[iOS] Destroy the engine prior to application termination. (#29295)

Author: zhongwuzw

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -284,6 +284,11 @@ - (void)setupNotificationCenterObservers {
                  name:UIApplicationWillResignActiveNotification
                object:nil];
 
+  [center addObserver:self
+             selector:@selector(applicationWillTerminate:)
+                 name:UIApplicationWillTerminateNotification
+               object:nil];
+
   [center addObserver:self
              selector:@selector(applicationDidEnterBackground:)
                  name:UIApplicationDidEnterBackgroundNotification
@@ -816,6 +821,11 @@ - (void)applicationWillResignActive:(NSNotification*)notification {
   [self goToApplicationLifecycle:@"AppLifecycleState.inactive"];
 }
 
+- (void)applicationWillTerminate:(NSNotification*)notification {
+  [self goToApplicationLifecycle:@"AppLifecycleState.detached"];
+  [self.engine destroyContext];
+}
+
 - (void)applicationDidEnterBackground:(NSNotification*)notification {
   TRACE_EVENT0("flutter", "applicationDidEnterBackground");
   [self surfaceUpdated:NO];

shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

@@ -124,6 +124,8 @@ - (void)handlePressEvent:(FlutterUIPressProxy*)press
 - (void)scrollEvent:(UIPanGestureRecognizer*)recognizer;
 - (void)updateViewportMetrics;
 - (void)onUserSettingsChanged:(NSNotification*)notification;
+- (void)applicationWillTerminate:(NSNotification*)notification;
+- (void)goToApplicationLifecycle:(nonnull NSString*)state;
 - (void)keyboardWillChangeFrame:(NSNotification*)notification;
 - (void)startKeyBoardAnimation:(NSTimeInterval)duration;
 - (void)ensureViewportMetricsIsCorrect;
@@ -214,6 +216,20 @@ - (void)testViewDidDisappearDoesntPauseEngineWhenNotTheViewController {
   OCMReject([viewControllerMock surfaceUpdated:[OCMArg any]]);
 }
 
+- (void)testAppWillTerminateViewDidDestroyTheEngine {
+  FlutterEngine* mockEngine = OCMPartialMock([[FlutterEngine alloc] init]);
+  [mockEngine createShell:@"" libraryURI:@"" initialRoute:nil];
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:mockEngine
+                                                                                nibName:nil
+                                                                                 bundle:nil];
+  id viewControllerMock = OCMPartialMock(viewController);
+  OCMStub([viewControllerMock goToApplicationLifecycle:@"AppLifecycleState.detached"]);
+  OCMStub([mockEngine destroyContext]);
+  [viewController applicationWillTerminate:nil];
+  OCMVerify([viewControllerMock goToApplicationLifecycle:@"AppLifecycleState.detached"]);
+  OCMVerify([mockEngine destroyContext]);
+}
+
 - (void)testViewDidDisappearDoesPauseEngineWhenIsTheViewController {
   id lifecycleChannel = OCMClassMock([FlutterBasicMessageChannel class]);
   FlutterEnginePartialMock* mockEngine = [[FlutterEnginePartialMock alloc] init];