Indicate Which SDL Window Has Input Focus

Visually indicating which SDL window has input focus can be useful for improving user experience. You can achieve this by altering the window's appearance or adding visual cues. Here are a few approaches you can take:

One simple way to indicate focus is by changing the window title when it gains or loses focus:

1void HandleWindowEvent(
2  SDL_WindowEvent& E, SDL_Window* window) {
3  if (E.event == SDL_WINDOWEVENT_FOCUS_GAINED) {
4    SDL_SetWindowTitle(window, "Focused");  
5  } else if (E.event == SDL_WINDOWEVENT_FOCUS_LOST) {
6    SDL_SetWindowTitle(window, "Unfocused");  
7  }
8}

You can also change the window's border color or background color. However, SDL does not directly support changing the border color, so you might need to render a colored border inside the window itself:

1#include <SDL.h>
2
3class Window {
4public:
5  Window() {
6    SDL_Init(SDL_INIT_VIDEO);
7
8    SDLWindow = SDL_CreateWindow(
9      "SDL Window",
10      100, 100, 800, 600,
11      SDL_WINDOW_SHOWN
12    );
13
14    SDL_Renderer* renderer = SDL_CreateRenderer(
15      SDLWindow, -1, 0);
16    SetFocusRenderer(renderer);
17  }
18
19  ~Window() {
20    SDL_DestroyWindow(SDLWindow);
21    SDL_Quit();
22  }
23
24  void SetFocusRenderer(SDL_Renderer* renderer) {
25    this->renderer = renderer;
26  }
27
28  void RenderFocusIndicator(bool hasFocus) {
29    SDL_SetRenderDrawColor(
30      renderer,
31      hasFocus ? 0 : 255,
32      hasFocus ? 255 : 0,
33      0, 255
34    ); 
35    SDL_RenderClear(renderer);
36    SDL_RenderPresent(renderer);
37  }
38
39  SDL_Window* SDLWindow{nullptr};
40  SDL_Renderer* renderer{nullptr};
41};
42
43void HandleWindowEvent(
44  SDL_WindowEvent& E, Window& window) {
45  if (E.event == SDL_WINDOWEVENT_FOCUS_GAINED) {
46    window.RenderFocusIndicator(true); 
47  } else if (E.event == SDL_WINDOWEVENT_FOCUS_LOST) {
48    window.RenderFocusIndicator(false); 
49  }
50}
51
52int main(int argc, char** argv) {
53  Window GameWindow;
54  SDL_Event Event;
55
56  while (true) {
57    while (SDL_PollEvent(&Event)) {
58      if (Event.type == SDL_QUIT) {
59        return 0;
60      }
61      if (Event.type == SDL_WINDOWEVENT) {
62        HandleWindowEvent(
63          Event.window, GameWindow);
64      }
65    }
66  }
67  
68  return 0;
69}

In this example, the background color of the window changes based on whether it has input focus. When the window gains focus, the background color changes to green; when it loses focus, the color changes to red.

Another method is to overlay text or graphics indicating focus status. This can be achieved using SDL's rendering functions to draw text or images on top of your window's content.

SDL provides a hint, SDL_HINT_VIDEO_MINIMIZE_ON_FOCUS_LOSS, which, when set to "0", prevents SDL windows from being minimized when they lose focus. This can be combined with other visual indicators to provide clear feedback to the user.

1SDL_SetHint(SDL_HINT_VIDEO_MINIMIZE_ON_FOCUS_LOSS, "0");

• Change Window Title: Update the window title based on focus status.
• Change Background Color: Use rendering to change the window's background color.
• Overlay Text/Graphics: Draw focus indicators on top of the window's content.
• Use SDL Hints: Adjust SDL hints to manage focus behavior.

By implementing these techniques, you can effectively indicate which SDL window currently has input focus, enhancing the user experience.