OC 创建单例

static BlockBackground *_sharedInstance = nil;

+ (BlockBackground*)sharedInstance
{
    if (_sharedInstance != nil) {
        return _sharedInstance;
    }

    @synchronized(self) {
        if (_sharedInstance == nil) {
            _sharedInstance = [[self alloc] init];
        }
    }
    
    return _sharedInstance;
}

+ (id)allocWithZone:(NSZone*)zone
{
    @synchronized(self) {
        if (_sharedInstance == nil) {
            _sharedInstance = [super allocWithZone:zone];
            return _sharedInstance;
        }
    }
    NSAssert(NO, @ "[BlockBackground alloc] explicitly called on singleton class.");
    return nil;
}

- (id)copyWithZone:(NSZone*)zone
{
    return self;
}

- (id)retain
{
    return self;
}

- (unsigned)retainCount
{
    return UINT_MAX;
}

- (oneway void)release
{
}

- (id)autorelease
{
    return self;
}