Managing Mouse Focus with SDL2

SDL also provides the SDL_GetMouseFocus() function, which returns the SDL_Window* with mouse focus, or a nullptr if no window has focus.

This example is very similar to the previous one, except we've updated our implementation to use SDL_GetMouseFocus() instead of SDL_GetWindowFlags().

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5Uint32 MonitorWindow(Uint32 Timer, void* Win) {
6  auto* Window{static_cast<SDL_Window*>(Win)};
7
8  if (Window == SDL_GetMouseFocus()) {
9    std::cout << "Focus\n";
10  } else {
11    std::cout << "No Focus\n";
12  }
13  return Timer;
14}
15
main Function|Show
16int main(int argc, char** argv) {/*...*/}

1No Focus
2Focus
3Focus

Whilst the SDL_GetWindowFlags() example lets us determine if a specific window has focus, SDL_GetMouseFocus() is primarily useful when we want to know which window has focus. This is typically used in applications where we manage multiple windows, including utility windows.

The following program creates two windows and, on every iteration of our application loop, reports which one of them has mouse focus:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5int main(int argc, char** argv) {
6  SDL_Init(SDL_INIT_VIDEO);
7  Window Window1;
8  Window Window2;
9
10  SDL_Event E;
11  while (true) {
12    while (SDL_PollEvent(&E)) {
13      // ...
14    }
15
16    SDL_Window* Focused{SDL_GetMouseFocus()};
17
18    if (Focused == Window1.SDLWindow) {
19      std::cout << "Window 1 has focus\n";
20    } else if (Focused == Window2.SDLWindow) {
21      std::cout << "Window 2 has focus\n";
22    } else {
23      std::cout << "Neither has focus\n";
24    }
25
26    Window1.Update();
27    Window2.Update();
28  }
29
30  SDL_Quit();
31  return 0;
32}

1Window 1 has focus
2Window 1 has focus
3Neither has focus
4Window 2 has focus

The most common way a user will switch their input focus is by left-clicking on the window with their mouse. By default, SDL does not report these clicks - you won't get a mouse click event when the user clicks to set input focus to your window.

This is typically a sensible default for games. For example, if we're working on a first-person shooter, we want players to be able to switch focus to our window by clicking on it, and we won't want that click to also fire their weapon.

However, we can change this behavior by setting the SDL_HINT_MOUSE_FOCUS_CLICKTHROUGH hint to "1":

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5
6int main(int argc, char** argv) {
7  SDL_SetHint(
8    SDL_HINT_MOUSE_FOCUS_CLICKTHROUGH,
9    "1"
10  );
11  SDL_Init(SDL_INIT_VIDEO);
12  Window GameWindow;
13
14  SDL_Event E;
15
16  while (true) {
17    while (SDL_PollEvent(&E)) {
18      switch (E.type) {
19      case SDL_WINDOWEVENT:
20        if (E.window.event ==
21          SDL_WINDOWEVENT_FOCUS_GAINED) {
22          std::cout << "Input Focus Event\n";
23        }
24        break;
25
26      case SDL_MOUSEBUTTONDOWN:
27        if (E.button.button ==
28          SDL_BUTTON_LEFT) {
29          std::cout << "Also a Click Event\n";
30        }
31        break;
32
33      default:
34        break;
35      }
36    }
37    GameWindow.Update();
38  }
39
40  SDL_Quit();
41  return 0;
42}

1Input Focus Event
2Also a Click Event

We can set it to "0" to restore the default behavior:

1SDL_SetHint(
2  SDL_HINT_MOUSE_FOCUS_CLICKTHROUGH,
3  "0"
4);

Mouse focus management is crucial for creating responsive multi-window applications in SDL2, allowing you to track which window the mouse is hovering over and respond appropriately. Here are the key topics covered:

• Using SDL_WindowEvent to detect when the mouse enters or leaves a window
• Checking window focus state using SDL_GetWindowFlags() and the SDL_WINDOW_MOUSE_FOCUS flag
• Using SDL_GetMouseFocus() to determine which window currently has mouse focus
• Customizing click-through behavior during focus changes with SDL_HINT_MOUSE_FOCUS_CLICKTHROUGH