nested callback hell in objective c - ios

Question

I am now working on an app that works with BLE, Backend server and location. I am facing a problem which I am not sure how to get out of which is what people call "Callback hell". The entire CoreBluetooth framework in iOS is based on a delegate pattern, which until you can use the CBPeripheral has to go to at least 3 callbacks:
DidConnectToPeripheral
DidDiscoverServices
DidDiscoverCharacteristics
But in fact there could be many more, and every action you take with the device will come back as a callback to one of those functions. Now when I want to "Rent" this ble product, I must connect to it, after connecting send a requests to the server and get the user's current location, after that all happens I have to write a value in the bluetooth device and get confirmation. This would not be so difficult, but unfortunately each and every one of those stages is failable, so error handling needs to be added. Not to mention implementing timeout.
I am sure I am not the only one to approach such issues so I looked around and I found 2 things that might help:
the Advanced NSOperations talk in the wwdc 2015, but after trying for 4 days to make it work, it seems like the code is too buggy.
Promisekit but I couldn't find a way to wrap CoreBluetooth.
How are people with even more complicated apps deal with this? in swift or objc.
Some sample problematic code:
-(void)startRentalSessionWithLock:(DORLock *)lock timeOut:(NSTimeInterval)timeout forSuccess:(void (^)(DORRentalSession * session))successBlock failure:(failureBlock_t)failureBlock{
//we set the block to determine what happens
NSAssert(lock.peripheral, #"lock has to have peripheral to connect to");
if (!self.rentalSession) {
self.rentalSession = [[DORRentalSession alloc] initWithLock:nil andSessionDict:#{} active:NO];
}
self.rentalSession.lock = lock;
[self connectToLock:self.rentalSession.lock.peripheral timeOut:timeout completionBlock:^(CBPeripheral *peripheral, NSError *error) {
self.BTConnectionCompleted = nil;
if (!error) {
[[INTULocationManager sharedInstance] requestLocationWithDesiredAccuracy:INTULocationAccuracyHouse timeout:1 delayUntilAuthorized:YES block:^(CLLocation *currentLocation, INTULocationAccuracy achievedAccuracy, INTULocationStatus status) {
if (status == INTULocationStatusSuccess || status == INTULocationStatusTimedOut) {
[self startServerRentalForSessionLockWithUserLocation:currentLocation.coordinate forSuccess:^(DORRentalSession *session) {
if (self.rentalSession.lock.peripheral && self.rentalSession.lock.peripheral.state == CBPeripheralStateConnected) {
[self.rentalSession.lock.peripheral setNotifyValue:YES forCharacteristic:self.rentalSession.lock.charectaristics.sensorCharacteristic];
}else{
//shouldnt come here
}
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
if (self.rentalSession.lock.peripheral.state == CBPeripheralStateConnected) {
!self.rentalSession.lock.open ? [self sendUnlockBLECommandToSessionLock] : nil;
if (successBlock) {
successBlock(session);
}
}else{
[self endCurrentRentalSessionWithLocation:self.rentalSession.lock.latLng andPositionAcc:#(1) Success:^(DORRentalSession *session) {
if (failureBlock) {
failureBlock([[NSError alloc] initWithDomain:DonkeyErrorDomain code:46 userInfo:#{NSLocalizedDescriptionKey:#"Could't connect to lock"}],200);
}
} failure:^(NSError *error, NSInteger httpCode) {
if (failureBlock) {
failureBlock([[NSError alloc] initWithDomain:DonkeyErrorDomain code:45 userInfo:#{NSLocalizedDescriptionKey:#"fatal error"}],200);
}
}];
}
});
} failure:^(NSError *error, NSInteger httpCode) {
if (failureBlock) {
failureBlock(error,httpCode);
}
}];
}else{
NSError *gpsError = [self donkeyGPSErrorWithINTULocationStatus:status];
if (failureBlock) {
failureBlock(gpsError,200);
}
}
}];
}else{
if (failureBlock) {
failureBlock(error,200);
}
}
}];
}

Answer 1

To get rid of this nested calls you can use GCD group + serial execution queue:
dispatch_queue_t queue = ddispatch_queue_create("com.example.queue", NULL);
dispatch_group_t group = dispatch_group_create();
// Add a task to the group
dispatch_group_async(group, queue, ^{
// Some asynchronous work
});
// Make dispatch_group_async and dispatch_group_sync calls here
// Callback to be executed when all scheduled tasks are completed.
dispatch_group_notify(serviceGroup,dispatch_get_main_queue(),^{
// Do smth when everything has finished
});
// wait for all tasks to complete
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
The other solution based on GCD groups is described here