fixes #245: create cache key ivar for unchanging use

@@ -41,6 +41,7 @@
 
 @property (nonatomic, strong) NSDictionary *infoDictionary;
 @property (nonatomic, strong) NSString *tileJSON;
+@property (nonatomic, strong) NSString *cacheKey;
 
 @end
 
@@ -84,6 +85,8 @@
 
         _tileJSON = tileJSON;
 
+        _cacheKey = [NSString stringWithFormat:@"Mapbox-%@%@", [_infoDictionary objectForKey:@"id"], ([_infoDictionary objectForKey:@"version"] ? [@"-" stringByAppendingString:[_infoDictionary objectForKey:@"version"]] : @"")];
+
         id dataObject = nil;
         
         if (mapView && (dataObject = [_infoDictionary objectForKey:@"data"]) && dataObject)
@@ -352,7 +355,7 @@
 
 - (NSString *)uniqueTilecacheKey
 {
-    return [NSString stringWithFormat:@"Mapbox-%@%@", [self.infoDictionary objectForKey:@"id"], ([self.infoDictionary objectForKey:@"version"] ? [@"-" stringByAppendingString:[self.infoDictionary objectForKey:@"version"]] : @"")];
+    return self.cacheKey;
 }
 
 - (NSString *)shortName