[core] Add `line-gradient` property

Porting of https://github.com/mapbox/mapbox-gl-js/pull/6303
See the link above for the description of the feature and
its limitations).

Based on patch from @lbud (Lauren Budorick).

2 files changed, 61 insertions, 1 deletions

diff --git a/src/mbgl/style/layers/line_layer.cpp b/src/mbgl/style/layers/line_layer.cpp
index 1b84c2d73e..1ddc690cc7 100644
--- a/src/mbgl/style/layers/line_layer.cpp
+++ b/src/mbgl/style/layers/line_layer.cpp
@@ -438,6 +438,33 @@ TransitionOptions LineLayer::getLinePatternTransition() const {
     return impl().paint.template get<LinePattern>().options;
 }
 
+ColorRampPropertyValue LineLayer::getDefaultLineGradient() {
+    return { {} };
+}
+
+ColorRampPropertyValue LineLayer::getLineGradient() const {
+    return impl().paint.template get<LineGradient>().value;
+}
+
+void LineLayer::setLineGradient(ColorRampPropertyValue value) {
+    if (value == getLineGradient())
+        return;
+    auto impl_ = mutableImpl();
+    impl_->paint.template get<LineGradient>().value = value;
+    baseImpl = std::move(impl_);
+    observer->onLayerChanged(*this);
+}
+
+void LineLayer::setLineGradientTransition(const TransitionOptions& options) {
+    auto impl_ = mutableImpl();
+    impl_->paint.template get<LineGradient>().options = options;
+    baseImpl = std::move(impl_);
+}
+
+TransitionOptions LineLayer::getLineGradientTransition() const {
+    return impl().paint.template get<LineGradient>().options;
+}
+
 using namespace conversion;
 
 optional<Error> LineLayer::setPaintProperty(const std::string& name, const Convertible& value) {
@@ -453,6 +480,7 @@ optional<Error> LineLayer::setPaintProperty(const std::string& name, const Conve
         LineBlur,
         LineDasharray,
         LinePattern,
+        LineGradient,
         LineOpacityTransition,
         LineColorTransition,
         LineTranslateTransition,
@@ -463,6 +491,7 @@ optional<Error> LineLayer::setPaintProperty(const std::string& name, const Conve
         LineBlurTransition,
         LineDasharrayTransition,
         LinePatternTransition,
+        LineGradientTransition,
     };
 
     Property property = Property::Unknown;
@@ -567,6 +596,16 @@ optional<Error> LineLayer::setPaintProperty(const std::string& name, const Conve
             property = Property::LinePatternTransition;
         }
         break;
+    case util::hashFNV1a("line-gradient"):
+        if (name == "line-gradient") {
+            property = Property::LineGradient;
+        }
+        break;
+    case util::hashFNV1a("line-gradient-transition"):
+        if (name == "line-gradient-transition") {
+            property = Property::LineGradientTransition;
+        }
+        break;
     
     }
 
@@ -669,6 +708,18 @@ optional<Error> LineLayer::setPaintProperty(const std::string& name, const Conve
         
     }
     
+    if (property == Property::LineGradient) {
+        Error error;
+        optional<ColorRampPropertyValue> typedValue = convert<ColorRampPropertyValue>(value, error, false, false);
+        if (!typedValue) {
+            return error;
+        }
+        
+        setLineGradient(*typedValue);
+        return nullopt;
+        
+    }
+    
 
     Error error;
     optional<TransitionOptions> transition = convert<TransitionOptions>(value, error);
@@ -726,6 +777,11 @@ optional<Error> LineLayer::setPaintProperty(const std::string& name, const Conve
         return nullopt;
     }
     
+    if (property == Property::LineGradientTransition) {
+        setLineGradientTransition(*transition);
+        return nullopt;
+    }
+    
 
     return Error { "layer doesn't support this property" };
 }
diff --git a/src/mbgl/style/layers/line_layer_properties.hpp b/src/mbgl/style/layers/line_layer_properties.hpp
index aeaf51698a..5fd349d38b 100644
--- a/src/mbgl/style/layers/line_layer_properties.hpp
+++ b/src/mbgl/style/layers/line_layer_properties.hpp
@@ -72,6 +72,9 @@ struct LinePattern : CrossFadedPaintProperty<std::string> {
     static std::string defaultValue() { return ""; }
 };
 
+struct LineGradient : ColorRampProperty {
+};
+
 class LineLayoutProperties : public Properties<
     LineCap,
     LineJoin,
@@ -89,7 +92,8 @@ class LinePaintProperties : public Properties<
     LineOffset,
     LineBlur,
     LineDasharray,
-    LinePattern
+    LinePattern,
+    LineGradient
 > {};
 
 } // namespace style