Does the SDL2 Application Loop Stop When the Window Loses Focus?

No, the main application while loop does not stop automatically just because the SDL window loses focus (i.e., the user clicks on another application's window).

Your C++ while (shouldContinue) loop continues executing its iterations regardless of the window's focus state by default. It will keep processing events (though fewer might occur), updating state, and rendering, even if the output isn't visible or is obscured by another window.

Detecting Focus Changes

However, SDL does generate events when the window gains or loses focus. You can detect these events within your event processing loop and choose to alter your application's behaviour accordingly.

The relevant event types are part of the SDL_WINDOWEVENT group:

• SDL_WINDOWEVENT_FOCUS_GAINED: The window has received keyboard and mouse focus.
• SDL_WINDOWEVENT_FOCUS_LOST: The window has lost keyboard and mouse focus.

You check for these like this:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h" // Assume this includes our Window class
4
5int main(int argc, char** argv) {
6  SDL_Init(SDL_INIT_VIDEO);
7  Window GameWindow;
8
9  bool shouldContinue{true};
10  bool hasFocus{true}; // Assume window starts with focus
11  SDL_Event Event;
12
13  while (shouldContinue) {
14    while (SDL_PollEvent(&Event)) {
15       if (Event.type == SDL_QUIT) {
16          shouldContinue = false;
17       }
18       // Check for window events
19       else if (Event.type == SDL_WINDOWEVENT) { 
20         switch (Event.window.event) {
21          case SDL_WINDOWEVENT_FOCUS_GAINED: 
22            hasFocus = true;
23            std::cout << "Window gained focus\\n";
24            break;
25          case SDL_WINDOWEVENT_FOCUS_LOST: 
26            hasFocus = false;
27            std::cout << "Window lost focus\\n";
28            break;
29         }
30       }
31       // ... handle other events like keyboard/mouse ...
32    }
33
34    // --- Application Logic Control ---
35    if (hasFocus) {
36      // Only update or perform certain actions if focused
37      // UpdateGameLogic();
38      // RenderActiveScene();
39    } else {
40      // Optionally perform different actions when not focused
41      // RenderPausedScreen();
42      // Reduce update frequency?
43    }
44
45    // We still need to manage loop speed, regardless of focus
46    SDL_Delay(16); // Example delay
47  }
48
49  SDL_Quit();
50  return 0;
51}

Controlling Behaviour Based on Focus

Many applications change their behaviour when they lose focus:

• Pause Gameplay: Most games pause automatically when the window loses focus. You can achieve this by skipping your game state update logic when hasFocus is false.
• Mute Audio: Games often mute or significantly lower audio volume when not in focus.
• Reduce Rendering/Update Rate: To save resources, you might significantly slow down or stop rendering updates when the window isn't focused or visible. You could introduce a much longer SDL_Delay() when hasFocus is false.
• Different Visuals: You might render a "Paused" overlay or a simplified graphic when out of focus.

So, while the loop itself doesn't stop, SDL provides the tools (SDL_WINDOWEVENT_FOCUS_LOST/GAINED) to detect focus changes, allowing you to intelligently control your application's activity based on whether the user is actively interacting with it.