ArcGIS Engine Developer Guide

VISUAL C++
Event circular reference issues
After a sink has performed an advise on the source, there is typically a COM
circular reference. This occurs because the source has an interface pointer to a
sink to fire events, and this keeps the sink alive. Similarly, a sink object has a
pointer back to the source so it can perform the unadvise at a later point. This
keeps the source alive. Therefore, these two objects will never be released and
may cause substantial memory leaks. There are a number of ways to tackle this
issue:
• Ensure the advise and unadvise are made on a method or Windows message
that is guaranteed to happen in pairs and is independent of an object’s life
cycle. For example, in a coclass that is also receiving Windows messages, use
the Windows messages OnCreate (WM_CREATE) and OnDestroy
(WM_DESTROY) to advise and unadvise.
• If an ATL dialog box class needs to listen to events, one approach is to make
the dialog box a private COM class and implement the events interface directly
on the dialog box. ATL allows this without much extra coding. This
approach is illustrated below. The dialog box class creates a CustomizeDialog
coclass and listens to ICustomizeDialogEvents. The OnInitDialog and OnDestroy
methods (corresponding to Windows messages) are used to advise and
unadvise on CustomizeDialog.
class CEngineControlsDlg :
public CAxDialogImpl,
public CComObjectRoot, // Make Dialog Class a COM Object as well.
public ICustomizeDialogEvents // Implement this interface directly on
this object.
CEngineControlsDlg() : m_dwCustDlgCookie(0) {} // Initialize cookie for
event listening.
// ... Event handlers and other standard dialog code has been removed ...
BEGIN_COM_MAP(CEngineControlsDlg)
COM_INTERFACE_ENTRY(ICustomizeDialogEvents) // Make sure QI works for
this event interface.
END_COM_MAP()
// ICustomizeDialogEvents implementation to receive events on this
dialog box.
STDMETHOD(OnStartDialog)();
STDMETHOD(OnCloseDialog)();
ICustomizeDialogPtr m_ipCustomizeDialog; // The source of events
DWORD m_dwCustDlgCookie; // Cookie for
CustomizeDialogEvents.
}
The dialog box needs to be created like a noncreatable COM object, rather
than on the stack as a local variable. This allocates the object on the heap and
allows it to be released through the COM reference counting mechanism.
Chapter 4 • Developer environments • 139