Keyboard Input & the Win32 message loop

Question

How do I handle key presses and key up events in the windows message loop? I need to be able to call two functions OnKeyUp(char c); and OnKeyDown(char c);.

Current literature I've found from googling has lead me to confusion over WM_CHAR or WM_KEYUP and WM_KEYDOWN, and is normally targeted at PDA or Managed code, whereas I'm using C++.

score 11 · Answer 1 · edited Sep 07 '19 at 15:45

A typical C++ message loop looks like this

MSG msg;
while (GetMessage(&msg, null, 0, 0))
{
    TranslateMessage(&msg);
    DispatchMessage(&msg);
}

The function of TranslateMessage is to generate WM_CHAR messages from WM_KEYDOWN messages, so if you want to see WM_CHAR messages you need to be sure to pass WM_KEYDOWN messages to it. If you don't care about WM_CHAR messages, you can skip that, and do something like this.

extern void OnKeyDown(WPARAM key);
extern void OnKeyUp(WPARAM key);

MSG msg;
while (GetMessage(&msg, null, 0, 0))
{
    if (msg.message == WM_KEYDOWN)
       OnKeyDown (msg.wParam);
    else if (msg.message == WM_KEYUP)
       OnKeyUp(msg.wParam);
    else
    {
       TranslateMessage(&msg);
       DispatchMessage(&msg);
    }
}

Notice that OnKeyDown and OnKeyUp messages are defined as taking a WPARAM rather than a char. That's because the values for WM_KEYDOWN and WM_KEYUP aren't limited to values that fit in a char. See WM_KEYDOWN

More:
Using Messages and Message Queues
https://learn.microsoft.com/en-us/windows/win32/winmsg/using-messages-and-message-queues

The problem here is that although I would now have code that runs key up and key down, I also have code that is not portable. I would need a way of converting WPARAM into something more portable, instead of taking all my crossplatform code and making it windows only so that I can test for virtual key values, hence 'char' being used. I also don't want to spend a day writtng code for every character to convert — Tom J Nowell, Mar 14 '10 at 19:46

score 7 · Accepted Answer · edited Jul 03 '14 at 11:58

Use char c = MapVirtualKey(param,MAPVK_VK_TO_CHAR); to convert virtual key codes to char, and process WM_KEYUP and WM_KEYDOWN and their wParams.

if (PeekMessage (&mssg, hwnd, 0, 0, PM_REMOVE))
{
    switch (mssg.message)
    {
        case WM_QUIT:
            PostQuitMessage (0);
            notdone = false;
            quit = true;
            break;

        case WM_KEYDOWN:
            WPARAM param = mssg.wParam;
            char c = MapVirtualKey (param, MAPVK_VK_TO_CHAR);
            this->p->Input ()->Keyboard ()->Listeners ()->OnKeyDown (c);
            break;

        case WM_KEYUP:
            WPARAM param = mssg.wParam;
            char c = MapVirtualKey (param, MAPVK_VK_TO_CHAR);
            this->p->Input ()->Keyboard ()->Listeners ()->OnKeyUp (c);
            break;
    }
    // dispatch the message
    TranslateMessage (&mssg);
    DispatchMessage (&mssg);
}

Keyboard Input & the Win32 message loop

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