Add missing sourceDidChange event (#3421)

platform/ios/app/MBXViewController.mm

@@ -2264,6 +2264,11 @@ CLLocationCoordinate2D randomWorldCoordinate(void) {
 
 // MARK: - MLNMapViewDelegate
 
+- (void)mapView:(MLNMapView *)mapView sourceDidChange:(MLNSource *)source
+{
+    NSLog(@"A source was updated: %@", source.identifier);
+}
+
 - (void)mapView:(MLNMapView *)mapView
     shaderWillCompile:(NSInteger)id
               backend:(NSInteger)backend

platform/ios/src/MLNMapView.mm

@@ -6933,8 +6933,13 @@ static void *windowScreenContext = &windowScreenContext;
 }
 
 - (void)sourceDidChange:(MLNSource *)source {
-    // no-op: we only show attribution after tapping the info button, so there's no
-    // interactive update needed.
+    if (!_mbglMap) {
+        return;
+    }
+
+    if ([self.delegate respondsToSelector:@selector(mapView:sourceDidChange:)]) {
+        [self.delegate mapView:self sourceDidChange:source];
+    }
 }
 
 - (void)didFailToLoadImage:(NSString *)imageName {

platform/ios/src/MLNMapViewDelegate.h

@@ -302,6 +302,14 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (void)mapView:(MLNMapView *)mapView didFinishLoadingStyle:(MLNStyle *)style;
 
+/**
+ Tells the delegate that the source changed.
+
+ @param mapView The map view that owns the source.
+ @param source The source that changed.
+ */
+- (void)mapView:(MLNMapView *)mapView sourceDidChange:(MLNSource *)source;
+
 /**
  Tells the delegate that the `mapView` is missing an image. The image should be added synchronously
  with ``MLNStyle/setImage:forName:`` to be rendered on the current zoom level. When loading icons

platform/ios/test/MLNMapViewDelegateIntegrationTests.swift

@@ -30,6 +30,8 @@ extension MLNMapViewDelegateIntegrationTests: MLNMapViewDelegate {
 
     func mapView(_: MLNMapView, didFinishLoading _: MLNStyle) {}
 
+    func mapView(_: MLNMapView, sourceDidChange _: MLNSource) {}
+
     func mapView(_: MLNMapView, didSelect _: MLNAnnotation) {}
 
     func mapView(_: MLNMapView, didDeselect _: MLNAnnotation) {}