DLL and drawing function disappearing - c++

Question

main.cpp (dll) http://pastebin.com/Z811YSz7
Funkcje.h http://pastebin.com/siZrUBh0
When i injected this dll to a game then i dont see letters.
When i maked drawing function in a while(1) then the letters are appearing and disappearing.
How to make the drawing function to be on the top of this game all the time.
#include <windows.h>
#include <vector>
#include "Funkcje.h"
#include <cstdio>
#include <string>
BOOL WINAPI DllMain(
HINSTANCE hinstDLL,
DWORD fdwreason,
LPVOID lpReversed)
{
switch (fdwreason)
{
case DLL_PROCESS_ATTACH:
PAINTSTRUCT ps;
HDC hdc;
for (size_t i = 0; i < getToplevelWindows().size(); i++){
hdc = GetDC(getToplevelWindows()[i]);
TextOut(hdc, 150, 150, L"TEST", 4);
ReleaseDC(getToplevelWindows()[i], hdc);
}
break;
}
return TRUE;
}

Answer 1

DllMain is only called once, when the DLL is loaded. This is not where you should be putting your code.
In order to properly "inject" code into a process (via DLL or any other mean) you must first determine what function or event in that process you want to hook.
In this case, I suppose you want to hook the window event WM_PAINT, a callback from the system sent to the window's procedure telling it that it should repaint its content for whatever reason. (This may not be always correct though, because a program can use other methods of updating a window content.)
So what you are going to do is find the window handle you want to hook, and replace its callback procedure with your own:
WNDPROC originalProc = SetWindowLongPtr(hWnd, GWLP_WNDPROC, (LONG_PTR)myHookProc);
But of course, not before having myHookProc defined and doing its job of hooking WM_PAINT:
LRESULT CALLBACK myHookProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
LRESULT originalResult = originalProc(hWnd, uMsg, wParam, lParam); // call original first
if (uMsg == WM_PAINT)
{
PAINTSTRUCT ps;
HDC hdc = GetDC(hWnd);
TextOut(hdc, 150, 150, L"TEST", 4);
ReleaseDC(hWnd, hdc);
}
return originalResult;
}
If you are dealing with multiple windows, you may want to use a std::unordered_map<HWND, WNDPROC> to keep relation between each window handle and its original procedure.