我有一个异步的方法，如何在多次并发后得到全部结束的通知

 dispatch_group_t serviceGroup = dispatch_group_create();

// Start the first service
dispatch_group_enter(serviceGroup);
[self.configService startWithCompletion:^(ConfigResponse *results, NSError* error){
    // Do something with the results
    configError = error;
    dispatch_group_leave(serviceGroup);
}];

// Start the second service
dispatch_group_enter(serviceGroup);
[self.preferenceService startWithCompletion:^(PreferenceResponse *results, NSError* error){
    // Do something with the results
    preferenceError = error;
    dispatch_group_leave(serviceGroup);
}];

dispatch_group_notify(serviceGroup,dispatch_get_main_queue(),^{
    // Assess any errors
    NSError *overallError = nil;
    if (configError || preferenceError)
    {
        // Either make a new error or assign one of them to the overall error
        overallError = configError ?: preferenceError;
    }
    // Now call the final completion block
    completion(overallError);
});