[ios, macos] Raise NSException if layer or source added more than once

When MGLSource and MGLLayer instances are added to the style, they lose
ownership of their std::unique_ptr<T> after it is moved to the mbgl
level. Subsequent attempts to add such instances result in a C++
exception.

This adds logic in the Darwin platform to raise a NSException if the
source and layer addToMapView methods are called more than once and the
pointer is invalid. In addition, the documentation in MGLStyle for
addSource: and addLayer: has been update to warn developers to avoid
adding the same instance twice and, for that matter, instances with the
same identifier.

1 files changed, 6 insertions, 0 deletions

diff --git a/platform/darwin/src/MGLStyleLayer.mm.ejs b/platform/darwin/src/MGLStyleLayer.mm.ejs
index 85a1af9970..c89912c1ff 100644
--- a/platform/darwin/src/MGLStyleLayer.mm.ejs
+++ b/platform/darwin/src/MGLStyleLayer.mm.ejs
@@ -112,6 +112,12 @@ namespace mbgl {
 
 - (void)addToMapView:(MGLMapView *)mapView
 {
+    if (_pendingLayer == nullptr) {
+        [NSException raise:@"MGLRedundantLayerException"
+            format:@"This instance %@ was already added to %@. Adding the same layer instance " \
+                    "to the style more than once is invalid.", self, mapView.style];
+    }
+
     [self addToMapView:mapView belowLayer:nil];
 }