[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, 15 insertions, 0 deletions

diff --git a/platform/darwin/src/MGLStyle.h b/platform/darwin/src/MGLStyle.h
index 10fad0196e..31808290f9 100644
--- a/platform/darwin/src/MGLStyle.h
+++ b/platform/darwin/src/MGLStyle.h
@@ -200,6 +200,9 @@ static const NSInteger MGLStyleDefaultVersion = 9;
 /**
  Adds a new source to the current style.
  
+ @note Adding the same source instance more than once or reusing source 
+    identifers in the sources you add will trigger exceptions.
+ 
  @param source The source to add to the current style.
  */
 - (void)addSource:(MGLSource *)source;
@@ -247,6 +250,9 @@ static const NSInteger MGLStyleDefaultVersion = 9;
 /**
  Adds a new layer on top of existing layers.
 
+ @note Adding the same layer instance more than once or reusing layer
+    identifers in the layers you add will trigger exceptions.
+
  @param layer The layer object to add to the map view. This object must be an
     instance of a concrete subclass of `MGLStyleLayer`.
  */
@@ -255,6 +261,9 @@ static const NSInteger MGLStyleDefaultVersion = 9;
 /**
  Inserts a new layer into the style at the given index.
  
+ @note Adding the same layer instance more than once or reusing layer
+    identifers in the layers you add will trigger exceptions.
+ 
  @param layer The layer to insert.
  @param index The index at which to insert the layer. An index of 0 would send
     the layer to the back; an index equal to the number of objects in the
@@ -272,6 +281,9 @@ static const NSInteger MGLStyleDefaultVersion = 9;
     inspectable in Interface Builder, or a manually constructed `NSURL`. This
     approach also avoids layer identifer name changes that will occur in the default
     style’s layers over time.
+ 
+    Adding the same layer instance more than once or reusing layer identifers in 
+    the layers you add will trigger exceptions.
 
  @param layer The layer to insert.
  @param sibling An existing layer in the style.
@@ -288,6 +300,9 @@ static const NSInteger MGLStyleDefaultVersion = 9;
     inspectable in Interface Builder, or a manually constructed `NSURL`. This
     approach also avoids layer identifer name changes that will occur in the default
     style’s layers over time.
+ 
+    Adding the same layer instance more than once or reusing layer identifers in
+    the layers you add will trigger exceptions.
 
  @param layer The layer to insert.
  @param sibling An existing layer in the style.