Make it possible for iOS largeTitle to collapse while scrolling. (#202)

iOS navbar largeTitle setting allows the navbar to be automatically collapsed when scrolling. For that to work iOS expects scrollable component to be rendered in the screen view hierarchy as a first children (or just to be looked up on the ancestor path using first children when navigating down). On top of that in RN we should use contentInsetAdjustmentBehavior="automatic" for SV's contentInsets to adjust as expected when the header scrolls and to allow for the SV background to be present below the navigation bar.

This works pretty well w/o any changes. However when testing I disovered one issue with the navigation bar rendering in a collapsed state on the first render. After that you could've scroll down to reveal large title bar but the initial state would be collapsed. As on iOS it is expected for large title enabled screens to initially render in the revealed state a workaround was required. It turned out that the header rendered in collapsed state because at the moment when navigation controller is created is has an empty list of view controllers. The list is empty because react first creates nnavigation controller and only then adds children to it. It turned out the fix was to populate viewControllers with a single item at first, then when controllers list is replaced with the children provided from react the header renders properly in the revealed state.

ios/RNSScreen.h

@@ -26,6 +26,13 @@ typedef NS_ENUM(NSInteger, RNSScreenStackAnimation) {
 
 @end
 
+@interface RNSScreen : UIViewController
+
+- (instancetype)initWithView:(UIView *)view;
+- (void)notifyFinishTransitioning;
+
+@end
+
 @interface RNSScreenManager : RCTViewManager
 @end
 

ios/RNSScreen.m

@@ -35,13 +35,6 @@
 
 @end
 
-@interface RNSScreen : UIViewController
-
-- (instancetype)initWithView:(UIView *)view;
-- (void)notifyFinishTransitioning;
-
-@end
-
 @interface RNSScreenView () <UIAdaptivePresentationControllerDelegate>
 @end
 

ios/RNSScreenStack.m

@@ -37,6 +37,12 @@
     _needUpdate = NO;
     [self addSubview:_controller.view];
     _controller.interactivePopGestureRecognizer.delegate = self;
+
+    // we have to initialize viewControllers with a non empty array for
+    // largeTitle header to render in the opened state. If it is empty
+    // the header will render in collapsed state which is perhaps a bug
+    // in UIKit but ¯\_(ツ)_/¯
+    [_controller setViewControllers:@[[UIViewController new]]];
   }
   return self;
 }
@@ -60,7 +66,6 @@
     if ([viewController isEqual:[_reactSubviews objectAtIndex:i - 1].controller]) {
       break;
     } else {
-      // TODO: send dismiss event
       [_dismissedScreens addObject:[_reactSubviews objectAtIndex:i - 1]];
     }
   }
@@ -94,23 +99,6 @@
   return _controller.viewControllers.count > 1;
 }
 
-- (void)markUpdated
-{
-  // We want 'updateContainer' to be executed on main thread after all enqueued operations in
-  // uimanager are complete. In order to achieve that we enqueue call on UIManagerQueue from which
-  // we enqueue call on the main queue. This seems to be working ok in all the cases I've tried but
-  // there is a chance it is not the correct way to do that.
-  if (!_needUpdate) {
-    _needUpdate = YES;
-    RCTExecuteOnUIManagerQueue(^{
-      RCTExecuteOnMainQueue(^{
-        _needUpdate = NO;
-        [self updateContainer];
-      });
-    });
-  }
-}
-
 - (void)markChildUpdated
 {
   // do nothing
@@ -128,14 +116,12 @@
     return;
   }
   [_reactSubviews insertObject:subview atIndex:atIndex];
-  [self markUpdated];
 }
 
 - (void)removeReactSubview:(RNSScreenView *)subview
 {
   [_reactSubviews removeObject:subview];
   [_dismissedScreens removeObject:subview];
-  [self markUpdated];
 }
 
 - (NSArray<UIView *> *)reactSubviews
@@ -146,6 +132,7 @@
 - (void)didUpdateReactSubviews
 {
   // do nothing
+  [self updateContainer];
 }
 
 - (void)setModalViewControllers:(NSArray<UIViewController *> *)controllers
@@ -188,9 +175,15 @@
   UIViewController *top = controllers.lastObject;
   UIViewController *lastTop = _controller.viewControllers.lastObject;
 
-  BOOL shouldAnimate = ((RNSScreenView *) lastTop.view).stackAnimation != RNSScreenStackAnimationNone;
+  // at the start we set viewControllers to contain a single UIVIewController
+  // instance. This is a workaround for header height adjustment bug (see comment
+  // in the init function). Here, we need to detect if the initial empty
+  // controller is still there
+  BOOL firstTimePush = ![lastTop isKindOfClass:[RNSScreen class]];
 
-  if (_controller.viewControllers.count == 0) {
+  BOOL shouldAnimate = !firstTimePush && ((RNSScreenView *) lastTop.view).stackAnimation != RNSScreenStackAnimationNone;
+
+  if (firstTimePush) {
     // nothing pushed yet
     [_controller setViewControllers:@[top] animated:NO];
   } else if (top != lastTop) {