Resolve dynamic background colors against the node's trait collection (behind experiment flag) (#2142)

* Resolve dynamic background colors against node's trait collection

Layer-backed nodes use CGColorRef instead of UIColor, so dynamic
(e.g. dark mode) background colors were not resolved for the current
trait collection. Resolve the UIColor via
resolvedColorWithTraitCollection: using the node's primitive trait
collection before extracting the CGColor, both when setting the
background color and when reapplying it on trait collection changes.

* Create ResolveBackgroundColorWithNodeTraits experiment

* Put resolve background colors for UIViewBridge behind an experiment flag

Author: ignacioparadisi

Source/ASDisplayNode.mm

@@ -458,10 +458,17 @@ - (void)asyncTraitCollectionDidChangeWithPreviousTraitCollection:(ASPrimitiveTra
         CGFloat cornerRadius = self->_cornerRadius;
         ASCornerRoundingType cornerRoundingType = self->_cornerRoundingType;
         UIColor *backgroundColor = self->_backgroundColor;
+        ASPrimitiveTraitCollection currentPrimitiveTraitCollection = self->_primitiveTraitCollection;
         self->__instanceLock__.unlock();
-        // TODO: we should resolve color using node's trait collection
-        // but Texture changes it from many places, so we may receive the wrong one.
-        CGColorRef cgBackgroundColor = backgroundColor.CGColor;
+        CGColorRef cgBackgroundColor;
+        if (ASActivateExperimentalFeature(ASExperimentalResolveBackgroundColorWithNodeTraits)) {
+          UITraitCollection *traitCollection = ASPrimitiveTraitCollectionToUITraitCollection(currentPrimitiveTraitCollection);
+          cgBackgroundColor = [backgroundColor resolvedColorWithTraitCollection:traitCollection].CGColor;
+        } else {
+          // TODO: we should resolve color using node's trait collection
+          // but Texture changes it from many places, so we may receive the wrong one.
+          cgBackgroundColor = backgroundColor.CGColor;
+        }
         if (!CGColorEqualToColor(self->_layer.backgroundColor, cgBackgroundColor)) {
           // Background colors do not dynamically update for layer backed nodes since they utilize CGColorRef
           // instead of UIColor. Non layer backed node also receive color to the layer (see [_ASPendingState -applyToView:withSpecialPropertiesHandling:]).

Source/ASExperimentalFeatures.h

@@ -35,6 +35,7 @@ typedef NS_OPTIONS(NSUInteger, ASExperimentalFeatures) {
   ASExperimentalLockTextRendererCache = 1 << 14,                            // exp_lock_text_renderer_cache
   ASExperimentalHierarchyDisplayDidFinishIsRecursive = 1 << 15,             // exp_hierarchy_display_did_finish_is_recursive
   ASExperimentalCheckBatchFetchingOnScroll = 1 << 16,                       // exp_check_batch_fetching_on_scroll
+  ASExperimentalResolveBackgroundColorWithNodeTraits = 1 << 17,             // exp_resolve_background_color_with_node_traits
   ASExperimentalFeatureAll = 0xFFFFFFFF
 };
 

Source/ASExperimentalFeatures.mm

@@ -28,7 +28,8 @@
                                       @"exp_no_text_renderer_cache",
                                       @"exp_lock_text_renderer_cache",
                                       @"exp_hierarchy_display_did_finish_is_recursive",
-                                      @"exp_check_batch_fetching_on_scroll"]));
+                                      @"exp_check_batch_fetching_on_scroll",
+                                      @"exp_resolve_background_color_with_node_traits"]));
 
   if (flags == ASExperimentalFeatureAll) {
     return allNames;

Source/Private/ASDisplayNode+UIViewBridge.mm

@@ -762,8 +762,10 @@ - (void)setBackgroundColor:(UIColor *)newBackgroundColor
   if (shouldApply) {
     UIColor *oldBackgroundColor = _backgroundColor;
     _backgroundColor = newBackgroundColor;
+    BOOL resolveWithNodeTraits = ASActivateExperimentalFeature(ASExperimentalResolveBackgroundColorWithNodeTraits);
+    UITraitCollection *traitCollection = resolveWithNodeTraits ? ASPrimitiveTraitCollectionToUITraitCollection(_primitiveTraitCollection) : nil;
     if (_flags.layerBacked) {
-      _layer.backgroundColor = _backgroundColor.CGColor;
+      _layer.backgroundColor = resolveWithNodeTraits ? [_backgroundColor resolvedColorWithTraitCollection:traitCollection].CGColor : _backgroundColor.CGColor;
     } else {
       /*
        NOTE: Setting to the view and layer individually is necessary.
@@ -775,7 +777,7 @@ - (void)setBackgroundColor:(UIColor *)newBackgroundColor
        */
       _view.backgroundColor = _backgroundColor;
       // Gather the CGColorRef from the view incase there are any changes it might apply to which CGColorRef is returned for dynamic colors
-      _layer.backgroundColor = _view.backgroundColor.CGColor;
+      _layer.backgroundColor = resolveWithNodeTraits ? [_backgroundColor resolvedColorWithTraitCollection:traitCollection].CGColor : _view.backgroundColor.CGColor;
     }
 
     if (![oldBackgroundColor isEqual:newBackgroundColor]) {