Prevent SDL from rendering when app is backgrounded

On some platforms, the game will crash if SDL attempts to render to the
screen/window while the game is backgrounded. To prevent this, stop
rendering when the game enters a background state and resume rendering
when the game enters a foreground state.

game/sdl/display.cpp

@@ -89,6 +89,7 @@ bool Display::Init()
     }
 
     renderer = SDL_CreateRenderer(window, 0, SDL_RENDERER_TARGETTEXTURE);
+    this->SetShouldRender(true);
 
     // Keep the screen size around
     s_siz.cx = cxScreen;
@@ -269,6 +270,9 @@ void Display::FrameComplete(int cfrmm, UpdateMap **apupd, Rect *arc,
 }
 
 void Display::RenderGameSurface() {
+    if (!m_fshouldRender)
+        return;
+
     // Create the texture
     texture = SDL_CreateTextureFromSurface(renderer, m_gameSurface);
 
@@ -311,4 +315,8 @@ float Display::Density() {
     return m_density;
 }
 
+void Display::SetShouldRender(bool fsr) {
+    m_fshouldRender = fsr;
+}
+
 } // namespace wi

game/sdl/host.cpp

@@ -220,6 +220,25 @@ bool ProcessSdlEvent(Event *pevt)
         }
         break;
 
+    case SDL_APP_DIDENTERFOREGROUND:
+        // Allow the display to render
+        gpdisp->SetShouldRender(true);
+
+        // SDL may have released its graphics context if the app was previously
+        // backgrounded. This leaves the screen black when the user returns.
+        // Hack: Draw dib and render
+        gpmfrmm->DrawFrame(true);
+        gpdisp->RenderGameSurface();
+
+        break;
+
+    case SDL_APP_WILLENTERBACKGROUND:
+        // Stop display rendering; SDL may release its graphics context when
+        // backgrounded, so we don't want to try to render to a non-existant context.
+        gpdisp->SetShouldRender(false);
+
+        break;
+
 	case SDL_QUIT:
         pevt->eType = appStopEvent;
         break;

game/sdl/htplatform.h

@@ -123,6 +123,7 @@ public:
     void SetFormMgrs(FormMgr *pfrmmSim, FormMgr *pfrmmInput);
     void RenderGameSurface();
     float Density();
+    void SetShouldRender(bool fsr);
 
 private:
     int m_imode;
@@ -145,6 +146,7 @@ private:
     Uint32 m_32bppColors[256];
     int m_pixelCount;
     float m_density;
+    bool m_fshouldRender;
 };
 
 #define kfDtClearLine 1

game/sdl/mac/AppDelegate.mm

@@ -31,11 +31,11 @@
 // as they do something else.
 
 - (void)applicationDidBecomeActive:(NSNotification *)notification {
-    // See host.cpp case SDL_APP_DIDENTERFOREGROUND
+    wi::HostAppDidEnterForeground();
 }
 
 - (void)applicationWillResignActive:(NSNotification *)notification {
-    // See host.cpp case SDL_APP_WILLENTERBACKGROUND
+    wi::HostAppWillEnterBackground();
 }
 #endif