iOS多段线覆盖导致内存问题

我在我的应用程序中使用mkmapview，它包含多个多段线覆盖和多个自定义注释，而运行应用程序时经常收到内存警告，我为此付出了很多努力

提前谢谢

for (int i = 0; i<[latArray count]-1; i++)
    {
        NSString *lat1 = [latArray objectAtIndex:i];
        NSString *longi1 = [longArray objectAtIndex:i];

        firstPlace = CLLocationCoordinate2DMake([lat1 doubleValue], [longi1 doubleValue]);

        NSString *lat2 = [latArray objectAtIndex:i+1];
        NSString *longi2 = [longArray objectAtIndex:i+1];

        secPlace = CLLocationCoordinate2DMake([lat2 doubleValue], [longi2 doubleValue]);

        [self directionRequest:firstPlace :secPlace index:i];
    }

-(void)directionRequest:(CLLocationCoordinate2D )firstCord :(CLLocationCoordinate2D )secCord index:(int)index
{

    @try
    {

        MKPlacemark *source=[[MKPlacemark alloc]initWithCoordinate:firstCord addressDictionary:[NSDictionary dictionaryWithObjectsAndKeys:@"",@"", nil]];

        MKMapItem *sourceMapItem=[[MKMapItem alloc]initWithPlacemark:source];
        [sourceMapItem setName:@""];

        MKPlacemark *destination=[[MKPlacemark alloc]initWithCoordinate:secCord addressDictionary:[NSDictionary dictionaryWithObjectsAndKeys:@"",@"", nil]];

        MKMapItem *destinationMapitem=[[MKMapItem alloc]initWithPlacemark:destination];
        [destinationMapitem setName:@""];

        MKDirectionsRequest *dirRequest=[[MKDirectionsRequest alloc]init];
        [dirRequest setSource:sourceMapItem];
        [dirRequest setDestination:destinationMapitem];
        //    dirRequest.requestsAlternateRoutes = YES;
        [dirRequest setTransportType:MKDirectionsTransportTypeAutomobile];

        MKDirections *direction=[[MKDirections alloc]initWithRequest:dirRequest];

        [direction calculateDirectionsWithCompletionHandler:^(MKDirectionsResponse *response, NSError *error)
         {

          //   NSLog(@"response = %@",response);
             NSArray *arrRoutes = [response routes];
             [arrRoutes enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop)
              {
                  MKRoute *rout = obj;
                  MKPolyline *line = [rout polyline];
                  [iMapView addOverlay:line];


              }];

         }];

    }
    @catch (NSException *exception)
    {

    }

}