HTTP requests simultaneously in Swift - swift

Question

On start my app I do some http requests, some heavy http requests (downloading some images) and some heavy tasks with UIGraphics (for example doing icon for GMSMarker from two UIImages and other operations with GraphicsContext). It costs some time, so I want to do all that tasks simultaneously. Can you show me best way make it?
On start I have to:
Download and write to local database all devices
Download and write to local database all geofences
Download and write to local database all users
Download and write to local database all positions
Download images for devices, users and geofences
Setup GMSMarkers for devices, users and geofences (after images for that objects will be available - for setting icon of marker)
Code of my login function (it works, but too slow):
func loginPressed(_ sender: UIButton) {
guard
let username = self.usernameTextField.text,
let password = self.passwordTextField.text,
!username.isEmpty,
!password.isEmpty
else {
return
}
self.loginButton.isEnabled = false
self.activityIndicator.startAnimating()
WebService.shared.connect(email: username, password: password) { error, loggedUser in
guard
error == nil,
let loggedUser = loggedUser
else {
self.showAlert(title: "Ошибка подключения", message: error?.localizedDescription ?? "", style: .alert)
self.activityIndicator.stopAnimating()
self.loginButton.isEnabled = true
return
}
DB.users.client.insert(loggedUser)
print("Start loading user photo...")
loggedUser.loadPhoto() { image in
if let image = image {
loggedUser.photo = UIImageJPEGRepresentation(image, 0.0)
}
print("User photo loaded...")
loggedUser.marker = UserMarker(loggedUser, at: CLLocation(latitude: 48.7193900, longitude: 44.50183))
DB.users.client.modify(loggedUser)
}
DB.geofences.server.getAll() { geofences in
DB.devices.server.getAll() { devices in
DB.positions.server.getAll() { positions in
for device in devices {
device.loadPhoto() { image in
if let image = image {
device.photo = UIImageJPEGRepresentation(image, 0.0)
}
if let position = positions.findById(device.positionId) {
device.marker = DeviceMarker(device, at: position)
}
device.attributes.battery = device.lastKnownBattery(in: positions)
}
}
geofences.forEach({$0.marker = GeofenceMarker($0)})
DB.geofences.client.updateAddress(geofences) { geofences in
if DEBUG_LOGS {
print("Geofences with updated addresses: ")
geofences.forEach({print("\($0.name), \($0.address ?? "")")})
}
DB.devices.client.insert(devices)
DB.geofences.client.insert(geofences)
DB.positions.client.insert(positions)
self.activityIndicator.stopAnimating()
WebService.shared.addObserver(DefaultObserver.shared)
self.performSegue(withIdentifier: "toMapController", sender: self)
}
}
}
}
}
}
Not sure it's good idea post here code snippets of all classes and objects, hope you'll get idea.
Any help would be appreciated.
P.S. In case you wonder what is DB, it's database, which consists of two parts - server side and client side for each group of objects, so first task is get all objects from server and write them in memory (in client database)
P.S. I've changed logic from "download everything" on login to "download all I need right now and download rest later". So now after I've got all devices, geofences and positions I'm performing segue to MapController, on which I show all those objects. Just after login I'm showing deviceMarkers (GMSMarker) with default iconView. SO question is - can I after show map with all objects start download photos of devices in background and refresh markers with that photos after that (in main thread of course)?

Answer 1

Network requests are asynchronous by default, so what you are asking for has already the intended behavior.
You can, however, make your life much more simple by using a Promises library such as then.
An example usage might look like:
login(email: "foo#bar.com", password: "pa$$w0rd")
.whenAll(syncDevices(), syncGeofences(), syncUsers(), syncPositions(), syncImages())
.onError { err in
// process error
}
.finally {
// update UI
}

Answer 2

Your login function is slow because you are downloading AND writing to disk (as you mentioned in your question) "all" the data in your closures (geofences, devices and/or positions). Furthermore, all your operations are being executed in the main thread. You should never do I/O (networking, writing to disk) in the main thread, as this thread is used mainly for UI updates. You should offload the expensive tasks to another thread using GCD.
Also, it is worth mentioning that writing to disk is a relatively slow operation, especially if you are doing it for EVERY item you are downloading.
I would recommend you to JUST download any data to be displayed and then use an async task in a DispatchQueue (GCD) to persist the data downloaded to disk AFTER you've displayed the data in your UI.
I am not sure what the DB.geofences.server.getAll() lines do for geofences, devices and positions (in regards to how you handle your networking or database fetching), so I cannot advice you on that. What I can advice you on, is to structure your code the following way:
When user logs in, do the validation against the DB (remote) and guard against a valid login. Transition to your next view controller next (don't execute all your logic), since I can see that you are delegating way too much responsibility to your login action (for your login button).
From that second view controller, get your data through networking calls asynchronously on another thread using a .UserInitiated priority (to get results fast).
After doing all your networking operations, call DispatchQueue.main.async { ... } to update your UI Asynchronously in the main thread with the data you just got.
AFTER you've displayed the downloaded data, you can persist it to your local DB, ideally using another DispatchQueue async task.
If anything I said above does not make any sense to you, please read AppCoda's article about GCD here and RayWenderlich's GCD article here. They will give you the basic knowledge about GCD in iOS. Once you've done that, come back and try to structure your code the way I recommended above.
I hope this helps!