Adds getClusterChildren for iOS and Android (rnmapbox#1495)

ios/RCTMGL/RCTMGLShapeSourceManager.m

@@ -202,4 +202,25 @@ - (UIView*)view
     }];
 }
 
+RCT_EXPORT_METHOD(getClusterChildren:(nonnull NSNumber*)reactTag
+                  clusterId:(nonnull NSNumber *)clusterId                  
+                  resolver:(RCTPromiseResolveBlock)resolve
+                  rejecter:(RCTPromiseRejectBlock)reject)
+{
+    [self.bridge.uiManager addUIBlock:^(__unused RCTUIManager *manager, NSDictionary<NSNumber*, UIView*> *viewRegistry) {
+        RCTMGLShapeSource* shapeSource = (RCTMGLShapeSource *)viewRegistry[reactTag];
+
+        NSArray<id<MGLFeature>> *shapes = [sтвhapeSource getClusterChildren: clusterId];
+
+        NSMutableArray<NSDictionary*> *features = [[NSMutableArray alloc] initWithCapacity:shapes.count];
+        for (int i = 0; i < shapes.count; i++) {
+            [features addObject:shapes[i].geoJSONDictionary];
+        }
+
+        resolve(@{
+                  @"data": @{ @"type": @"FeatureCollection", @"features": features }
+                  });
+    }];
+}
+
 @end

javascript/components/ShapeSource.js

@@ -291,6 +291,29 @@ class ShapeSource extends NativeBridgeComponent(AbstractSource) {
     return res.data;
   }
 
+  /**
+   * Returns the FeatureCollection from the cluster (on the next zoom level).
+   *
+   * @example
+   * const collection = await shapeSource.getClusterChildren(clusterId);
+   *
+   * @param  {number} clusterId - The id of the cluster to expand.
+   * @return {FeatureCollection}
+   */
+  async getClusterChildren(clusterId) {
+    const res = await this._runNativeCommand(
+      'getClusterChildren',
+      this._nativeRef,
+      [clusterId],
+    );
+
+    if (isAndroid()) {
+      return JSON.parse(res.data);
+    }
+
+    return res.data;
+  }
+
   setNativeProps(props) {
     const shallowProps = Object.assign({}, props);