application that is an Active document server.

  1. In your CView-derived class's header file, add the DECLARE_OLECMD_MAP macro to the class declaration.

    NoteNote:

    Use the CView-derived class because it is one of the classes in the Active document server that is in the WM_COMMAND message-routing chain.

    class CMyServerView : public CView
    {
    protected: // create from serialization only
       CMyServerView();
       DECLARE_DYNCREATE(CMyServerView)
       DECLARE_OLECMD_MAP()
    . . .
    };
    
  2. In the implementation file for the CView-derived class, add the BEGIN_OLECMD_MAP and END_OLECMD_MAP macros:

    BEGIN_OLECMD_MAP(CMyServerView, CView)
       
    END_OLECMD_MAP()
    
  3. To handle the standard OLE print command, add an ON_OLECMD macro to the command map specifying the OLE command ID for the standard print command and ID_FILE_PRINT for the WM_COMMAND ID. ID_FILE_PRINT is the standard print command ID used by AppWizard-generated MFC applications:

    BEGIN_OLECMD_MAP(CMyServerView, CView)
       ON_OLECMD(NULL,OLECMDID_PRINT,ID_FILE_PRINT)
    END_OLECMD_MAP()
    

Note that one of the standard OLE command macros, defined in afxdocob.h, could be used in place of the ON_OLECMD macro because OLECMDID_PRINT is a standard OLE command ID. The ON_OLECMD_PRINT macro will accomplish the same task as the ON_OLECMD macro shown above.

When a container application sends this server an OLECMDID_PRINT command through the server's IOleCommandTarget interface, the MFC printing command handler will be invoked in the server, causing the server to print the application. The Active document container's code to invoke the print command added in the steps above would look something like this:

void CContainerCntrItem::DoOleCmd()
{
   IOleCommandTarget *pCmd = NULL;
   HRESULT hr = E_FAIL;
   OLECMD ocm={OLECMDID_PRINT, 0};

   hr = m_lpObject->QueryInterface(IID_IOleCommandTarget,reinterpret_cast<void**>(&pCmd));
   if(FAILED(hr))
      return;

   hr = pCmd->QueryStatus(NULL, 1, &ocm, NULL);
   if(SUCCEEDED(hr) && (ocm.cmdf & OLECMDF_ENABLED))
   {
      //Command is available and enabled so call it
      COleVariant vIn;
      COleVariant vOut;
      hr = pCmd->Exec(NULL, OLECMDID_PRINT,
 OLECMDEXECOPT_DODEFAULT, &vIn, &vOut);
      ASSERT(SUCCEEDED(hr));
   }
   pCmd->Release();
}
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