Question

I’m trying to learn how to use and receive raw input messages, and so I devised the following program to test my understanding so far…

When I recieve the input message, I try to change the string which is displayed and place a new WM_PAINT message on the queue. But this message doesn’t seem to get called. Only when I resize the window does the text appear different.

Why is the WM_PAINT message not getting processed at all, until I resize the window for example?

#define _WIN32_WINNT 0x501
#include <Windows.h>
#include <assert.h>
#include <cstring>
#include <cstdio>

LRESULT CALLBACK WinProcedure(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam);

int WinMain(HINSTANCE hInst,HINSTANCE,LPSTR,int nCmdShow)
{
    WNDCLASS wc;
    HWND hwnd;

    wc.hInstance = hInst;
    wc.lpfnWndProc = WinProcedure;
    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hIcon = LoadIcon(NULL, IDI_WINLOGO);
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)COLOR_WINDOWFRAME;
    wc.lpszMenuName = NULL;
    wc.lpszClassName = "Untitled";

    if (!RegisterClass(&wc)) {
        // Error registering class
        return -1;
    }

    if (!(hwnd = CreateWindow(wc.lpszClassName, wc.lpszClassName, WS_OVERLAPPEDWINDOW, 
                    CW_USEDEFAULT,CW_USEDEFAULT,500,300,NULL,NULL,hInst,NULL))) {
        // Error creating window
        return -1;
    }

    ShowWindow(hwnd,nCmdShow); 
    UpdateWindow(hwnd);

    RAWINPUTDEVICE Rid[1];

    Rid[0].usUsagePage = 0x01; 
    Rid[0].usUsage = 0x06; 
    Rid[0].dwFlags = 0;   // adds HID keyboard and also ignores legacy keyboard messages
    Rid[0].hwndTarget = 0;

    if (RegisterRawInputDevices(Rid, 1, sizeof(Rid[0])) == FALSE) {
        return -5;
    }

    MSG msg;
    while (GetMessage(&msg,NULL,0,0) > 0) 
    { 
        TranslateMessage(&msg); 
        DispatchMessage(&msg); 
    }
    return 0;
}

LRESULT CALLBACK WinProcedure(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
    static char buffer[5000] = "hi";

    if (msg == WM_PAINT) {
        PAINTSTRUCT ps; 
        HDC dc; 
        RECT r; 
        GetClientRect(hwnd,&r); 
        dc=BeginPaint(hwnd,&ps); 
        DrawText(dc,buffer,-1,&r,DT_SINGLELINE|DT_CENTER|DT_VCENTER); 
        EndPaint(hwnd,&ps);
        return 0;
    }

    if (msg == WM_INPUT) {
        strcpy(buffer, "Recieved input.");
        //assert(false);
        PostMessage(hwnd, WM_PAINT, wparam, lparam);
        return 0;
    }

    if (msg == WM_DESTROY) {
        PostQuitMessage(0);
        return 0;
    }

    return DefWindowProc(hwnd, msg, wparam, lparam);
}

Answer 1

If you take a look at the documentation for the WM_PAINT message, it says very clearly:

The WM_PAINT message is generated by the system and should not be sent
by an application.

If something doesn’t work the way you expect, consult the documentation first. There’s a reason why they exist. Use them to your advantage.

The reason why just posting WM_PAINT doesn’t work because there’s way more to painting than just sending a WM_PAINT message. The system sets up the proper structures to allow the application to paint on the window, among other things, for example. Sending WM_PAINT to a window is only a samll part of the entire painting process.

What you probably want is the InvalidateRect() function, which tells the operating system that you want to repaint a certain region of a window. The OS then does the proper painting procedure.