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;}