[iOS] Fix random jitter in keyboard animation and correct spring animation target time.

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

@@ -34,6 +34,8 @@
 
 static constexpr int kMicrosecondsPerSecond = 1000 * 1000;
 static constexpr CGFloat kScrollViewContentSize = 2.0;
+static constexpr fml::TimeDelta kKeyboardAnimationUpdateViewportMetricsDelay =
+    fml::TimeDelta::FromMilliseconds(1);
 
 static NSString* const kFlutterRestorationStateAppData = @"FlutterRestorationStateAppData";
 
@@ -1661,15 +1663,19 @@ - (void)setupKeyboardAnimationVsyncClient {
         flutterViewController.get()->_viewportMetrics.physical_view_inset_bottom =
             flutterViewController.get()
                 .keyboardAnimationView.layer.presentationLayer.frame.origin.y;
-        [flutterViewController updateViewportMetricsIfNeeded];
+        [flutterViewController keyboardAnimationDelayUpdateViewportMetrics];
       }
     } else {
-      fml::TimeDelta timeElapsed = recorder.get()->GetVsyncTargetTime() -
+      // Because updateViewportMetrics will work in next frame and the frame will
+      // present in next next frame, so here should add a time of one frame interval
+      // to get correct animation target time.
+      fml::TimeDelta frameInterval = recorder->GetVsyncTargetTime() - recorder->GetVsyncStartTime();
+      fml::TimeDelta timeElapsed = recorder.get()->GetVsyncTargetTime() + frameInterval -
                                    flutterViewController.get().keyboardAnimationStartTime;
 
       flutterViewController.get()->_viewportMetrics.physical_view_inset_bottom =
           [[flutterViewController keyboardSpringAnimation] curveFunction:timeElapsed.ToSecondsF()];
-      [flutterViewController updateViewportMetricsIfNeeded];
+      [flutterViewController keyboardAnimationDelayUpdateViewportMetrics];
     }
   };
   flutter::Shell& shell = [_engine.get() shell];
@@ -1682,6 +1688,29 @@ - (void)setupKeyboardAnimationVsyncClient {
   [_keyboardAnimationVSyncClient await];
 }
 
+- (void)keyboardAnimationDelayUpdateViewportMetrics {
+  // Because the keyboard animation is driven by vsync callback in platform
+  // thread, and it is extremely closed to vsync callback in UI thread, so it maybe
+  // run before the vsync process callback in UI thread.
+  // And if this happens, will cause jitter behavior in rendering, so adding a
+  // tiny delay to keep the updateViewportMetrics signal run after the vsync
+  // process callback in UI thread to keep things smooth.
+  flutter::Shell& shell = [_engine.get() shell];
+  shell.GetTaskRunners().GetPlatformTaskRunner()->PostDelayedTask(
+      [weakSelf = [self getWeakPtr]] {
+        if (!weakSelf) {
+          return;
+        }
+        fml::scoped_nsobject<FlutterViewController> flutterViewController(
+            [(FlutterViewController*)weakSelf.get() retain]);
+        if (!flutterViewController) {
+          return;
+        }
+        [flutterViewController updateViewportMetricsIfNeeded];
+      },
+      kKeyboardAnimationUpdateViewportMetricsDelay);
+}
+
 - (void)invalidateKeyboardAnimationVSyncClient {
   [_keyboardAnimationVSyncClient invalidate];
   [_keyboardAnimationVSyncClient release];

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

@@ -139,6 +139,7 @@ - (void)setupKeyboardAnimationVsyncClient;
 - (UIView*)keyboardAnimationView;
 - (SpringAnimation*)keyboardSpringAnimation;
 - (void)setupKeyboardSpringAnimationIfNeeded:(CAAnimation*)keyboardAnimation;
+- (void)keyboardAnimationDelayUpdateViewportMetrics;
 - (void)ensureViewportMetricsIsCorrect;
 - (void)invalidateKeyboardAnimationVSyncClient;
 - (void)addInternalPlugins;
@@ -185,6 +186,26 @@ - (id)setupMockMainScreenAndView:(FlutterViewController*)viewControllerMock
   return mockView;
 }
 
+- (void)testKeyboardAnimationDelayUpdateViewportMetricsWillWorkCorrectly {
+  FlutterEngine* mockEngine = OCMPartialMock([[FlutterEngine alloc] init]);
+  [mockEngine createShell:@"" libraryURI:@"" initialRoute:nil];
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:mockEngine
+                                                                                nibName:nil
+                                                                                 bundle:nil];
+  FlutterViewController* viewControllerMock = OCMPartialMock(viewController);
+  CGRect viewFrame = UIScreen.mainScreen.bounds;
+  [self setupMockMainScreenAndView:viewControllerMock viewFrame:viewFrame convertedFrame:viewFrame];
+
+  [viewControllerMock keyboardAnimationDelayUpdateViewportMetrics];
+
+  // Expect the updateViewportMetrics will invoke after some time.
+  XCTestExpectation* expectation = [self expectationWithDescription:@"delay update viewport"];
+  OCMStub([viewControllerMock updateViewportMetricsIfNeeded]).andDo(^(NSInvocation* invocation) {
+    [expectation fulfill];
+  });
+  [self waitForExpectationsWithTimeout:5.0 handler:nil];
+}
+
 - (void)testViewDidLoadWillInvokeCreateTouchRateCorrectionVSyncClient {
   FlutterEngine* engine = [[FlutterEngine alloc] init];
   [engine runWithEntrypoint:nil];