Don't render in background (#7486)

ios/samples/gltf-viewer/gltf-viewer/FILViewController.mm

@@ -47,6 +47,9 @@ const float kToastDelayDuration = 2.0f;
 - (void)createRenderables;
 - (void)createLights;
 
+- (void)appWillResignActive:(NSNotification*)notification;
+- (void)appDidBecomeActive:(NSNotification*)notification;
+
 @end
 
 @implementation FILViewController {
@@ -73,6 +76,16 @@ const float kToastDelayDuration = 2.0f;
 
     self.title = @"https://google.github.io/filament/remote";
 
+    // Observe lifecycle notifications to prevent us from rendering in the background.
+    [NSNotificationCenter.defaultCenter addObserver:self
+                                           selector:@selector(appWillResignActive:)
+                                               name:UIApplicationWillResignActiveNotification
+                                             object:nil];
+    [NSNotificationCenter.defaultCenter addObserver:self
+                                           selector:@selector(appDidBecomeActive:)
+                                               name:UIApplicationDidBecomeActiveNotification
+                                             object:nil];
+
     // Arguments:
     // --model <path>
     //     path to glb or gltf file to load from documents directory
@@ -125,6 +138,14 @@ const float kToastDelayDuration = 2.0f;
     [self.view addSubview:_toastLabel];
 }
 
+- (void)appWillResignActive:(NSNotification*)notification {
+    [self stopDisplayLink];
+}
+
+- (void)appDidBecomeActive:(NSNotification*)notification {
+    [self startDisplayLink];
+}
+
 - (void)viewWillAppear:(BOOL)animated {
     [self startDisplayLink];
 }
@@ -330,6 +351,7 @@ const float kToastDelayDuration = 2.0f;
 }
 
 - (void)dealloc {
+    [NSNotificationCenter.defaultCenter removeObserver:self];
     delete _server;
     delete _automation;
     self.modelView.engine->destroy(_indirectLight);