Question

I asked my question yesterday but could not get a correct answer. now ill ask it more clearly.
using win api I created a window and a button. infront of the button there is a circle drawn in green color. once the button is pressed the circle should turn to red color.

(I’m using C++, and mingw compiler.)

here is my code

LRESULT CALLBACK WinProc(HWND hWnd,UINT msg,WPARAM wParam,LPARAM lParam)
{
 PAINTSTRUCT ps;
 HDC hDC;
 HBRUSH brusha;
 brusha=CreateSolidBrush(RGB(0,255,0));

 switch(msg)
{
    case WM_CREATE:
    {

        b=CreateWindowEx(WS_EX_CLIENTEDGE,
            "BUTTON",
            "red",
            WS_CHILD|WS_VISIBLE|
            BS_DEFPUSHBUTTON,
            350,
            100,
            100,
            40,
            hWnd,
            (HMENU)BUTTON,
            GetModuleHandle(NULL),
            NULL);
    }
    break;


    case WM_PAINT:
    {


    hDC=BeginPaint(hWnd,&ps);
    SelectObject(hDC,brusha);
    Ellipse(hDC, 20, 20, 100, 100);
    EndPaint(hWnd, &ps);

    }

    case WM_COMMAND:
      switch(LOWORD(wParam))
        {
            case BUTTON:
            {
            brusha=CreatSolideBrush(RGB(255,0,0));
            InvalidateRect( hWnd,0,false);

            }
        }
  break;


      case WM_DESTROY:
    {
        PostQuitMessage(0);
        return 0;
    }
    break;
}

return DefWindowProc(hWnd,msg,wParam,lParam);

  }

this compiles without errors. but nothing happens on button click

Answer 1

Well the problem is that you you always draw with a green brush. Look at your code

HBRUSH brusha;
brusha=CreateSolidBrush(RGB(0,255,0));

switch(msg)
{
    ...
    case WM_PAINT:
    {
    hDC=BeginPaint(hWnd,&ps);
    SelectObject(hDC,brusha);
    ...

In this code brusha is always going to be a green brush when you go into WM_PAINT.

You seem to think that just because you assign a red brush to the brusha variable in the WM_COMMAND part that somehow that is going to be remembered for the next paint, but that’s not true. Remember that in C++ variables are created afresh each time you enter a function, and destroyed each time you exit a function. So the way you have written the code cannot work.

Probably the simplest way is to make the hbrusha variable static. Static variables are not created and destroyed each time you enter and exit a funciton. Something like this

LRESULT CALLBACK WinProc(HWND hWnd,UINT msg,WPARAM wParam,LPARAM lParam)
{
    static HBRUSH brusha;

    switch(msg)
    {
    case WM_CREATE:
        brusha=CreateSolidBrush(RGB(0,255,0)); // set the brush green on create

        ...

        break;


    case WM_COMMAND:
        switch(LOWORD(wParam))
        {
        case BUTTON:
            brusha=CreatSolideBrush(RGB(255,0,0));
            InvalidateRect( hWnd,0,false);
        }
        break;

        ...

    }

    ...
}