[ios, macos] Put MGLStyleLayer inits in respective classes

Previously, we declared MGLStyleLayer initializers in the MGLStyleLayer
protocol as optional methods. This made it easy to opt in and out
of initializers that did or or did not make sense for the subclass.
However, this approach was dangerous since it was easy for an
application developer to initialize an MGLStyleLayer subclass with an
init method that was actually not implemented in that class causing
an exception at runtime.

This commit moves the init methods that each subclass supports to
each subclass so xcode (and the compiler) help the developer avoid
the previously possible runtime exception.

In addition, a new init method is added that takes the source layer
and passes that on to `mbgl::style:Layer` (but only on classes
where this is possible in core). This allows an application developer
to style a specific source layer (i.e. the contour lines of the mapbox
terrain vector source).

Finally, this refactors MGLStyleLayer classes to use an MGLSource instead of a
string identifier for the source when initializing the style.

1 files changed, 2 insertions, 2 deletions

diff --git a/platform/ios/app/MBXViewController.m b/platform/ios/app/MBXViewController.m
index 605d6b2da8..0f81643f73 100644
--- a/platform/ios/app/MBXViewController.m
+++ b/platform/ios/app/MBXViewController.m
@@ -484,7 +484,7 @@ static NSString * const MBXViewControllerAnnotationViewReuseIdentifer = @"MBXVie
     MGLGeoJSONSource *source = [[MGLGeoJSONSource alloc] initWithSourceIdentifier:@"ams" URL:geoJSONURL];
     [self.mapView.style addSource:source];
     
-    MGLFillStyleLayer *fillLayer = [[MGLFillStyleLayer alloc] initWithLayerIdentifier:@"test" sourceIdentifier:@"ams"];
+    MGLFillStyleLayer *fillLayer = [[MGLFillStyleLayer alloc] initWithLayerIdentifier:@"test" source:source];
     fillLayer.fillColor = [UIColor purpleColor];
     [self.mapView.style addLayer:fillLayer];
 }
@@ -495,7 +495,7 @@ static NSString * const MBXViewControllerAnnotationViewReuseIdentifer = @"MBXVie
     MGLRasterSource *rasterSource = [[MGLRasterSource alloc] initWithSourceIdentifier:@"my-raster-source" URL:rasterURL tileSize:512];
     [self.mapView.style addSource:rasterSource];
     
-    MGLRasterStyleLayer *rasterLayer = [[MGLRasterStyleLayer alloc] initWithLayerIdentifier:@"my-raster-layer" sourceIdentifier:@"my-raster-source"];
+    MGLRasterStyleLayer *rasterLayer = [[MGLRasterStyleLayer alloc] initWithLayerIdentifier:@"my-raster-layer" source:rasterSource];
     MGLStyleAttributeFunction *opacityFunction = [[MGLStyleAttributeFunction alloc] init];
     opacityFunction.stops = @{@20.0f: @1.0f,
                               @5.0f: @0.0f};