如何使 WM_PAINT 从 wndproc 外部运行

Question

I apologize if this is a dumb question, im just starting to learn winapi. here is the code in concern:

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
   

    WCHAR greeting[] = _T("line1");
    WCHAR greeting1[] = _T("another line");


    switch (message)
    {
    case WM_COMMAND:
        {
            int wmId = LOWORD(wParam);
            // Parse the menu selections:
            switch (wmId)
            {
            case IDM_ABOUT:
                DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
            
                break;
            case IDM_EXIT:
                DestroyWindow(hWnd);
                break;
         
            
            default:
                return DefWindowProc(hWnd, message, wParam, lParam);
            }
        }
        break;


    case WM_PAINT:
        {
        repaint = false;
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hWnd, &ps);

        switch (pageID) {

        case 1: {//display first string

            
            TextOut(hdc, 5, 5, greeting, _tcslen(greeting));
            break;

        }
        case 2: {//display the other string
            TextOut(hdc, 5, 100, greeting1, _tcslen(greeting1));
            break;
        }


        }
            EndPaint(hWnd, &ps);
        }
        break;


    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}
/*****************************************************************************/
// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
            pageID = 1;
            
            repaint = true;
            
            
        }
        if (LOWORD(wParam) == IDC_BUTTON1) {
            pageID = 2;
            return (INT_PTR)TRUE;

        }
        break;
    }
    return (INT_PTR)FALSE;
}

I apologize for the seemingly random variables that don't do anything, I was just trying to make this work any way possible, but no dice.

What I am trying to do, is to have one of the textouts run after a button is pressed in a dialog box. Unfortunately, the text isn't displayed until wndproc runs next time. UpdateWindow is useless, because I need to pass hWnd to it, but I can't do that when Im not in wndproc. Need a quick tip on how this should be done. Im sure everyone does it every day, just not obvious for me... Thanks!

Answer 1

Basically use InvalidateRect , but this comment

UpdateWindow is useless, because I need to pass hWnd to it, but I can't do that when Im not in wndproc.

exhibits kind of a fundamental misunderstanding of how Win32 programming works. You always need a window handle to a window you are working with. You get window handles when you create windows; you need to store those somewhere.

Anyway below is a minimal version of using buttons on one window to drive repainting of another window.

#include <windows.h>

HINSTANCE g_hInstance = 0;

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);
LRESULT CALLBACK SomeOtherWndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);

const int kBtn1ID = 101;
const int kBtn2ID = 102;
int pageID = 1;
HWND g_other_hwnd = NULL;

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{

    MSG msg = { 0 };
    WNDCLASS wc = { 0 };
    wc.lpfnWndProc = WndProc;
    wc.hInstance = hInstance;
    wc.hbrBackground = (HBRUSH)(COLOR_BACKGROUND);
    wc.lpszClassName = L"foobar";
    if (!RegisterClass(&wc))
        return 1;

    wc.lpszClassName = L"some_other_window";
    wc.lpfnWndProc = SomeOtherWndProc;
    if (!RegisterClass(&wc))
        return 1;

    g_hInstance = hInstance;

    if (!CreateWindow(L"foobar",
        L"foobar",
        WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        0, 0, 640, 480, 0, 0, hInstance, NULL))
        return 2;

    if (!(g_other_hwnd = CreateWindow(L"some_other_window",
        L"quux",
        WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        0, 0, 640, 480, 0, 0, hInstance, NULL)))
        return 2;

    while (GetMessage(&msg, NULL, 0, 0) > 0)
        DispatchMessage(&msg);

    return 0;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
    case WM_CREATE:
        CreateWindow(L"button", L"some button", BS_PUSHBUTTON | WS_VISIBLE | WS_CHILD, 10, 10, 150, 35, hWnd, (HMENU)kBtn1ID, g_hInstance, 0);
        CreateWindow(L"button", L"some other button", BS_PUSHBUTTON | WS_VISIBLE | WS_CHILD, 10, 55, 150, 35, hWnd, (HMENU)kBtn2ID, g_hInstance, 0);
        break;
    case WM_COMMAND: {
        int btn_id = LOWORD(wParam);
        switch (btn_id) {
        case kBtn1ID:
            pageID = 1;
            InvalidateRect(g_other_hwnd, NULL, TRUE);
            break;

        case kBtn2ID:
            pageID = 2;
            InvalidateRect(g_other_hwnd, NULL, TRUE);
            break;
        }
    }
    break;

    case WM_CLOSE:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;

}

LRESULT CALLBACK SomeOtherWndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{

    switch (message)
    {
    case WM_PAINT:
    {
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hWnd, &ps);

        static auto greeting = L"hello there.";
        static auto greeting1 = L"goodbye.";

        switch (pageID) {

        case 1: {//display first string


            TextOut(hdc, 5, 5, greeting, lstrlenW(greeting));
            break;

        }
        case 2: {//display the other string
            TextOut(hdc, 5, 100, greeting1, lstrlenW(greeting1));
            break;
        }

        }
        EndPaint(hWnd, &ps);
    }
    return 0;

    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;

}