Implement ScrollViewProviding

Author: kmichalikk

ios/RNSScreen.mm

@@ -1242,8 +1242,9 @@ - (void)overrideScrollViewBehaviorInFirstDescendantChainIfNeeded
 
 - (void)updateContentScrollViewEdgeEffectsIfExists
 {
-  [RNSScrollEdgeEffectApplicator applyToScrollView:[RNSScrollViewFinder findScrollViewInFirstDescendantChainFrom:self]
-                                      withProvider:self];
+  [RNSScrollEdgeEffectApplicator
+      applyToScrollView:[RNSScrollViewFinder findContentScrollViewWithFirstDescendantsChain:self]
+           withProvider:self];
 }
 
 #pragma mark - RNSSafeAreaProviding

ios/RNSScreenStack.mm

@@ -194,7 +194,7 @@ - (bool)onRepeatedTabSelectionOfTabScreenController:(RNSTabsScreenViewController
     return [[self popToRootViewControllerAnimated:true] count] > 0;
   } else if (tabScreenController.tabScreenComponentView.shouldUseRepeatedTabSelectionScrollToTopSpecialEffect) {
     UIScrollView *scrollView =
-        [RNSScrollViewFinder findScrollViewInFirstDescendantChainFrom:[[self topViewController] view]];
+        [RNSScrollViewFinder findContentScrollViewWithFirstDescendantsChain:[[self topViewController] view]];
     return [scrollView rnscreens_scrollToTop];
   }
 

ios/RNSScrollViewFinder.h

@@ -165,8 +165,9 @@ - (void)overrideScrollViewBehaviorInFirstDescendantChainIfNeeded
 
 - (void)updateContentScrollViewEdgeEffectsIfExists
 {
-  [RNSScrollEdgeEffectApplicator applyToScrollView:[RNSScrollViewFinder findScrollViewInFirstDescendantChainFrom:self]
-                                      withProvider:self];
+  [RNSScrollEdgeEffectApplicator
+      applyToScrollView:[RNSScrollViewFinder findContentScrollViewWithFirstDescendantsChain:self]
+           withProvider:self];
 }
 
 #pragma mark - Prop update utils

ios/RNSScrollViewFinder.mm

@@ -107,7 +107,7 @@ - (bool)tabScreenSelectedRepeatedly
     return [[self tabsSpecialEffectsDelegate] onRepeatedTabSelectionOfTabScreenController:self];
   } else if (self.tabScreenComponentView.shouldUseRepeatedTabSelectionScrollToTopSpecialEffect) {
     UIScrollView *scrollView =
-        [RNSScrollViewFinder findScrollViewInFirstDescendantChainFrom:[self tabScreenComponentView]];
+        [RNSScrollViewFinder findContentScrollViewWithFirstDescendantsChain:[self tabScreenComponentView]];
     return [scrollView rnscreens_scrollToTop];
   }
 

ios/bottom-tabs/RNSBottomTabsScreenComponentView.mm

@@ -0,0 +1,9 @@
+@protocol RNSContentScrollViewProviding
+/**
+ * Finds content ScrollView within provider's hierarchy. The content ScrollView serves as a main interaction on the
+ * given screen. Implementations should use `RNSScrollViewFinder.findContentScrollViewWithDelegatingToProvider` to
+ * continue continue the search when no more custom logic is necessary (e.g Stack determining the topmost screen (custom
+ * part) and forwarding the call directly to its view (going back to regular search)).
+ */
+- (nullable UIScrollView *)findContentScrollView;
+@end

ios/bottom-tabs/RNSTabsScreenViewController.mm

@@ -0,0 +1,25 @@
+#import <UIKit/UIKit.h>
+#import "RNSContentScrollViewProviding.h"
+
+@interface RNSScrollViewFinder : NSObject
+/**
+ * Searches for content ScrollView by traversing down the hierarchy using first subview, similar to UIKit behavior.
+ * It will fail if:
+ * - UIScrollView is not a first subview of view or one of its descendants in the hierarchy,
+ * - if UIScrollView's parent is not yet attached.
+ */
++ (nullable UIScrollView *)findContentScrollViewWithFirstDescendantsChain:(nullable UIView *)view;
+
+/**
+ * Looks for UIScrollView in a similar way to `findContentScrollViewWithFirstDescendantsChain`, until it finds
+ * `RNSContentScrollViewProviding`. Then, it delegates the task to the provider, and returns the results. This can
+ * overcome the problems of subviews' children not being mounted yet, or ScrollView being mounted at index different
+ * than 0.
+ *
+ * Caveat: when traversing the hierarchy, we don't check for conformance to protocol,
+ * but whether the view responds to `RNSContentScrollViewProviding.findContentScrollView`.
+ * This doesn't place locks and is faster.
+ */
++ (nullable UIScrollView *)findContentScrollViewWithDelegatingToProvider:(nullable UIView *)view;
+
+@end

ios/helpers/scroll-view/RNSContentScrollViewProviding.h

@@ -0,0 +1,41 @@
+#import "RNSScrollViewFinder.h"
+
+@implementation RNSScrollViewFinder
+
++ (UIScrollView *)findContentScrollViewWithFirstDescendantsChain:(UIView *)view
+{
+  UIView *currentView = view;
+
+  while (currentView != nil) {
+    if ([currentView isKindOfClass:UIScrollView.class]) {
+      return static_cast<UIScrollView *>(currentView);
+    } else if ([currentView.subviews count] > 0) {
+      currentView = currentView.subviews[0];
+    } else {
+      break;
+    }
+  }
+
+  return nil;
+}
+
++ (nullable UIScrollView *)findContentScrollViewWithDelegatingToProvider:(nullable UIView *)view
+{
+  UIView *currentView = view;
+
+  while (currentView != nil) {
+    if ([currentView isKindOfClass:UIScrollView.class]) {
+      return static_cast<UIScrollView *>(currentView);
+    } else if ([currentView respondsToSelector:@selector(findContentScrollView)]) {
+      return [static_cast<id<RNSContentScrollViewProviding>>(currentView) findContentScrollView];
+    } else if ([currentView.subviews count] > 0) {
+      currentView = currentView.subviews[0];
+    } else {
+      break;
+    }
+  }
+
+  return nil;
+}
+
+@end