Implement pause on focus lost for SDL frontend

src/citra/citra.cpp

@@ -150,6 +150,10 @@ int main(int argc, char **argv) {
 
     while (emu_window->IsOpen()) {
         Core::RunLoop();
+
+        // Pause on focus lost
+        if (!emu_window->HasFocus())
+            emu_window->WaitForFocus();
     }
 
     return 0;

src/citra/emu_window/emu_window_sdl2.cpp

@@ -50,6 +50,10 @@ bool EmuWindow_SDL2::IsOpen() const {
     return is_open;
 }
 
+bool EmuWindow_SDL2::HasFocus() const {
+    return has_focus;
+}
+
 void EmuWindow_SDL2::OnResize() {
     int width, height;
 
@@ -58,6 +62,24 @@ void EmuWindow_SDL2::OnResize() {
     NotifyFramebufferLayoutChanged(EmuWindow::FramebufferLayout::DefaultScreenLayout(width, height));
 }
 
+void EmuWindow_SDL2::WaitForFocus() {
+    while(!has_focus) {
+        SDL_Event event;
+        SDL_WaitEvent(&event);
+        if(event.type == SDL_WINDOWEVENT) {
+            switch (event.window.event) {
+            case SDL_WINDOWEVENT_FOCUS_GAINED:
+                has_focus=true;
+                break;
+            case SDL_WINDOWEVENT_CLOSE:
+                has_focus=true;
+                is_open = false;
+                break;
+            }
+        }
+    }
+}
+
 EmuWindow_SDL2::EmuWindow_SDL2() {
     keyboard_id = KeyMap::NewDeviceId();
 
@@ -136,6 +158,12 @@ void EmuWindow_SDL2::PollEvents() {
             case SDL_WINDOWEVENT_MINIMIZED:
                 OnResize();
                 break;
+            case SDL_WINDOWEVENT_FOCUS_GAINED:
+                has_focus = true;
+                break;
+            case SDL_WINDOWEVENT_FOCUS_LOST:
+                has_focus = false;
+                break;
             case SDL_WINDOWEVENT_CLOSE:
                 is_open = false;
                 break;

src/citra/emu_window/emu_window_sdl2.h

@@ -30,6 +30,12 @@ public:
     /// Whether the window is still open, and a close request hasn't yet been sent
     bool IsOpen() const;
 
+    /// Whether the window has focus
+    bool HasFocus() const;
+
+    /// Wait for the window to gain focus
+    void WaitForFocus();
+
     /// Load keymap from configuration
     void ReloadSetKeymaps() override;
 
@@ -52,6 +58,9 @@ private:
     /// Is the window still open?
     bool is_open = true;
 
+    /// Is the window has focus
+    bool has_focus = true;
+
     /// Internal SDL2 render window
     SDL_Window* render_window;