JavaScript EditorFree JavaScript Editor     Ajax Editor 

Main Page
  Previous Section Next Section

Making a Real-Time Event Loop

This type of real-time nonwaiting event loop is easy to make. All you need is a way to test if there is a message in the message queue. If there is, you can process it; otherwise, continue processing other game logic and repeat. The function that performs this test is called PeekMessage(). Its prototype is almost identical to GetMessage()'s, as shown here:

BOOL PeekMessage(
     LPMSG lpMsg,        // pointer to structure for message
     HWND hWnd,          // handle to window
     UINT wMsgFilterMin, // first message
     UINT wMsgFilterMax, // last message
     UINT wRemoveMsg);   // removal flags

This returns nonzero if a message is available.

The difference is in the last parameter, which controls how the messages should be retrieved from the message queue. The valid flags for wRemoveMsg are

  • PM_NOREMOVE— Messages are not removed from the queue after processing by PeekMessage().

  • PM_REMOVE— Messages are removed from the queue after processing by PeekMessage().

Taking these two possibilities into consideration, you can do one of two things: Use PeekMessage() with PM_NOREMOVE and, if there is a message, call GetMessage(); or use PM_REMOVE and use PeekMessage() itself to retrieve a message if there is one. Use the latter. Here's the core logic, changed to reflect this new technique in the main event loop:

    // test if there is a message in queue, if so get it
    if (PeekMessage(&msg,NULL,0,0,PM_REMOVE))
       // test if this is a quit
       if (msg.message == WM_QUIT)
    // translate any accelerator keys

    // send the message to the window proc
    }  // end if

    // main game processing goes here
}  // end while

I've highlighted important points in the code. The first section in bold is

if (msg.message == WM_QUIT) break;

This is how you must detect to bail out of the infinite while(TRUE) loop. Remember, when a WM_DESTROY message is processed in the WinProc, it's your job to send a WM_QUIT message via the call to PostQuitMessage(). The WM_QUIT then trickles through the event queue, and you can detect it so you can bail out of the main loop.

The last section of highlighted code simply indicates where you would put the call to your main game code loop. But remember, the call to Game_Main()—or whatever you call it—must return after one frame of animation or game logic. Otherwise, messages won't be processed by the main Windows event loop.

For an example of this new real-time structure that is more appropriate for game logic processing, take a look at the source DEMO2_4.CPP and the associated DEMO2_4.EXE on the CD-ROM. This structure will in fact be our model for the remainder of the book.

      Previous Section Next Section

    JavaScript EditorAjax Editor     JavaScript Editor