Implementing IServiceProvider in an MFC application - c++

Question

I need to implement the IServiceProvider interface in an open source MFC application; specifically my TTSApp application.
I am attempting to add support for the IAccessibleApplication interface, which is used by screen readers to obtain information about an application's name and version information.
It appears that Google Chrome implements the IServiceProvider interface via the AXPlatformNodeWin class, which is derived from the CComObjectRootEx class and other classes and interface. The problem is that an MFC application does not use the CComObjectRootEx class; it is used by ATL.
I have found the IServiceProviderImpl Class. Unfortunately, I cannot find any information on how it fits in the context of an application. Which class in my class hierarchy needs to be derived from the IServiceProviderImpl Class; my CWinApp derived class, my CDialogEx derived class, or some other class?

Answer 1

I learned a great deal while on my quest in search for an answer to this question. During the quest I fell down the rabbit hole (Alice's Adventures in Wonderland by Charles Lutwidge Dodgson, a.k.a. Lewis Carroll) only to find Cthulhu (The Call of Cthulhu by H. P. Lovecraft) waiting for me.
My initial research led me to the following macros defined in afxwin.h.
DECLARE_INTERFACE_MAP
BEGIN_INTERFACE_MAP
END_INTERFACE_MAP
BEGIN_INTERFACE_PART
END_INTERFACE_PART
The best documentation I could find for these macros is in the TN038: MFC/OLE IUnknown Implementation technical note. A good sample demonstrating the use of these macros and the implementation of the QueryService function is the TstCon sample.
Of course, this led to another question, what window do I need to do this for? To answer this question I looked at the source code of a certain screen reader to see how it uses the IAccessibleApplication interface.
The following function, though not the actual code used, demonstrates the technique (I cannot share the actual code since the screen reader is not open source).
std::wstring GetApplicationNameUsingTheIAccessibleApplicationInterface(
HWND hwnd, long idObject, long idChild)
{
CComPtr<IAccessible> acc;
CComVariant var;
auto hr = AccessibleObjectFromEvent(hwnd, idObject, idChild, &acc, &var);
if (hr != S_OK) return L"";
if (!acc) return L"";
CComQIPtr<IServiceProvider> serviceProvider = acc;
if (!serviceProvider) return L"";
CComQIPtr<IAccessibleApplication> application;
hr = serviceProvider->QueryService(
IID_IAccessible, __uuidof(IAccessibleApplication),
reinterpret_cast<void**>(&application));
if (FAILED(hr)) return L"";
if (!application) return L"";
CComBSTR appName;
hr = application->get_appName(&text);
if (FAILED(hr)) return L"";
return appName.m_str;
}
This function, or something like it, is called from our WinEventProc callback function in response to the EVENT_OBJECT_FOCUS event. This indicates that I need to do this for every window that can gain focus.
Armed with what I thought was the answer to my question, I dove in and implemented the IAccessibleApplication interface and added the necessary code to all of my focus-able windows. Much to my horror, my QueryService function was never called. When I debugged the screen reader to find out why, I discovered that the implicit QueryInterface implied by the following line of code failed.
CComQIPtr<IServiceProvider> serviceProvider = acc;
This led to a long and arduous quest to discover why the call to QueryInterface was failing.
I was working on a personal project at first so I could not call upon resources I have at my employer. Then, entirely by chance, I was assigned a task that required me to provide information on how to implement the IAccessible2 interface in a C++ application to a client who needed the information to help them make their applications more accessible. Hurrah, I was finally able to call upon coworkers for aid!
My coworker guided me down the correct path.
Create a customized version of the IAccessibleProxyImpl class and CAccessibleProxy class using source code obtained from atlacc.h.
Add a COM_INTERFACE_ENTRY for IAccessibleApplication in the COM_MAP (BEGIN_COM_MAP/END_COM_MAP) for my custom IAccessibleProxyImpl class.
Use the BEGIN_SERVICE_MAP, END_SERVICE_MAP, and SERVICE_ENTRY macros to provide an implementation of the IServiceProvider interface.
Provide an override for the CWnd::CreateAccessibleProxy function to cause my windows to use my custom accessible proxy and thus my implementation of the IAccessibleApplication interface.
Now the screen reader uses the application name I provide for the IAccessibleApplication interface for my application.
The application I did this for is open source. It is my TTSApp application. I have also made an example that demonstrates how to use a similar technique to support the IAccessible2 interface available here.
I am sharing this in the hopes that the information will prove to be helpful.