use mainScreenIfViewLoaded

Author: hellohuanlin

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

@@ -1370,12 +1370,12 @@ - (void)viewSafeAreaInsetsDidChange {
 
 // Set _viewportMetrics physical size.
 - (void)setViewportMetricsSize {
-  // TODO(hellohuanlin): Use [self mainScreenIfViewLoaded] instead of [UIScreen mainScreen].
-  // This requires adding the view to window during unit tests, which calls multiple engine calls
-  // that is hard to mock since they take/return structs. An alternative approach is to partial mock
-  // the FlutterViewController to make view controller life cycle methods no-op, and insert
-  // this mock into the responder chain.
-  CGFloat scale = [UIScreen mainScreen].scale;
+  UIScreen* mainScreen = [self mainScreenIfViewLoaded];
+  if (!mainScreen) {
+    return;
+  }
+
+  CGFloat scale = mainScreen.scale;
   _viewportMetrics.physical_width = self.view.bounds.size.width * scale;
   _viewportMetrics.physical_height = self.view.bounds.size.height * scale;
 }
@@ -1384,7 +1384,12 @@ - (void)setViewportMetricsSize {
 //
 // Viewport paddings represent the iOS safe area insets.
 - (void)setViewportMetricsPaddings {
-  CGFloat scale = [UIScreen mainScreen].scale;
+  UIScreen* mainScreen = [self mainScreenIfViewLoaded];
+  if (!mainScreen) {
+    return;
+  }
+
+  CGFloat scale = mainScreen.scale;
   _viewportMetrics.physical_padding_top = self.view.safeAreaInsets.top * scale;
   _viewportMetrics.physical_padding_left = self.view.safeAreaInsets.left * scale;
   _viewportMetrics.physical_padding_right = self.view.safeAreaInsets.right * scale;

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

@@ -871,6 +871,8 @@ - (void)testUpdateViewportMetricsIfNeeded_DoesNotInvokeEngineWhenShouldBeIgnored
   FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:mockEngine
                                                                                 nibName:nil
                                                                                  bundle:nil];
+  FlutterViewController* viewControllerMock = OCMPartialMock(viewController);
+  OCMStub([viewControllerMock mainScreenIfViewLoaded]).andReturn(UIScreen.mainScreen);
   mockEngine.viewController = viewController;
 
   id mockCoordinator = OCMProtocolMock(@protocol(UIViewControllerTransitionCoordinator));
@@ -890,6 +892,8 @@ - (void)testViewWillTransitionToSize_DoesDelayEngineCallIfNonZeroDuration {
   FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:mockEngine
                                                                                 nibName:nil
                                                                                  bundle:nil];
+  FlutterViewController* viewControllerMock = OCMPartialMock(viewController);
+  OCMStub([viewControllerMock mainScreenIfViewLoaded]).andReturn(UIScreen.mainScreen);
   mockEngine.viewController = viewController;
 
   // Mimic the device rotation with non-zero transition duration.
@@ -921,6 +925,8 @@ - (void)testViewWillTransitionToSize_DoesNotDelayEngineCallIfZeroDuration {
   FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:mockEngine
                                                                                 nibName:nil
                                                                                  bundle:nil];
+  FlutterViewController* viewControllerMock = OCMPartialMock(viewController);
+  OCMStub([viewControllerMock mainScreenIfViewLoaded]).andReturn(UIScreen.mainScreen);
   mockEngine.viewController = viewController;
 
   // Mimic the device rotation with zero transition duration.