Page 236 - DCAP408_WEB_PROGRAMMING

Windows Programming

Notes 14.9.2 A New Function, PeekMessage()

What we will do to solve this dilemma is replace our current GetMessage() function with a new
function, PeekMessage(). This function does essentially the same thing, but with one important
difference: it doesn’t wait for anything. PeekMessage() just looks into the message queue and
checks to see if any messages are waiting. If not, the program will continue on, allowing us to do
what we need.

Figure 14.1: The Structure of a PeekMessage() Loop

Create the Window

PeekMessage() No
While(TRUE) Handle DirectX Stuff
Is there a message?
Yes

TranslateMessage() DispatchMessage()

Message To
WindowProc()

Before we go any further, let’s take a good look at PeekMessage(). Here is it’s prototype.
BOOL PeekMessage(LPMSG lpMsg,
HWND hWnd,
UINT wMsgFilterMin,
UINT wMsgFilterMax,
UINT wRemoveMsg);
The first four parameters should be familiar to you. They are identical to the four parameters of
GetMessage(). However, the fifth one, wRemoveMsg, is new.
What it does is indicate whether or not the message retrieved from the event queue should stay
on the event queue or come off. We can put either PM_REMOVE or PM_NOREMOVE. The first
one takes the messages off the queue when they are read, while the second one leaves the
messages there for later retrieval. We will use the PM_REMOVE value here, and keep things
simple.
So how do we implement this into our program? Following is the main loop from the last
program we made, modified to use PeekMessage().
// Enter the infinite message loop
while(TRUE)
{
// Check to see if any messages are waiting in the queue
while(PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))

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