ios 判断字符串是否为空

- (BOOL) isBlankString:(NSString *)string {

    if (string == nil || string == NULL) {

        return YES;

    }

    if ([string isKindOfClass:[NSNull class]]) {

        return YES;

    }

    if ([[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0) {

        return YES;

    }

    return NO;

}