feat: linear gradient ios (#45434)

Summary:
- Adds `background` prop that supports CSS's linear gradient. Later this can be extended to support various other gradients and possibly CSS's background image (less motivation as better solutions exists for image)
- Uses `CAGradientlayer` to draw Linear Gradient layers. So it is GPU optimised under the hood.
- Style supports JS object to specify `LinearGradient`, so it can support Animated libraries.

## Changelog:
[IOS] [ADDED] - linear gradient

<!-- Help reviewers and the release process by writing your own changelog entry.

Pick one each for the category and type tags:

[ANDROID|GENERAL|IOS|INTERNAL] [BREAKING|ADDED|CHANGED|DEPRECATED|REMOVED|FIXED|SECURITY] - Message

For more details, see:
https://reactnative.dev/contributing/changelogs-in-pull-requests

Pull Request resolved: https://github.com/facebook/react-native/pull/45434

Test Plan:
- Check out `processBackground-test.js` for supported syntax testcases.
- Checkout example added in ViewExample.js

Although the PR is tested well but open to any changes/feedback on the approach taken.

Android PR - https://github.com/facebook/react-native/pull/45433. Separated the PRs to keep it easier to review. Both PRs can be reviewed individually.

Reviewed By: NickGerleman

Differential Revision: D60791581

Pulled By: joevilches

fbshipit-source-id: 051088fdf68d9fe20c0c306f1f1c591cbd77f3d5

packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm

@@ -33,6 +33,7 @@ using namespace facebook::react;
   __weak CALayer *_borderLayer;
   CALayer *_boxShadowLayer;
   CALayer *_filterLayer;
+  NSMutableArray<CAGradientLayer *> *_gradientLayers;
   BOOL _needsInvalidateLayer;
   BOOL _isJSResponder;
   BOOL _removeClippedSubviews;
@@ -458,6 +459,11 @@ using namespace facebook::react;
     }
   }
 
+  // `linearGradient`
+  if (oldViewProps.backgroundImage != newViewProps.backgroundImage) {
+    needsInvalidateLayer = YES;
+  }
+
   // `boxShadow`
   if (oldViewProps.boxShadow != newViewProps.boxShadow) {
     needsInvalidateLayer = YES;
@@ -830,6 +836,56 @@ static RCTBorderStyle RCTBorderStyleFromBorderStyle(BorderStyle borderStyle)
     [self.layer addSublayer:_filterLayer];
   }
 
+  [self clearExistingGradientLayers];
+  auto backgroundImage = _props->backgroundImage;
+  if (backgroundImage.empty()) {
+    return;
+  }
+  for (const auto &gradient : backgroundImage) {
+    CAGradientLayer *gradientLayer = [CAGradientLayer layer];
+    NSMutableArray *colors = [NSMutableArray array];
+    NSMutableArray *locations = [NSMutableArray array];
+    for (const auto &colorStop : gradient.colorStops) {
+      if (colorStop.position.has_value()) {
+        auto location = @(colorStop.position.value());
+        UIColor *color = RCTUIColorFromSharedColor(colorStop.color);
+        [colors addObject:(id)color.CGColor];
+        [locations addObject:location];
+      }
+    }
+    gradientLayer.startPoint = CGPointMake(gradient.startX, gradient.startY);
+    gradientLayer.endPoint = CGPointMake(gradient.endX, gradient.endY);
+
+    if (locations.count > 0) {
+      gradientLayer.locations = locations;
+    }
+    gradientLayer.colors = colors;
+    gradientLayer.frame = layer.bounds;
+
+    // border styling to work with gradient layers
+    if (useCoreAnimationBorderRendering) {
+      gradientLayer.borderWidth = layer.borderWidth;
+      gradientLayer.borderColor = layer.borderColor;
+      gradientLayer.cornerRadius = layer.cornerRadius;
+      gradientLayer.cornerCurve = layer.cornerCurve;
+    } else {
+      CAShapeLayer *maskLayer = [CAShapeLayer layer];
+      CGPathRef path = RCTPathCreateWithRoundedRect(
+          self.bounds,
+          RCTGetCornerInsets(RCTCornerRadiiFromBorderRadii(borderMetrics.borderRadii), UIEdgeInsetsZero),
+          nil);
+      maskLayer.path = path;
+      CGPathRelease(path);
+      gradientLayer.mask = maskLayer;
+    }
+
+    // border layer should appear above gradient layers to make sure that the border is visible
+    gradientLayer.zPosition = _borderLayer.zPosition - 1;
+
+    [self.layer addSublayer:gradientLayer];
+    [_gradientLayers addObject:gradientLayer];
+  }
+
   [_boxShadowLayer removeFromSuperlayer];
   _boxShadowLayer = nil;
   if (!_props->boxShadow.empty()) {
@@ -848,6 +904,18 @@ static RCTBorderStyle RCTBorderStyleFromBorderStyle(BorderStyle borderStyle)
   }
 }
 
+- (void)clearExistingGradientLayers
+{
+  if (_gradientLayers == nil) {
+    _gradientLayers = [NSMutableArray new];
+    return;
+  }
+  for (CAGradientLayer *gradientLayer in _gradientLayers) {
+    [gradientLayer removeFromSuperlayer];
+  }
+  [_gradientLayers removeAllObjects];
+}
+
 #pragma mark - Accessibility
 
 - (NSObject *)accessibilityElement

packages/rn-tester/js/examples/LinearGradient/LinearGradientExample.js

@@ -126,4 +126,35 @@ exports.examples = [
       );
     },
   },
+  {
+    title: 'Gradient with uniform border style',
+    render(): React.Node {
+      return (
+        <GradientBox
+          testID="linear-gradient-with-uniform-borders"
+          style={{
+            experimental_backgroundImage:
+              'linear-gradient(to bottom right, yellow, green);',
+            borderRadius: 16,
+          }}
+        />
+      );
+    },
+  },
+  {
+    title: 'Gradient with non-uniform border style',
+    render(): React.Node {
+      return (
+        <GradientBox
+          testID="linear-gradient-with-non-uniform-borders"
+          style={{
+            experimental_backgroundImage:
+              'linear-gradient(to bottom right, yellow, green);',
+            borderTopRightRadius: 8,
+            borderTopLeftRadius: 80,
+          }}
+        />
+      );
+    },
+  },
 ];

packages/rn-tester/js/utils/RNTesterList.ios.js

@@ -298,6 +298,10 @@ const APIs: Array<RNTesterModuleInfo> = ([
     key: 'FilterExample',
     module: require('../examples/Filter/FilterExample'),
   },
+  {
+    key: 'LinearGradient',
+    module: require('../examples/LinearGradient/LinearGradientExample'),
+  },
   {
     key: 'MixBlendModeExample',
     module: require('../examples/MixBlendMode/MixBlendModeExample'),