iOS network activity indicator manager

ADVNetworkActivityIndicator.h

//
//  Copyright © 2011 Yuri Kotov
//

#import <Foundation/Foundation.h>

@interface ADVNetworkActivityIndicator : NSObject

+ (ADVNetworkActivityIndicator *) sharedIndicator;

- (void) start;
- (void) stop;

@end

ADVNetworkActivityIndicator.m

//
//  Copyright © 2011 Yuri Kotov
//

#import "ADVNetworkActivityIndicator.h"

static inline void CreateDispatchSourceAndSetHandler(dispatch_source_t *source, dispatch_block_t handler)
{
    *source = dispatch_source_create(DISPATCH_SOURCE_TYPE_DATA_ADD, 0, 0, dispatch_get_main_queue());
    dispatch_source_set_event_handler(*source, handler);
    dispatch_resume(*source);
}

@implementation ADVNetworkActivityIndicator
{
    signed long _count;
    dispatch_source_t _increment;
    dispatch_source_t _decrement;
}

#pragma mark - ADVNetworkActivityIndicator
+ (ADVNetworkActivityIndicator *) sharedIndicator
{
    static dispatch_once_t predicate;
    static ADVNetworkActivityIndicator *instance;
    dispatch_once(&predicate, ^{ instance = [self new]; });
    return instance;
}

- (void) start
{
    dispatch_source_merge_data(_increment, 1ul);
}

- (void) stop
{
    dispatch_source_merge_data(_decrement, 1ul);
}

#pragma mark - NSObject
- (id) init
{
    self = [super init];
    if (self)
    {
        __block __unsafe_unretained ADVNetworkActivityIndicator *this = self;
        CreateDispatchSourceAndSetHandler(&_increment, ^{
            if (0 == this->_count)
            {
                [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];
            }
            this->_count += dispatch_source_get_data(this->_increment);
        });
        CreateDispatchSourceAndSetHandler(&_decrement, ^{
            this->_count -= dispatch_source_get_data(this->_decrement);
            NSCAssert(-1 < this->_count, @"Unbalanced call to -[%@ stop]", [this class]);
            if (0 == this->_count)
            {
                [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
            }
        });
    }
    return self;
}

- (void) dealloc
{
    dispatch_release(_decrement);
    dispatch_release(_increment);
    #if !__has_feature(objc_arc)
    [super dealloc];
    #endif
}

@end