[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).

5 files changed, 72 insertions, 2 deletions

diff --git a/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/LineLayer.java b/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/LineLayer.java
index d6519c991f..9130bfd76f 100644
--- a/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/LineLayer.java
+++ b/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/LineLayer.java
@@ -478,6 +478,34 @@ public class LineLayer extends Layer {
     nativeSetLinePatternTransition(options.getDuration(), options.getDelay());
   }
 
+  /**
+   * Get the LineGradient property
+   *
+   * @return property wrapper value around String
+   */
+  @SuppressWarnings("unchecked")
+  public PropertyValue<String> getLineGradient() {
+    checkThread();
+    return (PropertyValue<String>) new PropertyValue("line-gradient", nativeGetLineGradient());
+  }
+
+  /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @return int representation of a rgba string color
+   * @throws RuntimeException thrown if property isn't a value
+   */
+  @ColorInt
+  public int getLineGradientAsInt() {
+    checkThread();
+    PropertyValue<String> value = getLineGradient();
+    if (value.isValue()) {
+      return rgbaToColor(value.getValue());
+    } else {
+      throw new RuntimeException("line-gradient was set as a Function");
+    }
+  }
+
   @Keep
   private native Object nativeGetLineCap();
 
@@ -574,6 +602,9 @@ public class LineLayer extends Layer {
   @Keep
   private native void nativeSetLinePatternTransition(long duration, long delay);
 
+  @Keep
+  private native Object nativeGetLineGradient();
+
   @Override
   @Keep
   protected native void finalize() throws Throwable;
diff --git a/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/PropertyFactory.java b/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/PropertyFactory.java
index cd7bd473f3..c619d0141a 100644
--- a/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/PropertyFactory.java
+++ b/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/PropertyFactory.java
@@ -395,6 +395,36 @@ public class PropertyFactory {
   }
 
   /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @param value a int color value
+   * @return property wrapper around String color
+   */
+  public static PropertyValue<String> lineGradient(@ColorInt int value) {
+    return new PaintPropertyValue<>("line-gradient", colorToRgbaString(value));
+  }
+
+  /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @param value a String value
+   * @return property wrapper around String
+   */
+  public static PropertyValue<String> lineGradient(String value) {
+    return new PaintPropertyValue<>("line-gradient", value);
+  }
+
+  /**
+   * Defines a gradient with which to color a line feature. Can only be used with GeoJSON sources that specify `"lineMetrics": true`.
+   *
+   * @param expression an expression statement
+   * @return property wrapper around an expression statement
+   */
+  public static PropertyValue<Expression> lineGradient(Expression expression) {
+    return new PaintPropertyValue<>("line-gradient", expression);
+  }
+
+  /**
    * The opacity at which the icon will be drawn.
    *
    * @param value a Float value
diff --git a/platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/layer.junit.ejs b/platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/layer.junit.ejs
index 8acfc88dfa..a319d52ea3 100644
--- a/platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/layer.junit.ejs
+++ b/platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/layer.junit.ejs
@@ -118,7 +118,7 @@ public class <%- camelize(type) %>LayerTest extends BaseActivityTest {
 
 <% } -%>
 <% for (const property of properties) { -%>
-<% if (property.name != 'heatmap-color') { -%>
+<% if (property['property-type'] !== 'color-ramp') { -%>
 <% if (property.transition) { -%>
 
   @Test
diff --git a/platform/android/src/style/layers/line_layer.cpp b/platform/android/src/style/layers/line_layer.cpp
index af4e24523e..f143ecc236 100644
--- a/platform/android/src/style/layers/line_layer.cpp
+++ b/platform/android/src/style/layers/line_layer.cpp
@@ -236,6 +236,12 @@ namespace android {
         layer.as<mbgl::style::LineLayer>()->LineLayer::setLinePatternTransition(options);
     }
 
+    jni::Object<jni::ObjectTag> LineLayer::getLineGradient(jni::JNIEnv& env) {
+        using namespace mbgl::android::conversion;
+        Result<jni::jobject*> converted = convert<jni::jobject*>(env, layer.as<mbgl::style::LineLayer>()->LineLayer::getLineGradient());
+        return jni::Object<jni::ObjectTag>(*converted);
+    }
+
 
     jni::Class<LineLayer> LineLayer::javaClass;
 
@@ -287,7 +293,8 @@ namespace android {
             METHOD(&LineLayer::getLineDasharray, "nativeGetLineDasharray"),
             METHOD(&LineLayer::getLinePatternTransition, "nativeGetLinePatternTransition"),
             METHOD(&LineLayer::setLinePatternTransition, "nativeSetLinePatternTransition"),
-            METHOD(&LineLayer::getLinePattern, "nativeGetLinePattern"));
+            METHOD(&LineLayer::getLinePattern, "nativeGetLinePattern"),
+            METHOD(&LineLayer::getLineGradient, "nativeGetLineGradient"));
     }
 
 } // namespace android
diff --git a/platform/android/src/style/layers/line_layer.hpp b/platform/android/src/style/layers/line_layer.hpp
index 84ecc77139..9eef1349cb 100644
--- a/platform/android/src/style/layers/line_layer.hpp
+++ b/platform/android/src/style/layers/line_layer.hpp
@@ -74,6 +74,8 @@ public:
     jni::Object<jni::ObjectTag> getLinePattern(jni::JNIEnv&);
     void setLinePatternTransition(jni::JNIEnv&, jlong duration, jlong delay);
     jni::Object<TransitionOptions> getLinePatternTransition(jni::JNIEnv&);
+
+    jni::Object<jni::ObjectTag> getLineGradient(jni::JNIEnv&);
     jni::jobject* createJavaPeer(jni::JNIEnv&);
 
 }; // class LineLayer