interface allows applications to handle incoming or outgoing messages
selectively while waiting for responses from synchronous calls. The ability to filter
messages is often useful when a dialog box is being displayed or a lengthy
operation is in progress. Performance can be improved when applications handle some
messages and defer others.
When to Implement
in place of OLE's default message handling when there is a need to
selectively handle incoming or outgoing messages, while at the same time waiting for
responses from synchronous calls.
Although not as significant an issue as with 16-bit applications, you may
still need to implement IMessageFilter
as a way of solving deadlocks. IMessageFilter
is still needed to find out if an application is blocking, so that you can
task-switch to that application and give the user an opportunity to deal with the
blocking application. For example, if you have Word talking to Excel, with
Excel running in the background in formula mode, Excel won't check all calls,
thereby blocking further action. IMessageFilter
would put up a dialog indicating which task is blocking and provide the user
with an opportunity to deal with the deadlock.
For applications that do not implement IMessageFilter
, incoming and outgoing calls to OLE are handled automatically by OLE's
default message handling procedures. OLE provides default message handling for each
of the conditions that would be handled by an IMessageFilter
method. For incoming calls, OLE dispatches all calls, regardless of their
logical thread ID. For incoming Windows messages or Macintosh events, the default
behavior is as though IMessageFilter::MessagePending
had returned PENDINGMSG_WAITDEFPROCESS. This means that task-switching and
window-activation messages are dispatched, as well as WM_TIMER messages. Other
input messages are discarded and OLE continues to wait for the reply.
When to Use
You don't call this interface directly. It's provided by the application and
called by the system.
Methods in Vtable Order
||Provides a single entry point for incoming calls.
||Provides application with opportunity to display dialog offering retry or
cancel options or to switch tasks.
||Indicates a Windows message has arrived while OLE is waiting to respond to a
In the WIN32 Programmer's Reference
: "Messages and Message Queues"
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