Must multimedia players check the message queue and invoke DispatchMessage?

[Ove Kaaven]

Den 08. april 2011 13:15, skrev Joerg-Cyril.Hoehle at t-systems.com:
> DispatchMessage is not always needed.  Unfortunately, I don't grok
> yet when DispatchMessage is required.
> 
> http://blogs.msdn.com/b/oldnewthing/archive/2004/06/08/150929.aspx
> "Everybody who has messed with window messaging knows that GetMessage
> and PeekMessage retrieve queued messages, which are dispatched to
> windows via DispatchMessage."
> 
> None the clearer. If I just retrieved them (they were obviously
> "dispatched" to my queue), why must I dispatch again?

No, they were *not* dispatched to your queue, they were queued (posted).
DispatchMessage calls the appropriate window procedure, it is not used
to queue messages.

Sending and queuing is *not* the same. The rule is basically:

- If the message was queued with Post*Message, then
GetMessage/PeekMessage returns it and you should pass it to DispatchMessage.
- If the message was sent with Send*Message, then GetMessage/PeekMessage
handles (dispatches) it, does not return it, and thus DispatchMessage
would never even see it.

Consider sent messages synchronous, and posted/queued messages as
asynchronous. Windows may reorder queued (asynchronous) messages by
message priority class, so that e.g. GetMessage may return a WM_PAINT
only after all other (higher-priority) messages have been returned and
dispatched.

> I.e. what's the kind of work that the function calling GetMessage
> Should/can do vs. the work to be done by whatever function DispatchMessage
> eventually calls?

The function calling GetMessage isn't supposed to do anything beyond
calling DispatchMessage.