[iOS] centralized network activity indicator counter

UIApplication+NetworkActivityIndicator.m

@interface UIApplication (NetworkActivityIndicator)

- (void)increaseActiveRequests;
- (void)decreaseActiveRequests;

@end

@implementation UIApplication (NetworkActivityIndicator)

static int counter = 0;

- (void)increaseActiveRequests {
	[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:(++counter > 0)];
}

- (void)decreaseActiveRequests {
	if (--counter < 0) counter = 0;
	[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:(counter > 0)];
}

@end