[WIP] [ios, macos] Calculate number of tiles for an offline region

platform/darwin/src/MGLOfflineRegion.h

@@ -2,6 +2,8 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+@class MGLTileSource;
+
 /**
  An object conforming to the `MGLOfflineRegion` protocol determines which
  resources are required by an `MGLOfflinePack` object. At present, only
@@ -10,6 +12,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @protocol MGLOfflineRegion <NSObject>
 
+/**
+ The number of tiles needed to load one of the style’s sources within the region.
+ */
+-(uint64_t)countTilesForTileSource:(MGLTileSource *)tileSource;
+
 @end
 
 NS_ASSUME_NONNULL_END

platform/darwin/src/MGLTilePyramidOfflineRegion.mm

@@ -7,6 +7,8 @@
 #import "MGLOfflineRegion_Private.h"
 #import "MGLGeometry_Private.h"
 #import "MGLStyle.h"
+#import "MGLTileSource.h"
+#import "MGLVectorTileSource.h"
 
 @interface MGLTilePyramidOfflineRegion () <MGLOfflineRegion_Private>
 
@@ -53,6 +55,20 @@ - (instancetype)initWithStyleURL:(NSURL *)styleURL bounds:(MGLCoordinateBounds)b
     return self;
 }
 
+-(uint64_t)countTilesForTileSource:(MGLTileSource *)tileSource {
+    auto tilePyramidOfflineRegion = [self offlineRegionDefinition];
+
+    mbgl::style::SourceType sourceType;
+
+    if ([tileSource isKindOfClass:[MGLVectorTileSource class]]) {
+        sourceType = mbgl::style::SourceType::Vector;
+    } else  {
+        sourceType = mbgl::style::SourceType::Raster;
+    }
+
+    return tilePyramidOfflineRegion.tileCount(sourceType, 512, {static_cast<unsigned char>(tilePyramidOfflineRegion.minZoom), static_cast<unsigned char>(tilePyramidOfflineRegion.maxZoom)});
+}
+
 - (instancetype)initWithOfflineRegionDefinition:(const mbgl::OfflineRegionDefinition &)definition {
     NSURL *styleURL = [NSURL URLWithString:@(definition.styleURL.c_str())];
     MGLCoordinateBounds bounds = MGLCoordinateBoundsFromLatLngBounds(definition.bounds);

platform/darwin/test/MGLOfflineRegionTests.m

@@ -29,4 +29,6 @@ - (void)testEquality {
     XCTAssertEqual(original.maximumZoomLevel, original.maximumZoomLevel, @"Maximum zoom level has changed.");
 }
 
+// TODO: Add tileCount test(s)
+
 @end