+ (void)process:(void(^)(NSError *error))callback
{
    static KSEncore *encore;
    static dispatch_once_t token;
    dispatch_once(&token, ^{ encore = [KSEncore new]; });

    [encore queue:callback block:^{
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // ...
            dispatch_async(dispatch_get_main_queue(), ^{
                [encore flush:^(void (^callback)(NSError *error)) {
                    callback(error);
                }];
            });
        });
    }];
}